publicationDate,title,abstract,id
2017-01-16,Electric field modulation of the non-linear areal magnetic anisotropy energy,"We study the ferromagnetic layer thickness dependence of the
voltage-controlled magnetic anisotropy (VCMA) in gated CoFeB/MgO
heterostructures with heavy metal underlayers. When the effective CoFeB
thickness is below ~1 nm, the VCMA efficiency of Ta/CoFeB/MgO heterostructures
considerably decreases with decreasing CoFeB thickness. We find that a high
order phenomenological term used to describe the thickness dependence of the
areal magnetic anisotropy energy can also account for the change in the areal
VCMA efficiency. In this structure, the higher order term competes against the
common interfacial VCMA, thereby reducing the efficiency at lower CoFeB
thickness. The areal VCMA efficiency does not saturate even when the effective
CoFeB thickness exceeds ~1 nm. We consider the higher order term is related to
the strain that develops at the CoFeB/MgO interface: as the average strain of
the CoFeB layer changes with its thickness, the electronic structure of the
CoFeB/MgO interface varies leading to changes in areal magnetic anisotropy
energy and VCMA efficiency.",1701.04150v1
2018-10-01,Determination of spin Hall angle in heavy metal/CoFeB-based heterostructures with interfacial spin-orbit fields,"Magnetization dynamics in W/CoFeB, CoFeB/Pt and W/CoFeB/Pt multilayers was
investigated using spin-orbit-torque ferromagnetic resonance (SOT-FMR)
technique. An analytical model based on magnetization dynamics due to SOT was
used to fit heavy metal (HM) thickness dependence of symmetric and
antisymmetric components of the SOT-FMR signal. The analysis resulted in a
determination of the properties of HM layers, such as spin Hall angle and spin
diffusion length. The spin Hall angle of -0.36 and 0.09 has been found in the
W/CoFeB and CoFeB/Pt bilayers, respectively, which add up in the case of
W/CoFeB/Pt trilayer. More importantly, we have determined effective interfacial
spin-orbit fields at both W/CoFeB and CoFeB/Pt interfaces, which are shown to
cancel Oersted field for particular thicknesses of the heavy metal layers,
leading to pure spin-current-induced dynamics and indicating the possibility
for a more efficient magnetization switching.",1810.00641v1
2011-07-01,"High frequency magnetic behavior through the magnetoimpedance effect in CoFeB/(Ta, Ag, Cu) multilayered ferromagnetic thin films","We studied the dynamics of magnetization through an investigation of the
magnetoimpedance effect in CoFeB/(Ta, Ag, Cu) multilayered thin films grown by
magnetron sputtering. Impedance measurements were analyzed in terms of the
mechanisms responsible for their variations at different frequency intervals
and the magnetic and structural properties of the multilayers. Analysis of the
mechanisms responsible for magnetoimpedance according to frequency and external
magnetic field showed that for the CoFeB/Cu multilayer, ferromagnetic resonance
(FMR) contributes significantly to the magnetoimpedance effect at frequencies
close to 470 MHz. This frequency is low when compared to the results obtained
for CoFeB/Ta and CoFeB/Ag multilayers and is a result of the anisotropy
distribution and non-formation of regular bilayers in this sample. The MImax
values occurred at different frequencies according to the used non-magnetic
metal. Variations between 25% and 30% were seen for a localized frequency band,
as in the case of CoFeB/Ta and CoFeB/Ag, as well as for a wide frequency range,
in the case of CoFeB/Cu.",1107.0204v1
2023-01-12,Cryogenic temperature deposition of high-performance CoFeB/MgO/CoFeB magnetic tunnel junctions on 300 mm wafers,"We developed a cryogenic temperature deposition process for high-performance
CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) on 300 mm thermally oxidized
silicon wafers. The effect of the deposition temperature of the CoFeB layers on
the nanostructure, magnetic and magneto-transport properties of the MTJs were
investigated in detail. When CoFeB was deposited at 100 K, the MTJs exhibited a
perpendicular magnetic anisotropy (PMA) of 214 uJ/m2 and a voltage-controlled
magnetic anisotropy (VCMA) coefficient of -45 fJ/Vm, corresponding to 1.4- and
1.7-fold enhancements in PMA and VCMA, respectively, compared to the case of
room-temperature deposition of CoFeB. The improvement in the MTJ properties was
not simply due to the morphology of the MTJ films. The interface-sensitive
magneto-transport properties indicated that interfacial qualities such as
intermixing and oxidation states at the MgO/CoFeB interfaces were improved by
the cryogenic temperature deposition. Cryogenic-temperature sputtering
deposition is expected to be a standard manufacturing process for
next-generation magnetoresistive random-access memory.",2301.04823v1
2007-02-09,X-ray photoemission study of CoFeB/MgO thin film bi-layers,"We present results from an X-ray photoemission spectroscopy (XPS) study of
CoFeB/MgO bi-layers where we observe process-dependent formation of B, Fe, and
Co oxides at the CoFeB/MgO interface due to oxidation of CoFeB during MgO
deposition. Vacuum annealing reduces the Co and Fe oxides but further
incorporates B into the MgO forming a composite MgBxOy layer. Inserting an Mg
layer between CoFeB and MgO introduces an oxygen sink, providing increased
control over B content in the barrier.",0702232v1
2005-04-03,Dependence of Giant Tunnel Magnetoresistance of Sputtered CoFeB/MgO/CoFeB Magnetic Tunnel Junctions on MgO Barrier Thickness and Annealing Temperatur,"We investigated the dependence of giant tunnel magnetoresistance (TMR) on the
thickness of an MgO barrier and on the annealing temperature of sputtered
CoFeB/MgO/CoFeB magnetic tunnel junctions deposited on SiO2/Si wafers. The
resistance-area product exponentially increases with MgO thickness, indicating
that the quality of MgO barriers is high in the investigated thickness range of
1.15-2.4 nm. High-resolution transmission electron microscope images show that
annealing at 375 C results in the formation of crystalline CoFeB/MgO/CoFeB
structures, even though CoFeB electrodes are amorphous in the as-sputtered
state. The TMR ratio increases with annealing temperature and is as high as
260% at room temperature and 403% at 5 K.",0504051v1
2021-03-30,Thermal annealing enhancement of Josephson critical currents in ferromagnetic CoFeB,"The electrical and structural properties of Co40Fe40B20 (CoFeB) alloy are
tunable with thermal annealing. This is key in the optimization of CoFeB-based
spintronic devices, where the advantageously low magnetic coercivity, high spin
polarization, and controllable magnetocrystalline anisotropy are utilised. So
far, there has been no report on superconducting devices based on CoFeB. Here,
we report Nb/CoFeB/Nb Josephson devices and demonstrate an enhancement of the
critical current by up to 700% following thermal annealing due to increased
structural ordering of the CoFeB. The results demonstrate that CoFeB is a
promising material for the development of superconducting spintronic devices.",2103.16136v1
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
2023-08-17,Interfacial Fe segregation and its influence on magnetic properties of CoFeB/MgFeO multilayers,"We investigated the effect of Fe segregated from partially Fe-substituted MgO
(MgFeO) on the magnetic properties of CoFeB/MgFeO multilayers. X-ray
photoelectron spectroscopy (XPS) as well as magnetic measurements revealed that
the segregated Fe was reduced to metal and exhibited ferromagnetism at the
CoFeB/MgFeO interface. The CoFeB/MgFeO multilayer showed more than 2-fold
enhancement in perpendicular magnetic anisotropy (PMA) energy density compared
with a standard CoFeB/MgO multilayer. The PMA energy density was further
enhanced by inserting an ultrathin MgO layer in between CoFeB and MgFeO layers.
Ferromagnetic resonance measurement also revealed a remarkable reduction of
magnetic damping in the CoFeB/MgFeO multilayers.",2308.08876v1
2012-08-29,CoFeB Thickness Dependence of Thermal Stability Factor in CoFeB/MgO Perpendicular Magnetic Tunnel Junctions,"Thermal stability factor (delta) of recording layer was studied in
perpendicular anisotropy CoFeB/MgO magnetic tunnel junctions (p-MTJs) with
various CoFeB recording layer thicknesses and junction sizes. In all series of
p-MTJs with different thicknesses, delta is virtually independent of the
junction sizes of 48-81 nm in diameter. The values of delta increase linearly
with increasing the recording layer thickness. The slope of the linear fit is
explained well by a model based on nucleation type magnetization reversal.",1208.5828v1
2016-10-24,Spin orbit effects in CoFeB/MgO hetereostructures with heavy metal underlayers,"We study effects originating from the strong spin orbit coupling in CoFeB/MgO
heterostructures with heavy metal (HM) underlayers. The perpendicular magnetic
anisotropy at the CoFeB/MgO interface, the spin Hall angle of the heavy metal
layer, current induced torques and the Dzyaloshinskii-Moriya interaction at the
HM/CoFeB interfaces are studied for films in which the early 5d transition
metals are used as the HM underlayer. We show how the choice of the HM layer
influences these intricate spin orbit effects that emerge within the bulk and
at interfaces of the heterostructures.",1610.07473v1
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
2017-07-11,Interface Dzyaloshinskii-Moriya interaction in the interlayer exchange antiferromagnetic coupled Pt/CoFeB/Ru/CoFeB systems,"Interfacial Dzyaloshinskii-Moriya interaction (iDMI) in interlayer exchange
coupled (IEC) Pt/Co$_{20}$Fe$_{60}$B$_{20}$(1.12
nm)/Ru/Co$_{20}$Fe$_{60}$B$_{20}$(1.12 nm) systems have been studied
theoretically and experimentally. Vibrating sample magnetometer has been used
to measure their magnetization at saturation and their interlayer exchange
coupling constants. These latter are found to be of an antiferromagnetic nature
for the investigated Ru range thickness (0.5-1 nm). Their dynamic magnetic
properties were studied using Brillouin light scattering (BLS) technique. The
BLS measurements reveal pronounced non-reciprocal spin waves propagation. In
contrast to the calculations for symmetrical IEC CoFeB layers, this
experimental nonreciprocity is Ru thickness and thus coupling strength
dependent. Therefore, to explain the experimental behaviour, a theoretical
model based on the perpendicular interface anisotropy difference between the
bottom and top CoFeB layers has been developed. We show that the Ru thickness
dependence of the spin wave non-reciprocity is well reproduced by considering a
constant iDMI and different perpendicular interfacial anisotropy fields between
the top and bottom CoFeB layers. This anisotropy difference has been confirmed
by the investigation of the CoFeB thickness dependence of effective
magnetization of Pt/CoFeB/Ru and Ru/CoFeB/MgO individual layers, where a linear
behaviour has been observed.",1707.03427v1
2017-09-21,Low Gilbert Damping Constant in Perpendicularly Magnetized W/CoFeB/MgO Films with High Thermal Stability,"Perpendicular magnetic materials with low damping constant and high thermal
stability have great potential for realizing high-density, non-volatile, and
low-power consumption spintronic devices, which can sustain operation
reliability for high processing temperatures. In this work, we study the
Gilbert damping constant ({\alpha}) of perpendicularly magnetized W/CoFeB/MgO
films with a high perpendicular magnetic anisotropy (PMA) and superb thermal
stability. The {\alpha} of these PMA films annealed at different temperatures
is determined via an all-optical Time-Resolved Magneto-Optical Kerr Effect
method. We find that {\alpha} of these W/CoFeB/MgO PMA films decreases with
increasing annealing temperature, reaches a minimum of {\alpha} = 0.016 at an
annealing temperature of 350 {\deg}C, and then increases to 0.024 after
post-annealing at 400 {\deg}C. The minimum {\alpha} observed at 350 {\deg}C is
rationalized by two competing effects as the annealing temperature becomes
higher: the enhanced crystallization of CoFeB and dead-layer growth occurring
at the two interfaces of the CoFeB layer. We further demonstrate that {\alpha}
of the 400 {\deg}C-annealed W/CoFeB/MgO film is comparable to that of a
reference Ta/CoFeB/MgO PMA film annealed at 300 {\deg}C, justifying the
enhanced thermal stability of the W-seeded CoFeB films.",1709.07483v1
2018-01-30,"Crystalline structure and XMCD studies of Co40Fe40B20 grown on Bi2Te3, BiTeI and Bi2Se3","Epitaxial films of Co40Fe40B20 (further - CoFeB) were grown on Bi2Te3(001)
and Bi2Se3(001) substrates by laser molecular beam epitaxy (LMBE) technique at
200-400C. Bcc-type crystalline structure of CoFeB with (111) plane parallel to
(001) plane of Bi2Te3 was observed, in contrast to polycrystalline CoFeB film
formed on Bi2Se3(001) at RT using high-temperature seeding layer. Therefore,
structurally ordered ferromagnetic thin films were obtained on the topological
insulator surface for the first time. Using high energy electron diffraction
(RHEED) 3D reciprocal space mapping, epitaxial relations of main
crystallographic axes for the CoFeB/ Bi2Te3 heterostructure were revealed. MOKE
and AFM measurements showed the isotropic azimuthal in-plane behavior of
magnetization vector in CoFeB/ Bi2Te3, in contrast to 2nd order magnetic
anisotropy seen in CoFeB/Bi2Se3. XPS measurements showed more stable behavior
of CoFeB grown on Bi2Te3 to the oxidation, in compare to CoFeB grown on Bi2Se3.
XAS and XMCD measurements of both concerned nanostructures allowed calculation
of spin and orbital magnetic moments for Co and Fe. Additionally, crystalline
structure and XMCD response of the CoFeB/BiTeI and Co55Fe45/BiTeI systems were
studied, epitaxial relations of main crystallographic axes were found, and spin
and orbital magnetic moments were calculated.",1801.10061v1
2005-09-28,Antiferromagnetically coupled CoFeB/Ru/CoFeB trilayers,"This work reports on the magnetic interlayer coupling between two amorphous
CoFeB layers, separated by a thin Ru spacer. We observe an antiferromagnetic
coupling which oscillates as a function of the Ru thickness x, with the second
antiferromagnetic maximum found for x=1.0 to 1.1 nm. We have studied the
switching of a CoFeB/Ru/CoFeB trilayer for a Ru thickness of 1.1 nm and found
that the coercivity depends on the net magnetic moment, i.e. the thickness
difference of the two CoFeB layers. The antiferromagnetic coupling is almost
independent on the annealing temperatures up to 300 degree C while an annealing
at 350 degree C reduces the coupling and increases the coercivity, indicating
the onset of crystallization. Used as a soft electrode in a magnetic tunnel
junction, a high tunneling magnetoresistance of about 50%, a well defined
plateau and a rectangular switching behavior is achieved.",0509749v1
2016-12-09,Influence of intermixing at the Ta/CoFeB interface on spin Hall angle in Ta/CoFeB/MgO heterostructures,"We investigate the spin Hall effect in perpendicularly magnetized
Ta/Co40Fe40B20/MgO trilayers with Ta underlayers thicker than the spin
diffusion length. The crystallographic structures of the Ta layer and Ta/CoFeB
interface are examined in detail using X-ray diffraction and transmission
electron microscopy. The thinnest Ta underlayer is amorphous, whereas for
thicker Ta layers a disoriented tetragonal beta-phase appears. Effective
spin-orbit torques are calculated based on harmonic Hall voltage measurements
performed in a temperature range between 15 and 300 K. To account for the
temperature dependence of damping-like and field-like torques, we extend the
spin diffusion model by including an additional contribution from the Ta/CoFeB
interface. Based on this approach, the temperature dependence of the spin Hall
angle in the Ta underlayer and at Ta/CoFeB interface are determined separately.
The results indicate an almost temperature-independent spin Hall angle of
theta_SH-N = -0.2 in Ta and a strongly temperature-dependent theta_SH-I for the
intermixed Ta/CoFeB interface.",1612.03020v1
2005-09-28,"Switching of sub-micrometer sized, antiferromagnetically coupled CoFeB/Ru/CoFeB trilayers","This work reports on the magnetic reversal of sub-micrometer sized elements
consisting of an CoFeB/Ru/CoFeB artificial ferrimagnet (AFi). The elements were
patterned into ellipses having a width of approximately 250 to 270nm and a
varying aspect ratio between 1.3 and 8. The coercivity was found to decrease
with an increasing imbalance of the magnetic moment of the two
antiferromagnetically coupled layers and is therefore strongly affected by an
increase of effective anisotropy due to the antiferromagnetic coupling of the
two layers. With respect to a single layer of amorphous CoFeB, patterned in
comparable elements, the AFi has an increased coercivity. Switching asteroids
comparable to single layers were only observed for samples with a high net
moment.",0509750v3
2006-02-26,Strong temperature dependence of antiferromagnetic coupling in CoFeB/Ru/CoFeB,"The temperature dependence of saturation and spin-flop fields for artificial
ferrimagnets (AFi) based on antiparallel coupled CoFeB/Ru/CoFeB trilayers has
been investigated in a temperature range between 80K and 600K. The results
presented in this paper are relevant for magnetic devices using this system,
e.g. magnetic-random access memory based on spin-flop switching. In good
accordance to the theory, the saturation field Hsat behaves like Hsat ~ H_0
(T/T_0)/sinh(T/T_0) with a characteristic temperature of T_0 = 150K. Within
this model, the Fermi velocity for the Ru layer is of the order of 10^5m/s,
therefore, explaining the strong variation of the coupling strength with the
temperature in Ru based AFi. Furthermore, a strong uniaxial anisotropy of K_u =
2x10^3 J/m^3 with a small angular distribution of the anisotropy axes is
observed for the AFi trilayers based on amorphous CoFeB alloys.",0602609v2
2006-06-20,Giant tunnel magnetoresistance and high annealing stability in CoFeB/MgO/CoFeB magnetic tunnel junctions with synthetic pinned layer,"We investigated the relationship between tunnel magnetoresistance (TMR) ratio
and the crystallization of CoFeB layers through annealing in magnetic tunnel
junctions (MTJs) with MgO barriers that had CoFe/Ru/CoFeB synthetic ferrimagnet
pinned layers with varying Ru spacer thickness (tRu). The TMR ratio increased
with increasing annealing temperature (Ta) and tRu, reaching 361% at Ta = 425C,
whereas the TMR ratio of the MTJs with pinned layers without Ru spacers
decreased at Ta over 325C. Ruthenium spacers play an important role in forming
an (001)-oriented bcc CoFeB pinned layer, resulting in a high TMR ratio through
annealing at high temperatures.",0606503v1
2013-04-17,Tunneling magneto thermocurrent in CoFeB/MgO/CoFeB based magnetic tunnel junctions,"We study the tunneling magneto thermopower and tunneling magneto
thermocurrent of CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJ). The devices
show a clear change of the thermoelectric properties upon reversal of the
magnetisation of the CoFeB layers from parallel to the antiparallel
orientation. When switching from parallel to antiparallel the thermopower
increases by up to 55% where as the thermocurrent drops by 45%. These
observations can be well explained by the Onsager relations taking into account
the tunneling magneto resistance of the MTJ. These findings contrast previous
studies on AlO based MTJ systems, revealing tunneling magneto thermo power but
no tunneling magneto thermocurrent.",1304.4798v1
2014-01-18,Enhancement of Perpendicular Magnetic Anisotropy and Transmission of Spin-Hall-Effect-Induced Spin Currents by a Hf Spacer Layer in W/Hf/CoFeB/MgO Layer,"We report that strong perpendicular magnetic anisotropy of the ferromagnetic
layer in a W/CoFeB/MgO multilayer structure can be established by inserting a
Hf layer as thin as 0.25 nm between the W and CoFeB layers. The Hf spacer also
allows transmission of spin currents generated by an in-plane charge current in
the W layer to apply strong spin torque on the CoFeB, thereby enabling
current-driven magnetic switching. The antidamping-like and field-like
components of the spin torque exerted on a 1 nm CoFeB layer are of comparable
magnitudes in this geometry. Both components originate from the spin Hall
effect in the underlying W layer.",1401.4617v1
2014-12-12,Asymmetric spin-wave dispersion due to Dzyaloshinskii-Moriya interaction in an ultrathin Pt/CoFeB film,"Employing Brillouin spectroscopy, strong interfacial Dzyaloshinskii-Moriya
interactions have been observed in an ultrathin Pt/CoFeB film. Our
micromagnetic simulations show that spin-wave nonreciprocity due to asymmetric
surface pinning is insignificant for the 0.8nmthick CoFeB film studied. The
observed high asymmetry of the monotonic spin wave dispersion relation is thus
ascribed to strong Dzyaloshinskii-Moriya interactions present at the Pt/CoFeB
interface. Our findings should further enhance the significance of CoFeB as an
important material for magnonic, spintronic and skyrmionic applications.",1412.3907v2
2017-04-11,CoFeAlB alloy with low damping and low magnetization for spin transfer torque switching,"We investigate the effect of Al doping on the magnetic properties of the
alloy CoFeB. Comparative measurements of the saturation magnetization, the
Gilbert damping parameter $\alpha$ and the exchange constant as a function of
the annealing temperature for CoFeB and CoFeAlB thin films are presented. Our
results reveal a strong reduction of the magnetization for CoFeAlB in
comparison to CoFeB. If the prepared CoFeAlB films are amorphous, the damping
parameter $\alpha$ is unaffected by the Al doping in comparison to the CoFeB
alloy. In contrast, in the case of a crystalline CoFeAlB film, $\alpha$ is
found to be reduced. Furthermore, the x-ray characterization and the evolution
of the exchange constant with the annealing temperature indicate a similar
crystallization process in both alloys. The data proves the suitability of
CoFeAlB for spin torque switching properties where a reduction of the switching
current in comparison with CoFeB is expected.",1704.03326v1
2019-02-20,CoFeB/MgO/CoFeB structures with orthogonal easy axes: perpendicular anisotropy and damping,"We report on the Gilbert damping parameter $\alpha$, the effective
magnetization $4\pi M_{eff}$, and the asymmetry of the $g$-factor in
bottom-CoFeB(0.93~nm)/MgO(0.90--1.25~nm)/CoFeB(1.31~nm)-top as-deposited
systems.
  Magnetization of CoFeB layers exhibits a specific noncollinear configuration
with orthogonal easy axes and with $4\pi M_{eff}$ values of $+2.2$ kG and
$-2.3$ kG for the bottom and top layers, respectively. We show that $4\pi
M_{eff}$ depends on the asymmetry $g_\perp - g_\parallel$ of the $g$-factor
measured in the perpendicular and the in-plane directions revealing a highly
nonlinear relationship. In contrast, the Gilbert damping is practically the
same for both layers. Annealing of the films results in collinear easy axes
perpendicular to the plane for both layers. However, the linewidth is strongly
increased due to enhanced inhomogeneous broadening.",1902.07563v1
2017-02-17,Spin-orbit torque in MgO/CoFeB/Ta/CoFeB/MgO symmetric structure with interlayer antiferromagnetic coupling,"Spin current generated by spin Hall effect in the heavy metal would diffuse
up and down to adjacent ferromagnetic layers and exert torque on their
magnetization, called spin-orbit torque. Antiferromagnetically coupled
trilayers, namely the so-called synthetic antiferromagnets (SAF), are usually
employed to serve as the pinned layer of spintronic devices based on spin
valves and magnetic tunnel junctions to reduce the stray field and/or increase
the pinning field. Here we investigate the spin-orbit torque in
MgO/CoFeB/Ta/CoFeB/MgO perpendicularly magnetized multilayer with interlayer
antiferromagnetic coupling. It is found that the magnetization of two CoFeB
layers can be switched between two antiparallel states simultaneously. This
observation is replicated by the theoretical calculations by solving
Stoner-Wohlfarth model and Landau-Lifshitz-Gilbert equation. Our findings
combine spin-orbit torque and interlayer coupling, which might advance the
magnetic memories with low stray field and low power consumption.",1702.05331v1
2019-09-27,Observation of spin-orbit magnetoresistance in CoFeB/heavy metal/MgO with existence of both spin Hall effect and Edelstein effect,"In this paper, we report the observation of spin-orbit magnetoresistance
(SOMR) in ferromagnetic metal/heavy metal/MgO system. We measure the
magnetoresistance as the function of the thickness of heavy metal (HM) for
CoFeB/HM/MgO and CoFeB/HM films where HM = Pt and Ta. Besides the conventional
spin Hall magnetoresistance (SMR) peak, the evidence of the SOMR is indicated
by another peak of the MR ratio when the thickness of HM is around 1 ~ 2 nm for
CoFeB/HM/MgO films, which is absent for CoFeB/HM films. We speculate the SOMR
observed in our experiment originates from the spin-orbit coupling at the
HM/MgO interface. We give the boundary conditions of our samples and calculate
the theoretical magnetoresistance based on spin diffusion equation. Based on
the theoretical results, we can explain the two peaks we observe separately
comes from the spin current generated by spin Hall effect and by Edelstein
effect.",1909.12811v3
2021-08-11,Enhancement of in-plane anisotropy in MoS2/CoFeB bilayers,"Transition metal dichalcogenides (TMD) possess novel properties which makes
them potential candidates for various spintronic applications. Heterostructures
of TMD with magnetic thin film have been extensively considered for
spin-orbital torque, enhancement of perpendicular magnetic anisotropy etc.
However, the effect of TMD on magnetic anisotropy in heterostructures of
in-plane magnetization has not been studied so far. Further the effect of the
TMD on the domain structure and magnetization reversal of the ferromagnetic
system is another important aspect to be understood. In this context we study
the effect of MoS2, a well-studied TMD material, on magnetic properties of
CoFeB in MoS2/CoFeB heterostructures. The reference CoFeB film possess a weak
in-plane anisotropy. However, when the CoFeB is deposited on MoS2 the in-plane
anisotropy is enhanced as observed from magneto optic Kerr effect (MOKE)
microscopy as well as ferromagnetic resonance (FMR). Magnetic domain structure
and magnetization reversal have also been significantly modified for the
MoS2/CoFeB bilayer as compared to the reference CoFeB layer. Frequency and
angle dependent FMR measurement show that the magnetic anisotropy of CoFeB
increases with increase in thickness of MoS2 in the MoS2/CoFeB
heterostructures.",2108.05130v1
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
2023-06-05,Spin-orbit torque generation in bilayers composed of CoFeB and epitaxial SrIrO$_{3}$ grown on an orthorhombic DyScO$_{3}$ substrate,"We report on the highly efficient spin-orbit torque (SOT) generation in
epitaxial SrIrO$_{3}$(SIO), which is grown on an orthorhombic DyScO$_{3}$(110)
substrate. By conducting harmonic Hall measurement in
Co$_{20}$Fe$_{60}$B$_{20}$ (CoFeB)/SIO bilayers, we characterize two kinds of
the SOTs, i.e., dampinglike (DL) and fieldlike ones to find that the former is
much larger than the latter. By comparison with the Pt control sample with the
same CoFeB thickness, the observed DL SOT efficiency $\xi$$_{DL}$ of SIO
($\sim$0.32) is three times higher than that of Pt ($\sim$0.093). The
$\xi$$_{DL}$ is nearly constant as a function of the CoFeB thickness,
suggesting that the SIO plays a crucial role in the large SOT generation. These
results on the CoFeB/SIO bilayers highlight that the epitaxial SIO is promising
for low-current and reliable spin-orbit torque-controlled devices.",2306.02567v1
2005-09-28,Magnetic properties of antiferromagnetically coupled CoFeB/Ru/CoFeB,"This work reports on the thermal stability of two amorphous CoFeB layers
coupled antiferromagnetically via a thin Ru interlayer. The saturation field of
the artificial ferrimagnet which is determined by the coupling, J, is almost
independent on the annealing temperature up to more than 300 degree C. An
annealing at more than 325 degree C significantly increases the coercivity, Hc,
indicating the onset of crystallization.",0509753v1
2008-12-03,Correlation between magnetism and spin-dependent transport in CoFeB alloys,"We report a correlation between the spin polarization of the tunneling
electrons (TSP) and the magnetic moment of amorphous CoFeB alloys. Such a
correlation is surprising since the TSP involves s-like electrons close to the
Fermi level (EF), while the magnetic moment mainly arises due to all
d-electrons below EF. We show that probing the s and d-bands individually
provides clear and crucial evidence for such a correlation to exist through s-d
hybridization, and demonstrate the tuneability of the electronic and magnetic
properties of CoFeB alloys.",0812.0679v1
2011-10-03,Spin transfer torque oscillator based on asymmetric magnetic tunnel junctions,"We present a study of the spin transfer torque oscillator based on
CoFeB/MgO/CoFeB asymmetric magnetic tunnel junctions. We observe microwave
precession in junctions with different thickness of the free magnetization
layer. Taking advantage of the ferromagnetic interlayer exchange coupling
between the free and reference layer in the MTJ and perpendicular interface
anisotropy in thin CoFeB electrode we demonstrate the nanometer scale device
that can generate high frequency signal without external magnetic field
applied. The amplitude of the oscillation exceeds 10 nV/Hz^0.5 at 1.5 GHz.",1110.0295v1
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
2017-12-21,Exchange-torque-induced excitation of perpendicular standing spin waves in nanometer-thick YIG films,"Spin waves in ferrimagnetic yttrium iron garnet (YIG) films with ultralow
magnetic damping are relevant for magnon-based spintronics and low-power
wave-like computing. The excitation frequency of spin waves in YIG is rather
low in weak external magnetic fields because of its small saturation
magnetization, which limits the potential of YIG films for high-frequency
applications. Here, we demonstrate how exchange-coupling to a CoFeB film
enables efficient excitation of high-frequency perpendicular standing spin
waves (PSSWs) in nanometer-thick (80 nm and 295 nm) YIG films using uniform
microwave magnetic fields. In the 295-nm-thick YIG film, we measure intense
PSSW modes up to 10th order. Strong hybridization between the PSSW modes and
the ferromagnetic resonance mode of CoFeB leads to characteristic anti-crossing
behavior in broadband spin-wave spectra. A dynamic exchange torque at the
YIG/CoFeB interface explains the excitation of PSSWs. The localized torque
originates from exchange coupling between two dissimilar magnetization
precessions in the YIG and CoFeB layers. As a consequence, spin waves are
emitted from the YIG/CoFeB interface and PSSWs form when their wave vector
matches the perpendicular confinement condition. PSSWs are not excited when the
exchange coupling between YIG and CoFeB is suppressed by a Ta spacer layer.
Micromagnetic simulations confirm the exchange-torque mechanism.",1712.08204v1
2020-11-14,Observation of multi-skyrmion objects created by size and density control in Ta/CoFeB/MgO films,"Magnetic skyrmions are chiral spin textures with a nontrivial topology that
offer a potential for future magnetic memory and storage devices. The
controlled formation and adjustment of size and density of magnetic skyrmions
in Ta/CoFeB/MgO trilayers is demonstrated. It is the ideal candidate for the
use as a bottom electrode integration into CoFeB/MgO/CoFeB magnetic tunnel
junctions. Varying the CoFeB thickness close to the out-of-plane to in-plane
magnetic phase transition, we find that subtle energy contributions enable the
skyrmion formation in a narrow thickness window, corresponding to only around
10 pm variation in CoFeB thickness. Using magneto-optical imaging with
quantitative image processing, variations in skyrmion diameter and distribution
below the Abbe limit can be analyzed. We demonstrate a high degree of diameter
and density control. Zero-field stable skyrmions can be set with proper
magnetic field initialization. This demonstrated tunability and degree of
comprehension of skyrmion formation, paves the way for future skyrmion based
magnetic memory. Moreover, we demonstrate a controlled merging of individual
skyrmions to complex topological objects. We compare our results with the
baby-Skyrme model, developed to describe the soliton nature, for any
topological charge n, and demonstrate the ability to form multi-skyrmion
objects. These objects will be interesting for fundamental mathematical studies
of the topological behavior of solitons in the future.",2011.07336v2
2020-11-23,Asymmetric depinning of chiral domain walls in ferromagnetic trilayers,"We show that the coupling between two ferromagnetic layers separated by a
nonmagnetic spacer can be used to control the depinning of domain walls and
induce unidirectional domain wall propagation. We investigated CoFeB/Ti/CoFeB
trilayers where the easy axis of the magnetization of the top CoFeB layer is
out-of-plane and that of the bottom layer is in-plane. Using Magneto-optic Kerr
effect microscopy, we find that the depinning of a domain wall in the
perpendicularly magnetized CoFeB layer is influenced by the orientation of the
magnetization of the in-plane layer, which gives rise to a field-driven
asymmetric domain expansion. This effect occurs due to the magnetic coupling
between the internal magnetization of the domain wall and the magnetization of
the in-plane CoFeB layer, which breaks the symmetry of up-down and down-up
homochiral N\'eel domain walls in the perpendicular CoFeB layer. Micromagnetic
simulations support these findings by showing that the interlayer coupling
either opposes or favors the Dzyaloshinskii-Moriya interaction in the domain
wall, thereby generating an imbalance in the depinning fields. This effect also
allows for artificially controlling the chirality and dynamics of domain walls
in magnetic layers lacking a strong Dzyaloshinskii-Moriya interaction.",2011.11290v1
2021-02-16,Magnonic band structure in CoFeB/Ta/NiFe meander-shaped magnetic bilayers,"In this work, we investigate the spin-wave propagation in three-dimensional
nanoscale CoFeB/Ta/NiFe meander structures fabricated on a structured SiO2/Si
substrate. The magnonic band structure has been experimentally determined by
wavevector-resolved Brillouin light scattering (BLS) spectroscopy and a set of
stationary modes interposed by two dispersive modes of Bloch type have been
identified. The results could be understood by micromagnetic and finite element
simulations of the mode distributions in both real space and the frequency
domain. The dispersive modes periodically oscillate in frequency over the
Brillouin zones and correspond to modes, whose spatial distributions extend
over the entire sample and are either localized exclusively in the CoFeB layer
or the entire CoFeB/Ta/NiFe magnetic bilayer. Stationary modes are mainly
concentrated in the vertical segments of the CoFeB and NiFe layers and show
negligible amplitudes in the horizon-tal segments. The findings are compared
with those of single-layer CoFeB meander structures with the same geometry
parameters, which reveals the influence of the dipolar coupling between the two
ferromagnetic layers on the magnonic band structure.",2102.08223v2
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
2016-03-28,Electric-field modulation of exchange stiffness in MgO/CoFeB with perpendicular anisotropy,"We observe magnetic domain structures of MgO/CoFeB with a perpendicular
magnetic easy axis under an electric field. The domain structure shows a maze
pattern with electric-field dependent isotropic period. We find that the
electric-field modulation of the period is explained by considering the
electric-field modulation of the exchange stiffness constant in addition to the
known magnetic anisotropy modulation.",1603.08280v1
2006-10-18,Effect of high annealing temperature on giant tunnel magnetoresistance ratio of CoFeB/MgO/CoFeB magnetic tunnel junctions,"We report tunnel magnetoresistance (TMR) ratios as high as 472% at room
temperature and 804% at 5 K in pseudo-spin valve (SV) CoFeB/MgO/CoFeB magnetic
tunnel junctions (MTJs) annealed at 450oC, which is approaching the
theoretically predicted value. By contrast, the TMR ratios for exchange-biased
(EB) SV MTJs with a MnIr antiferromagnetic layer are found to drop when they
are annealed at 450oC. Energy dispersive X-ray analysis shows that annealing at
450oC induces interdiffusion of Mn and Ru atoms into the MgO barrier and
ferromagnetic layers in EB-SV MTJs. Mechanisms behind the different annealing
behavior are discussed.",0610526v1
2011-08-24,Sub-Poissonian shot noise in CoFeB/MgO/CoFeB-based magnetic tunneling junctions,"We measured the shot noise in the CoFeB/MgO/CoFeB-based magnetic tunneling
junctions with a high tunneling magnetoresistance ratio (over 200% at 3 K).
Although the Fano factor in the anti-parallel configuration is close to unity,
it is observed to be typically 0.91\pm0.01 in the parallel configuration. It
indicates the sub-Poissonian process of the electron tunneling in the parallel
configuration due to the relevance of the spin-dependent coherent transport in
the low bias regime.",1108.4831v1
2011-09-27,Determination of spin-dependent Seebeck coefficients of CoFeB/MgO/CoFeB magnetic tunnel junction nanopillars,"We investigate the spin-dependent Seebeck coefficient and the tunneling
magneto thermopower of CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJ) in the
presence of thermal gradients across the MTJ. Thermal gradients are generated
by an electric heater on top of the nanopillars. The thermo power voltage
across the MTJ is found to scale linearly with the heating power and reveals
similar field dependence as the tunnel magnetoresistance. The amplitude of the
thermal gradient is derived from calibration measurements in combination with
finite element simulations of the heat flux. Based on this, large
spin-dependent Seebeck coefficients of the order of (240 \pm 110) \muV/K are
derived. From additional measurements on MTJs after dielectric breakdown, a
tunneling magneto thermopower up to 90% can be derived for 1.5 nm MgO based MTJ
nanopillars.",1109.5912v1
2013-01-09,Low-frequency and shot noises in CoFeB/MgO/CoFeB magnetic tunneling junctions,"The low-frequency and shot noises in spin-valve CoFeB/MgO/CoFeB magnetic
tunneling junctions were studied at low temperature. The measured 1/f noise
around the magnetic hysteresis loops of the free layer indicates that the main
origin of the 1/f noise is the magnetic fluctuation, which is discussed in
terms of a fluctuation-dissipation relation. Random telegraph noise (RTN) is
observed to be symmetrically enhanced in the hysteresis loop with regard to the
two magnetic configurations. We found that this enhancement is caused by the
fluctuation between two magnetic states in the free layer. Although the 1/f
noise is almost independent of the magnetic configuration, the RTN is enhanced
in the antiparallel configuration. These findings indicate the presence of
spin-dependent activation of RTN. Shot noise reveals the spin-dependent
coherent tunneling process via a crystalline MgO barrier.",1301.2161v1
2013-05-31,"Effect of Thermal Annealing on Boron Diffusion, Micro-structural, Electrical and Magnetic properties of Laser Ablated CoFeB Thin Films","We report on Boron diffusion and subsequent crystallization of
Co$_{40}$Fe$_{40}$B$_{20}$ (CoFeB) thin films on SiO$_2$/Si(001) substrate
using pulsed laser deposition. Secondary ion mass spectroscopy reveals Boron
diffusion at the interface in both amorphous and crystalline phase of CoFeB.
High-resolution transmission electron microscopy reveals a small fraction of
nano-crystallites embedded in the amorphous matrix of CoFeB. However, annealing
at 400$^\circ$C results in crystallization of CoFe with \textit{bcc} structure
along (110) orientation. As-deposited films are non-metallic in nature with the
coercivity (H$_c$) of 5Oe while the films annealed at 400$^\circ$C are metallic
with a H$_c$ of 135Oe.",1305.7335v1
2015-03-02,Exchange bias effect in martensitic epitaxial Ni-Mn-Sn thin films applied to pin CoFeB/MgO/CoFeB magnetic tunnel junctions,"The exchange bias effect is commonly used to shift the coercive field of a
ferromagnet. This technique is crucial for the use of magnetic tunnel junctions
as logic or memory devices. Therefore, an independent switching of the two
ferromagnetic electrodes is necessary to guarantee a reliable readout. Here, we
demonstrate that the intrinsic exchange bias effect of Ni-Mn-Sn can be used to
apply a unidirectional anisotropy to magnetic tunnel junctions. For this, we
use epitaxial Ni-Mn-Sn films as pinning layers for microfabricated
CoFeB/MgO/CoFeB magnetic tunnel junctions. We compare the exchange bias field
($H_{\text{EB}}$) measured after field cooling in $-10$\,kOe external field by
magnetization measurements with $H_{\text{EB}}$ obtained from tunnel
magnetoresistance measurements. Consistent for both methods we find an exchange
bias of about $H_{\text{EB}}=130$\,Oe at 10\,K, which decreases with increasing
temperature and vanishes above 70\,K.",1503.00440v2
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
2015-09-14,Thickness-Dependent Magnetoelasticity and its Effects on Perpendicular Magnetic Anisotropy in Ta|CoFeB|MgO Thin Films,"We report measurements of the in-plane magnetoelastic coupling in ultra-thin
Ta|CoFeB|MgO layers as a function of uniaxial strain, conducted using a
four-point bending apparatus. For annealed samples, we observe a strong
dependence on the thickness of the CoFeB layer in the range 1.3-2.0 nm, which
can be modeled as arising from a combination of effective surface and volume
contributions to the magnetoelastic coupling. We point out that if similar
thickness dependence exists for magnetoelastic coupling in response to biaxial
strain, then the standard N\'eel model for the magnetic anisotropy energy
acquires a term inversely proportional to the magnetic layer thickness. This
contribution can significantly change the overall magnetic anisotropy, and
provides a natural explanation for the strongly nonlinear dependence of
magnetic anisotropy energy on magnetic layer thickness that is commonly
observed for ultrathin annealed CoFeB|MgO films with perpendicular magnetic
anisotropy.",1509.04134v1
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
2016-10-08,Perpendicularly magnetized CoFeB multilayers with tunable interlayer exchange for synthetic ferrimagnets,"A study of the multilayer system MgO/CoFeB(1.1nm)/Ta($t$)/CoFeB(0.8nm)/MgO is
presented, where the two CoFeB layers are separated by a Ta interlayer of
varying thickness $t$. The magnetization properties deduced from complementary
techniques such as superconducting quantum interference magnetometry,
ferromagnetic resonance frequency measurements and Brillouin light scattering
spectroscopy can be tuned by changing the Ta thickness between $t$=0.25 nm, 0.5
nm and 0.75 nm. For $t$=0.5 nm, a ferromagnetic coupling is observed, whereas
for t=0.75 nm, the antiferromagnetic coupling needed to construct a synthetic
ferrimagnet is realized. In the later case, the shape of magnetic domain walls
between two ferrimagnetic alignments or between a ferro- and a ferrimagnetic
alignment is very different. This behavior can be interpreted as a result of
the change in dipolar as well as interlayer exchange energy and domain wall
pinning, which is an important conclusion for the realization of data storage
devices based on synthetic ferri- and antiferromagnets.",1610.02550v1
2017-01-05,Excitation and detection of short-waved spin waves in ultrathin Ta/CoFeB/MgO-layer system suitable for spin-orbit-torque magnonics,"We report on the excitation and detection of short-waved spin waves with wave
vectors up to about $40\,\mathrm{rad}\,\mu\mathrm{m}^{-1}$ in spin-wave
waveguides made from ultrathin, in-plane magnetized Co$_{8}$Fe$_{72}$B$_{20}$
(CoFeB). The CoFeB is incorporated in a layer stack of Ta/CoFeB/Mgo, a layer
system featuring large spin orbit torques and a large perpendicular magnetic
anisotropy constant. The short-waved spin waves are excited by nanometric
coplanar waveguides and are detected via spin rectification and microfocussed
Brillouin light scattering spectroscopy. We show that the large perpendicular
magnetic anisotropy benefits the spin-wave lifetime greatly, resulting in a
lifetime comparable to bulk systems without interfacial damping. The presented
results pave the way for the successful extension of magnonics to ultrathin
asymmetric layer stacks featuring large spin orbit torques.",1701.01399v1
2017-01-31,"Lack of correlation between the spin mixing conductance and the ISHE-generated voltages in CoFeB/Pt,Ta bilayers","We investigate spin pumping phenomena in polycrystalline CoFeB/Pt and
CoFeB/Ta bilayers and the correlation between the effective spin mixing
conductance $g^{\uparrow\downarrow}_{\rm eff}$ and the obtained voltages
generated by the spin-to-charge current conversion via the inverse spin Hall
effect in the Pt and Ta layers. For this purpose we measure the in-plane
angular dependence of the generated voltages on the external static magnetic
field and we apply a model to separate the spin pumping signal from the one
generated by the spin rectification effect in the magnetic layer. Our results
reveal a dominating role of anomalous Hall effect for the spin rectification
effect with CoFeB and a lack of correlation between
$g^{\uparrow\downarrow}_{\rm eff}$ and inverse spin Hall voltages pointing to a
strong role of the magnetic proximity effect in Pt in understanding the
observed increased damping. This is additionally reflected on the presence of a
linear dependency of the Gilbert damping parameter on the Pt thickness.",1701.09110v1
2017-03-21,Influence of magnetic field and ferromagnetic film thickness on domain pattern transfer in multiferroic heterostructures,"Domains in BaTiO$_3$ induces a regular modulation of uniaxial magnetic
anisotropy in CoFeB via an inverse magnetostriction effect. As a result, the
domain structures of the CoFeB wedge film and BaTiO$_3$ substrate correlate
fully and straight ferroelectric domain boundaries in BaTiO$_3$ pin magnetic
domain walls in CoFeB. We use x-ray photoemission electron microscopy and
magneto-optical Kerr effect microscopy to characterize the spin structure of
the pinned domain walls. In a rotating magnetic field, abrupt and reversible
transitions between two domain wall types occur, namely, narrow walls where the
magnetization vectors align head-to-tail and much broader walls with
alternating head-to-head and tail-to-tail magnetization configurations. We
characterize variations of the domain wall spin structure as a function of
magnetic field strength and CoFeB film thickness and compare the experimental
results with micromagnetic simulations.",1703.07227v1
2018-05-31,The current-induced spin-orbit torque and field-free switching from Mo-based magnetic heterostructures,"Magnetic heterostructure Mo/CoFeB/MgO has strong perpendicular magnetic
anisotropy and thermal stability. Through current-induced hysteresis loop shift
measurements, we show that the dampinglike spin-orbit torque (SOT) efficiency
of Mo/CoFeB/MgO heterostructure is $\xi_{DL}\approx -0.003\pm 0.001$ and fairly
independent of the annealing temperature from 300$^\circ$C to 400$^\circ$C.
Though $|\xi_{DL}|$ is small while compare to those from Ta or W-based
heterostructures, reversible current-induced SOT switching of a
thermally-stable Mo/CoFeB/MgO heterostruture can still be achieved.
Furthermore, we observe field-free current-induced switching from a
Mo/CoFeB/MgO structure with the Mo layer being wedge-deposited. Our results
indicate that even for a weak spin-orbit interaction 4d transition metal such
as Mo, it is still possible to generate sufficient spin current for
conventional SOT switching and to realize field-free current-induced switching
by structural engineering.",1805.12322v3
2014-06-08,Perpendicular magnetic anisotropy and magnetization process in CoFeB/Pd multilayer films,"Perpendicular magnetic anisotropy (PMA) and dynamic magnetization reversal
process in [CoFeB $t$ nm/Pd 1.0 nm]$_n$ ($t$ = 0.4, 0.6, 0.8, 1.0, and 1.2 nm;
$n$ = 2 - 20) multilayer films have been studied by means of magnetic
hysteresis and Kerr effect measurements. Strong and controllable PMA with an
effective uniaxial anisotropy up to 7.7$\times$ 10$^6$ J.m$^{-3}$ and a
saturation magnetization as low as 200 emu/cc are achieved. Surface/interfacial
anisotropy of CoFeB/Pd interfaces, the main contribution to the PMA, is
separated from the effective uniaxial anisotropy of the films, and appears to
increase with the number of the CoFeB/Pd bilayers. Observation of the magnetic
domains during a magnetization reversal process using polar magneto-optical
Kerr microscopy shows the detailed behavior of nucleation and displacement of
the domain walls.",1406.2028v1
2021-01-26,Voltage-controlled antiferromagnetism in magnetic tunnel junctions,"We demonstrate a voltage-controlled exchange bias effect in CoFeB/MgO/CoFeB
magnetic tunnel junctions that is related to the interfacial Fe(Co)Ox formed
between the CoFeB electrodes and the MgO barrier. The unique combination of
interfacial antiferromagnetism, giant tunneling magnetoresistance, and sharp
switching of the perpendicularly-magnetized CoFeB allows sensitive detection of
the exchange bias. It is found that the exchange bias field can be isothermally
controlled by magnetic fields at low temperatures. More importantly, the
exchange bias can also be effectively manipulated by the electric field applied
to the MgO barrier due to the voltage-controlled antiferromagnetic anisotropy
in this system.",2101.11044v1
2023-07-27,Interfacial Resonance States-Induced Negative Tunneling Magneto-resistance in Orthogonally-Magnetized CoFeB/MgO/CoFeB,"Magnetic tunneling junctions (MTJs) are essential for non-volatile
magneto-resistive random access memory (MRAM) applications. Here, we report the
observation of a large negative tunneling magneto-resistance (TMR) in the
CoFeB/MgO/CoFeB system with an orthogonally-magnetized configuration. Through
the thickness modulation of the MgO barrier, the negative TMR component can be
enhanced up to 20% under a negative voltage bias. Moreover, the tunnel
anisotropic magneto-resistance measurements unveil that the negative TMR
component likely arises from the interfacial resonance states (IRS) in the
minority band of the bottom ferromagnetic layer. Complementary first principle
calculations further quantify the IRS location and strength with respect to the
Fermi level position. Our work not only confirm the vital role of IRS in the
electrical transport of MTJ, but also provide valuable insights for the design
of new-generation voltage-controlled MRAM and related spintronic applications.",2307.14807v1
2024-01-26,Efficient Control of Magnetization Dynamics Via W/CuO$_\text{x}$ Interface,"Magnetization dynamics, which determine the speed of magnetization switching
and spin information propagation, play a central role in modern spintronics.
Gaining its control will satisfy the different needs of various spintronic
devices. In this work, we demonstrate that the surface oxidized Cu
(CuO$_\text{x}$) can be employed for the tunability of magnetization dynamics
of ferromagnet (FM)/heavy metal (HM) bilayer system. The capping CuO$_\text{x}$
layer in CoFeB/W/CuO$_\text{x}$ trilayer reduces the magnetic damping value in
comparison with the CoFeB/W bilayer. The magnetic damping even becomes lower
than that of the CoFeB/CuO$_\text{x}$ by ~ 16% inferring the stabilization of
anti-damping phenomena. Further, the reduction in damping is accompanied by a
very small reduction in the spin pumping-induced output DC voltage in the
CoFeB/W/CuO$_\text{x}$ trilayer. The simultaneous observation of anti-damping
and spin-to-charge conversion can be attributed to the orbital Rashba effect
observed at the HM/CuO$_\text{x}$ interface. Our experimental findings
illustrate that the cost-effective CuO$_\text{x}$ can be employed as an
integral part of modern spintronics devices owing to its rich underneath
spin-orbital physics.",2401.14708v1
2019-04-23,Spin injection and pumping generated by a direct current flowing through a magnetic tunnel junction,"A charge flow through a magnetic tunnel junction (MTJ) leads to the
generation of a spin-polarized current which exerts a spin-transfer torque
(STT) on the magnetization. When the density of applied direct current exceeds
some critical value, the STT excites high-frequency magnetization precession in
the ""free"" electrode of MTJ. Such precession gives rise to microwave output
voltage and, furthermore, can be employed for spin pumping into adjacent normal
metal or semiconductor. Here we describe theoretically the spin dynamics and
charge transport in the CoFeB/MgO/CoFeB/Au tunneling heterostructure connected
to a constant-current source. The magnetization dynamics in the free CoFeB
layer with weak perpendicular anisotropy is calculated by numerical integration
of the Landau-Lifshitz-Gilbert-Slonczewski equation accounting for both STT and
voltage controlled magnetic anisotropy associated with the CoFeB|MgO interface.
It is shown that a large-angle magnetization precession, resulting from
electrically induced dynamic spin reorientation transition, can be generated in
a certain range of relatively low current densities. An oscillating spin
current, which is pumped into the Au overlayer owing to such precession, is
then evaluated together with the injected spin current. Considering both the
driving spin-polarized charge current and the pumped spin current, we also
describe the charge transport in the CoFeB/Au bilayer with the account of
anomalous and inverse spin Hall effects. An electric potential difference
between the lateral sides of the CoFeB/Au bilayer is calculated as a function
of distance from the CoFeB|MgO interface. It is found that this transverse
voltage signal in Au is large enough for experimental detection, which
indicates significant efficiency of the proposed current-driven spin injector.",1904.10361v1
2016-05-20,Elementary specific spin and orbital moments of ultrathin CoFeB amorphous films on GaAs(100),"Nanoscale CoFeB amorphous films have been synthesized on GaAs(100) and
studied with X-ray magnetic circular dichroism (XMCD) and transmission electron
microscopy (TEM). We have found that the ratios of the orbital to spin magnetic
moments of both the Co and Fe in the ultrathin amorphous film have been
enhanced by more than 300% compared with those of the bulk crystalline Co and
Fe, and in specifically, a large orbital moment of 0.56*10^-6 B from the Co
atoms has been observed and at the same time the spin moment of the Co atoms
remains comparable to that of the bulk hcp Co. The results indicate that the
large uniaxial magnetic anisotropy (UMA) observed in the ultrathin CoFeB film
on GaAs(100) is related to the enhanced spin-orbital coupling of the Co atoms
in the CoFeB. This work offers experimental evidences of the correlation
between the UMA and the elementary specific spin and orbital moments in the
CoFeB amorphous film on the GaAs(100) substrate, which is significant for
spintronics applications.",1605.06358v1
2020-07-24,"Determination of the spin Hall angle, spin mixing conductance and spin diffusion length in Ir/CoFeB for spin-orbitronic devices","Iridium is a very promising material for spintronic applications due to its
interesting magnetic properties such as large RKKY exchange coupling as well as
its large spin-orbit coupling value. Ir is for instance used as a spacer layer
for perpendicular synthetic antiferromagnetic or ferrimagnet systems. However,
only a few studies of the spintronic parameters of this material have been
reported. In this paper, we present inverse spin Hall effect - spin pumping
ferromagnetic resonance measurements on CoFeB/Ir based bilayers to estimate the
values of the effective spin Hall angle, the spin diffusion length within
iridium, and the spin mixing conductance in the CoFeB/Ir bilayer. In order to
have reliable results, we performed the same experiments on CoFeB/Pt bilayers,
which behavior is well known due to numerous reported studies. Our experimental
results show that the spin diffusion length within iridium is 1.3 nm for
resistivity of 250 n$\Omega$.m, the spin mixing conductance $g_{eff}^{\uparrow
\downarrow}$ of the CoFeB/Ir interface is 30 nm$^{-2}$, and the spin Hall angle
of iridium has the same sign than the one of platinum and is evaluated at 26%
of the one of platinum. The value of the spin Hall angle found is 7.7% for Pt
and 2% for Ir. These relevant parameters shall be useful to consider Ir in new
concepts and devices combining spin-orbit torque and spin-transfer torque.",2007.12413v1
2010-06-21,Magnonic spin-wave modes in CoFeB antidot lattices,"In this manuscript time-resolved magneto-optical Kerr effect experiments on
structured CoFeB films are presented. The geometries considered are two
dimensional square lattices of micrometer-sized antidots, fabricated by a
focused ion beam. The spin-wave spectra of these magnonic crystals show a novel
precessional mode, which can be related to a Bloch state at the zone boundary.
Additionally, another magnetic mode of different nature appears, whose
frequency displays no dependence on the externally applied magnetic field.
These findings are interpreted as delocalized and localized modes,
respectively.",1006.4038v1
2012-02-23,Domain structure in CoFeB thin films with perpendicular magnetic anisotropy,"Domain structures in CoFeB-MgO thin films with a perpendicular easy
magnetization axis were observed by magneto-optic Kerr-effect microscopy at
various temperatures. The domain wall surface energy was obtained by analyzing
the spatial period of the stripe domains and fitting established domain models
to the period. In combination with SQUID measurements of magnetization and
anisotropy energy, this leads to an estimate of the exchange stiffness and
domain wall width in these films. These parameters are essential for
determining whether domain walls will form in patterned structures and devices
made of such materials.",1202.5128v1
2012-08-27,Spin-wave modes and band structure of rectangular CoFeB antidot lattices,"We present an investigation of rectangular antidot lattices in a CoFeB film.
Magnonic band structures are numerically calculated, and band gaps are
predicted which shift in frequency by 0.9 GHz when rotating the external field
from the long to the short axis of the unit cell. We demonstrate by
time-resolved experiments that magnonic dipolar surface modes are split in
frequency by 0.6 GHz which agrees well with the theoretical prediction. These
findings provide the basis for directional spin-wave filtering with magnonic
devices.",1208.5339v1
2013-07-24,Measurement of magnetization using domain compressibility in CoFeB films with perpendicular anisotropy,"We present a method to map the saturation magnetization of soft ultrathin
films with perpendicular anisotropy, and we illustrate it to assess the
compositional dependence of the magnetization of CoFeB(1 nm)/MgO films. The
method relies on the measurement of the dipolar repulsion of parallel domain
walls that define a linear domain. The film magnetization is linked to the
field compressibility of the domain. The method also yields the minimal
distance between two walls before their merging, which sets a practical limit
to the storage density in spintronic devices using domain walls as storage
entities.",1307.6479v1
2016-08-08,Temperature dependence of the spin Hall angle and switching current in the nc-W(O)/CoFeB/MgO system with perpendicular magnetic anisotropy,"We investigated the temperature dependence of the switching current for a
perpendicularly magnetized CoFeB film deposited on a nanocrystalline tungsten
film with large oxygen content: nc-W(O). The spin Hall angle
$|\Theta_\mathrm{SH}| \approx 0.22$ is independent of temperature, whereas the
switching current increases strongly at low temperature. We show that the
nc-W(O) is insensitive to annealing. It thus can be a good choice for the
integration of spin Hall driven writing of information in magnetic memory or
logic devices that require a high-temperature annealing process during
fabrication.",1608.02528v1
2017-06-20,Scaling of the Hall effects beyond the quantum resistance threshold in oxidized CoFeB,"The ordinary and the extraordinary Hall effects were studied in gradually
oxidized amorphous CoFeB ferromagnets over six orders of resistivity from the
metallic to the strongly insulating regime. Polarity of the extraordinary Hall
effect reverses, and the amplitude of both the ordinary and the extraordinary
Hall effects increases quadratically with resistivity when resistance exceeds
the quantum resistance threshold. The absolute value of the extraordinary Hall
effect scales linearly with the ordinary one in the entire range over eight
orders of magnitude between the metallic and the insulating states. The
behavior differs qualitatively and quantitatively from theoretically predicted
and experimentally known in other materials.",1706.06392v1
2021-01-28,Voltage Controlled Spin-Orbit Torque Switching in W/CoFeB/MgO,"Voltage control of magnetism and spintronics have been highly desirable, but
rarely realized. In this work, we show voltage-controlled spin-orbit torque
(SOT) switching in W/CoFeB/MgO films with perpendicular magnetic anisotropy
(PMA) with voltage administered through SrTiO3 with a high dielectric constant.
We show that a DC voltage can significantly lower PMA by 45%, reduce switching
current by 23%, and increase the damping-like torque as revealed by the first
and second-harmonic measurements. These are characteristics that are
prerequisites for voltage-controlled and voltage-select SOT switching
spintronic devices.",2101.12281v1
2023-11-16,Unidirectional propagation of zero-momentum magnons,"We report on experimental observation of unidirectional propagation of
zero-momentum magnons in synthetic antiferromagnet consisting of strained
CoFeB/Ru/CoFeB trilayer. Inherent non-reciprocity of spin waves in synthetic
antiferromagnets with uniaxial anisotropy results in smooth and monotonous
dispersion relation around Gamma point, where the direction of the phase
velocity is reversed, while the group velocity direction is conserved. The
experimental observation of this phenomenon by intensity-, phase-, and
time-resolved Brillouin light scattering microscopy is corroborated by
analytical models and micromagnetic simulations.",2311.10044v1
2014-10-14,Low frequency noise peak near magnon emission energy in magnetic tunnel junctions,"We report on the low frequency (LF) noise measurements in magnetic tunnel
junctions (MTJs) below 4 K and at low bias, where the transport is strongly
affected by scattering with magnons emitted by hot tunnelling electrons, as
thermal activation of magnons from the environment is suppressed. For both
CoFeB/MgO/CoFeB and CoFeB/AlO$_{x}$/CoFeB MTJs, enhanced LF noise is observed
at bias voltage around magnon emission energy, forming a peak in the bias
dependence of noise power spectra density, independent of magnetic
configurations. The noise peak is much higher and broader for unannealed
AlO$_{x}$-based MTJ, and besides Lorentzian shape noise spectra in the
frequency domain, random telegraph noise (RTN) is visible in the time traces.
During repeated measurements the noise peak reduces and the RTN becomes
difficult to resolve, suggesting defects being annealed. The Lorentzian shape
noise spectra can be fitted with bias-dependent activation of RTN, with the
attempt frequency in the MHz range, consistent with magnon dynamics. These
findings suggest magnon-assisted activation of defects as the origin of the
enhanced LF noise.",1410.3586v2
2015-05-05,Large spin Hall magnetoresistance and its correlation to the spin-orbit torque in W/CoFeB/MgO structures,"The spin-orbit interaction in heavy metal/ferromagnet/oxide structures has
been extensively investigated because it can be employed in manipulation of the
magnetization direction by in-plane current. This implies the existence of an
inverse effect, in which the conductivity in such structures should depend on
the magnetization orientation. In this work, we report a systematic study of
the magnetoresistance (MR) of the W/CoFeB/MgO structures and its correlation to
the current-induced torque to the magnetization. We observe that the MR is
independent of the angle between magnetization and current direction, but is
determined by the relative magnetization orientation with respect to the spin
direction accumulated by spin Hall effect, which is the same symmetry of
so-called spin Hall magnetoresistance. The MR of ~1% in W/CoFeB/MgO samples is
considerably larger than those in other structures of Ta/CoFeB/MgO or
Pt/Co/AlOx, which indicates a larger spin Hall angle of W. Moreover, the
similar W thickness dependence of the MR and the current-induced magnetization
switching efficiency demonstrates that they share the same underlying physics,
which allows one to utilize the MR in non-magnet/ferromagnet structure in order
to understand closely related other spin-orbit coupling effects such as inverse
spin Hall effect, spin-orbit spin transfer torques, etc.",1505.00899v1
2020-04-07,Electrical spin injection into InGaAs/GaAs quantum wells: a comparison between MgO tunnel barriers grown by sputtering and molecular beam epitaxy methods,"An efficient electrical spin injection into an InGaAs/GaAs quantum well light
emitting diode is demonstrated thanks to a CoFeB/MgO spin injector. The
textured MgO tunnel barrier is fabricated by two different techniques:
sputtering and molecular beam epitaxy (MBE). The maximal spin injection
efficiency is comparable for both methods. Additionally, the effect of
annealing is also investigated for the two types of samples. Both samples show
the same trend: an increase of the electroluminescence circular polarization
(Pc) with the increase of annealing temperature, followed by a saturation of Pc
beyond 350{\deg}C annealing. Since the increase of Pc starts well below the
crystallization temperature of the full CoFeB bulk layer, this trend could be
mainly due to an improvement of chemical structure at the top CoFeB/MgO
interface. This study reveals that the control of CoFeB/MgO interface is
essential important for an optimal spin injection into semiconductor.",2004.03292v1
2020-06-04,Heisenberg Exchange and Dzyaloshinskii-Moriya Interaction in Ultrathin CoFeB Single and Multilayers,"We present results of the analysis of Brillouin Light Scattering (BLS)
measurements of spin waves performed on ultrathin single and multirepeat CoFeB
layers with adjacent heavy metal layers. From a detailed study of the spin-wave
dispersion relation, we independently extract the Heisenberg exchange
interaction (also referred to as symmetric exchange interaction), the
Dzyaloshinskii-Moriya interaction (DMI, also referred to as antisymmetric
exchange interaction), and the anisotropy field. We find a large DMI in CoFeB
thin films adjacent to a Pt layer and nearly vanishing DMI for CoFeB films
adjacent to a W layer. Furthermore, the residual influence of the dipolar
interaction on the dispersion relation and on the evaluation of the Heisenberg
exchange parameter is demonstrated. In addition, an experimental analysis of
the DMI on the spin-wave lifetime is presented. All these parameters play a
crucial role in designing skyrmionic or spin-orbitronic devices.",2006.02690v1
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
2020-09-29,Structural Phase Dependent Giant Interfacial Spin Transparency in W/CoFeB Thin Film Heterostructure,"Pure spin current has transfigured the energy-efficient spintronic devices
and it has the salient characteristic of transport of the spin angular
momentum. Spin pumping is a potent method to generate pure spin current and for
its increased efficiency high effective spin-mixing conductance (Geff) and
interfacial spin transparency (T) are essential. Here, a giant T is reported in
Sub/W(t)/Co20Fe60B20(d)/SiO2(2 nm) heterostructures in \beta-tungsten (\beta-W)
phase by employing all-optical time-resolved magneto-optical Kerr effect
technique. From the variation of Gilbert damping with W and CoFeB thicknesses,
the spin diffusion length of W and spin-mixing conductances are extracted.
Subsequently, T is derived as 0.81 \pm 0.03 for the \beta-W/CoFeB interface. A
sharp variation of Geff and T with W thickness is observed in consonance with
the thickness-dependent structural phase transition and resistivity of W. The
spin memory loss and two-magnon scattering effects are found to have negligible
contributions to damping modulation as opposed to spin pumping effect which is
reconfirmed from the invariance of damping with Cu spacer layer thickness
inserted between W and CoFeB. The observation of giant interfacial spin
transparency and its strong dependence on crystal structures of W will be
important for pure spin current based spin-orbitronic devices.",2009.14143v1
2022-06-10,Large anomalous unidirectional magnetoresistance in a single ferromagnetic layer,"Unidirectional magnetoresistance (UMR) in a ferromagnetic bilayer due to the
spin Hall effects (SHEs) provides a facile means of probing in-plane
magnetization to avoid complex magnetic tunnel junctions. However, the UMR
signal is very weak and usually requires a lock-in amplifier for detection even
in the bilayer involving Ta or Pt with a large spin Hall angle (SHA). Here we
report a type of UMR, termed as the anomalous UMR (AUMR), in a single CoFeB
layer without any adjacent SHE layers, where the UMR signal is about 10 times
larger than that in Ta/CoFeB structures and can be detected by using
conventional dc multimeters in the absence of lock-in amplifiers. We further
demonstrate that the extracted AUMR by excluding thermal contributions shows
reversal signs for the CoFeB and NiFe single layers with opposite SHAs,
indicating that the AUMR may originate from the self-generated spin
accumulation interacting with magnetization through the giant
magnetoresistance-like mechanism. These results suggest that the AUMR
contributes UMR signals larger than the interfacial spin Hall UMR in the
CoFeB-involved systems, providing a convenient and reliable approach to detect
in-plane magnetization for the two-terminal spintronic devices.",2206.04851v1
2022-06-14,Probing of large interfacial contribution to spin orbit coupling in CoFeB/Ta heterostructure by ultrafast THz emission spectroscopy,"Ultrafast THz radiation generation from ferromagnetic/nonmagnetic bilayer
heterostructure-based spintronic emitters generally exploits the conversion
from spin- to charge-current within the nonmagnetic layer and its interface
with the ferromagnetic layer. Various possible sub-contributions to the
underlying mechanism of inverse spin Hall effect for the THz emission from such
structures, need to be exploited for not only investigating the intricacies at
the fundamental level in the material properties themselves but also for
improving their performance for broadband and high-power THz emission. Here, we
report ultrafast THz emission from CoFeB/Ta bilayer at varying sample
temperatures in a large range to unravel the role of intrinsic and extrinsic
spin to charge conversion processes. In addition to an enhancement in the THz
emission, its temperature dependence shows a THz signal polarity reversal if
the CoFeB/Ta sample is annealed at an elevated temperature. We extract the
behaviour of the spin Hall resistivity, determine the intrinsic spin Hall
conductivity contribution in it and compare those with the standard Fe/Pt
system. Our results clearly demonstrate a giant interfacial contribution to the
overall spin Hall angle arising from the modified interface in the annealed
CoFeB/Ta, where a sign reversal in the corresponding spin Hall angle is
manifested from the THz amplitude variation with the temperature.",2206.06718v2
2022-08-31,Perpendicular magnetic anisotropy in as-deposited CoFeB/MgO thin films,"Fabrication of perpendicularly magnetized ferromagnetic films on various
buffer layers, especially on numerous newly discovered spin-orbit torque (SOT)
materials to construct energy-efficient spin-orbitronic devices, is a
long-standing challenge. Even for the widely used CoFeB/MgO structures,
perpendicular magnetic anisotropy (PMA) can only be established on limited
buffer layers through post-annealing above 300 {\deg}C. Here, we report that
the PMA of CoFeB/MgO films can be established reliably on various buffer layers
in the absence of post-annealing. Further results show that precise control of
MgO thickness, which determines oxygen diffusion in the underneath CoFeB layer,
is the key to obtaining the as-deposited PMA. Interestingly, contrary to
previous understanding, post-annealing does not influence the well-established
as-deposited PMA significantly but indeed enhances unsaturated PMA with a thick
MgO layer by modulating oxygen distributions, rather than crystallinity or Co-
and Fe-O bonding. Moreover, our results indicate that oxygen diffusion also
plays a critical role in the PMA degradation at high temperature. These results
provide a practical approach to build spin-orbitronic devices based on various
high-efficient SOT materials.",2208.14913v1
2023-11-20,Interplay between moment-dependent and field-driven unidirectional magnetoresistance in CoFeB/InSb/CdTe heterostructures,"Magnetoresistance effects are crucial for understanding the charge/spin
transport as well as propelling the advancement of spintronic applications.
Here we report the coexistence of magnetic moment-dependent (MD) and magnetic
field-driven (FD) unidirectional magnetoresistance (UMR) effects in
CoFeB/InSb/CdTe heterostructures. The strong spin-orbital coupling of InSb and
the matched impedance at the CoFeB/InSb interface warrant a distinct MD-UMR
effect at room temperature, while the interaction between the in-plane magnetic
field and the Rashba effect at the InSb/CdTe interface induces the marked
FD-UMR signal that dominates the high-field region. Moreover, owning to the
different spin transport mechanisms, these two types of nonreciprocal charge
transport show opposite polarities with respect to the magnetic field
direction, which further enable an effective phase modulation of the
angular-dependent magnetoresistance. Besides, the demonstrations of both the
tunable UMR response and two-terminal spin-orbit torque-driven magnetization
switching validate our CoFeB/InSb/CdTe system as a suitable integrated building
block for multifunctional spintronic device design.",2311.11843v1
2024-03-05,Orbital torque switching in perpendicularly magnetized materials,"The orbital Hall effect in light materials has attracted considerable
attention for developing novel orbitronic devices. Here we investigate the
orbital torque efficiency and demonstrate the switching of the perpendicularly
magnetized materials through the orbital Hall material (OHM), i.e., Zirconium
(Zr). The orbital torque efficiency of approximately 0.78 is achieved in the Zr
OHM with the perpendicularly magnetized [Co/Pt]3 sample, which significantly
surpasses that of the perpendicularly magnetized CoFeB/Gd/CoFeB sample
(approximately 0.04). Such notable difference is attributed to the different
spin-orbit correlation strength between the [Co/Pt]3 sample and the
CoFeB/Gd/CoFeB sample, which has been confirmed through the theoretical
calculations. Furthermore, the full magnetization switching of the [Co/Pt]3
sample with a switching current density of approximately 2.6x106 A/cm2 has been
realized through Zr, which even outperforms that of the W spin Hall material.
Our finding provides a guideline to understand orbital torque efficiency and
paves the way to develop energy-efficient orbitronic devices.",2403.03043v1
2005-10-20,Dependence of tunnel magnetoresistance in MgO based magnetic tunnel junctions on Ar pressure during MgO sputtering,"We investigated dependence of tunnel magnetoresistance effect in
CoFeB/MgO/CoFeB magnetic tunnel junctions on Ar pressure during MgO-barrier
sputtering. Sputter deposition of MgO-barrier at high Ar pressure of 10 mTorr
resulted in smooth surface and highly (001) oriented MgO. Using this MgO as a
tunnel barrier, tunnel magnetoresistance (TMR) ratio as high as 355% at room
temperature (578% at 5K) was realized after annealing at 325 C or higher, which
appears to be related to a highly (001) oriented CoFeB texture promoted by the
smooth and highly oriented MgO. Electron-beam lithography defined
deep-submicron MTJs having a low-resistivity Au underlayer with the
high-pressure deposited MgO showed high TMR ratio at low resistance-area
product (RA) below 10 ohm-um^2 as 27% at RA = 0.8 ohm-um^2, 77% at RA = 1.1
ohm-um^2, 130% at RA = 1.7 ohm-um^2, and 165% at RA = 2.9 ohm-um^2.",0510531v1
2005-10-20,Current-driven magnetization switching in CoFeB/MgO/CoFeB magnetic tunnel junctions,"Current-driven magnetization switching in low-resistance
Co40Fe40B20/MgO/Co40Fe40B20 magnetic tunnel junctions (MTJs) is reported. The
critical-current densities Jc required for current-driven switching in samples
annealed at 270C and 300C are found to be as low as 7.8 x 10^5 A/cm^2 and 8.8 x
10^5 A/cm^2 with accompanying tunnel magnetoresistance (TMR) ratios of 49% and
73 %, respectively. Further annealing of the samples at 350C increases TMR
ratio to 160 %, while accompanying Jc increases to 2.5 x 10^6 A/cm^2. We
attribute the low Jc to the high spin-polarization of tunnel current and small
MsV product of the CoFeB single free layer, where Ms is the saturation
magnetization and V the volume of the free layer.",0510538v1
2006-09-13,Current-induced magnetization switching in MgO barrier based magnetic tunnel junctions with CoFeB/Ru/CoFeB synthetic ferrimagnetic free layer,"We report the intrinsic critical current density (Jc0) in current-induced
magnetization switching and the thermal stability factor (E/kBT, where E, kB,
and T are the energy potential, the Boltzmann constant, and temperature,
respectively) in MgO based magnetic tunnel junctions with a
Co40Fe40B20(2nm)/Ru(0.7-2.4nm)/Co40Fe40B20(2nm) synthetic ferrimagnetic (SyF)
free layer. We show that Jc0 and E/kBT can be determined by analyzing the
average critical current density as a function of coercivity using the
Slonczewski's model taking into account thermal fluctuation. We find that high
antiferromagnetic coupling between the two CoFeB layers in a SyF free layer
results in reduced Jc0 without reducing high E/kBT.",0609306v2
2009-08-15,Thermal-magnetic noise measurement of spin-torque effects on ferromagnetic resonance in MgO-based magnetic tunnel junctions,"Thermal-magnetic noise at ferromagnetic resonance (T-FMR) can be used to
measure magnetic perpendicular anisotropy of nanoscale magnetic tunnel
junctions (MTJs). For this purpose, T-FMR measurements were conducted with an
external magnetic field up to 14 kOe applied perpendicular to the film surface
of MgO-based MTJs under a dc bias. The observed frequency-field relationship
suggests that a 20 A CoFeB free layer has an effective demagnetization field
much smaller than the intrinsic bulk value of CoFeB, with 4PiMeff = (6.1 +/-
0.3) kOe. This value is consistent with the saturation field obtained from
magnetometry measurements on extended films of the same CoFeB thickness.
In-plane T-FMR on the other hand shows less consistent results for the
effective demagnetization field, presumably due to excitations of more complex
modes. These experiments suggest that the perpendicular T-FMR is preferred for
quantitative magnetic characterization of nanoscale MTJs.",0908.2164v1
2011-04-04,Tunneling magneto thermo power in magnetic tunnel junction nanopillars,"We study the tunneling magneto thermo power (TMTP) in CoFeB/MgO/CoFeB
magnetic tunnel junction nanopillars. Thermal gradients across the junctions
are generated by a micropatterned electric heater line. Thermo power voltages
up to a few tens of \muV between the top and bottom contact of the nanopillars
are measured which scale linearly with the applied heating power and hence with
the applied temperature gradient. The thermo power signal varies by up to 10
\muV upon reversal of the relative magnetic configuration of the two CoFeB
layers from parallel to antiparallel. This signal change corresponds to a large
spin-dependent Seebeck coefficient of the order of 100 \muV/K and a large TMTP
change of the tunnel junction of up to 90%.",1104.0537v2
2012-08-28,Magnetic field sensor with voltage-tunable sensing properties,"We report on a magnetic field sensor based on CoFeB/MgO/CoFeB magnetic tunnel
junctions. By taking advantage of the perpendicular magnetic anisotropy of the
CoFeB/MgO interface, the magnetization of the sensing layer is tilted
out-of-plane which results in a linear response to in-plane magnetic fields.
The application of a bias voltage across the MgO tunnel barrier of the field
sensor affects the magnetic anisotropy and thereby its sensing properties. An
increase of the maximum sensitivity and simultaneous decrease of the magnetic
field operating range by a factor of two is measured. Based on these results,
we propose a voltage-tunable sensor design that allows for active control of
the sensitivity and the operating filed range with the strength and polarity of
the applied bias voltage.",1208.5588v1
2013-11-15,Electric-field-induced strain-mediated magnetoelectric effect in CoFeB-MgO magnetic tunnel junctions,"Magnetoelectric coupling between magnetic and electric dipoles is one of the
cornerstones of modern physics towards developing the most energy-efficient
magnetic data storage. Conventionally, magnetoelectric coupling is achieved in
single-phase multiferroics or in magnetoelectric composite nanostructures
consisting of ferromagnetic and ferroelectric/piezoelectric materials. Here, we
demonstrate an electric-field-induced strain-mediated magnetoelectric effect in
ultrathin CoFeB/MgO magnetic tunnel junction employing non-piezoelectric
material, which is a vitally important structure for spintronic devices, by
using dynamical magnetoelectric and piezoresponse force microscopy measurement
techniques. We show that the applied electric-field induces strain in a few
atomic layers of dielectric MgO which is transferred to magnetostrictive CoFeB
layer, resulting in a magnetoelectric effect of magnitude up to 80.8 V cm-1
Oe-1 under -0.5 V. The demonstrated strain-mediated magnetoelectric effect with
an electric field in magnetic tunnel junctions is a significant step towards
exploring magnetoelectrically controlled spintronic devices for low-power and
high density magnetic data storage applications.",1311.3794v1
2014-04-17,Large and robust electrical spin injection into GaAs at zero magnetic field using an ultrathin CoFeB/MgO injector,"We demonstrate a large electrical spin injection into GaAs at zero magnetic
field thanks to an ultrathin perpendicularly magnetized CoFeB contact of a few
atomic planes (1.2 nm). The spin-polarization of electrons injected into GaAs
was examined by the circular polarization of electroluminescence from a Spin
Light Emitting Diode with embedded InGaAs/GaAs quantum wells. The
electroluminescence polarization as a function of the magnetic field closely
traces the out-of-plane magnetization of the CoFeB/MgO injector. A circular
polarization degree of the emitted light as large as 20% at 25 K is achieved at
zero magnetic field. Moreover the electroluminescence circular polarization is
still about 8% at room temperature.",1404.4527v1
2014-07-24,Unambiguous separation of the inverse spin Hall and anomalous Nernst Effects within a ferromagnetic metal using the spin Seebeck effect,"The longitudinal spin Seebeck effect is measured on the ferromagnetic
insulator Fe$_3$O$_4$ with the ferromagnetic metal
Co$_{0.2}$Fe$_{0.6}$B$_{0.2}$ (CoFeB) as the spin detector. By using a
non-magnetic spacer material between the two materials (Ti), it is possible to
decouple the two ferromagnetic materials and directly observe pure spin flow
from Fe$_3$O$_4$ into CoFeB. It is shown, that in a single ferromagnetic metal
the inverse spin Hall effect (ISHE) and anomalous Nernst effect (ANE) can occur
simultaneously with opposite polarity. Using this and the large difference in
the coercive fields between the two magnets, it is possible to unambiguously
separate the contributions of the spin Seebeck effect from the ANE and observe
the degree to which each effect contributes to the total response. These
experiments show conclusively that the ISHE and ANE in CoFeB are separate
phenomena with different origins and can coexist in the same material with
opposite response to a thermal gradient.",1407.6758v1
2015-05-30,Effect of annealing temperature on exchange stiffness of CoFeB thin films,"We investigate the exchange stiffness constants of 28-nm-thick CoFeB film
using Brillouin light scattering. Series of CoFeB films are prepared on the
MgO(001) substrate with or without additional 5-nm thick MgO buffer layer, the
effect of the annealing temperature on the exchange stiffness constants are
studied. We found that the exchange stiffness constant of 400oC annealed sample
with MgO buffer increased by 10 % form the 200oC annealed sample (=0.73 +/-
0.01 x 10-11 J/m), while the exchange stiffness constant of without MgO buffer
layer sample increase by 6 % from the as-grown sample (=1.11 +/- 0.02 x 10-11
J/m).",1506.00129v1
2016-02-08,Nanomagnetic logic with non-uniform states of clocking,"Nanomagnetic logic transmits information along a path of nanomagnets. The
basic mechanism to drive such a transmission, known as clocking, can be
achieved by exploiting the spin-Hall effect, as recently observed in
experiments on Ta/CoFeB/MgO multilayers [D. Bhowmik, et al., Nat. Nano. 9, 59
(2013)]. This paper shows the fundamental mechanism of the spin-Hall driven
clocking by using a full micromagnetic framework and considering two different
devices, Ta/CoFeB/MgO and Pt/CoFeB/MgO. The former is used for a direct
comparison of the numerical results with the experiments while the latter
permits to predict the effect of the Dzyaloshinskii-Moriya interaction (DMI) in
the clocking mechanism. Results show that the clocking state is non-uniform and
it is characterized by the presence of domains separated by Bloch (N\'eel)
domain walls depending on the absence (presence) of the DMI. Our findings point
out that for the design of nanomagnetic logic a full micromagnetic approach is
necessary.",1602.02528v1
2016-04-18,Temperature dependence of spin-orbit torques in W/CoFeB bilayers,"We report on the temperature and layer thickness variation of spin-orbit
torques in perpendicularly magnetized W/CoFeB bilayers. Harmonic Hall voltage
measurements reveal dissimilar temperature evolutions of longitudinal and
transverse effective magnetic field components. The transverse effective field
changes sign at 250 K for a 2 nm thick W buffer layer, indicating a much
stronger contribution from interface spin-orbit interactions compared to, for
example, Ta. Transmission electron microscopy measurements reveal that
considerable interface mixing between W and CoFeB is primarily responsible for
this effect.",1604.05204v1
2016-04-19,Probing the Dzyaloshinskii-Moriya interaction in CoFeB ultrathin films using domain wall creep and Brillouin light spectroscopy,"We have characterized the strength of the interfacial Dyzaloshinskii-Moriya
interaction (DMI) in ultrathin perpendicularly magnetized CoFeB/MgO films,
grown on different underlayers of W, TaN, and Hf, using two experimental
methods. First, we determined the effective DMI field from measurements of
field-driven domain wall motion in the creep regime, where applied in-plane
magnetic fields induce an anisotropy in the wall propagation that is correlated
with the DMI strength. Second, Brillouin light spectroscopy was employed to
quantify the frequency non-reciprocity of spin waves in the CoFeB layers, which
yielded an independent measurement of the DMI. By combining these results, we
show that DMI estimates from the different techniques only yield qualitative
agreement, which suggests that open questions remain on the underlying models
used to interpret these results.",1604.05475v1
2018-01-17,Interference induced enhancement of magneto-optical Kerr effect in ultrathin magnetic films,"We have studied the magneto-optical spectra of ultrathin magnetic films
deposited on Si substrates coated with an oxide layer (SiOx). We find that the
Kerr rotation angle and the ellipticity of ~1 nm thick CoFeB thin films, almost
transparent to visible light, show a strong dependence on the thickness of the
SiOx layer. The Kerr signal from the 1 nm CoFeB thin film can be larger than
that of ~100 nm thick CoFeB films for a given SiOx thickness and light
wavelength. The enhancement of the Kerr signal occurs when optical interference
takes place within the SiOx layer. Interestingly, under such resonance
condition, the measured Kerr signal is in some cases larger than the estimation
despite the good agreement of the measured and calculated reflection amplitude.
We infer the discrepancy originates from interface states that are distinct
from the bulk characteristics. These results show that optical interference
effect can be utilized to study the magneto-optical properties of ultrathin
films.",1801.05539v1
2018-03-19,Enhanced spin-orbit torque via interface engineering in Pt/CoFeB/MgO heterostructures,"Spin-orbit torque facilitates efficient magnetization switching via an
in-plane current in perpendicularly magnetized heavy metal/ferromagnet
heterostructures. The efficiency of spin-orbit-torque-induced switching is
determined by the charge-to-spin conversion arising from either bulk or
interfacial spin-orbit interactions, or both. Here, we demonstrate that the
spin-orbit torque and the resultant switching efficiency in Pt/CoFeB systems
are significantly enhanced by an interfacial modification involving Ti
insertion between the Pt and CoFeB layers. Spin pumping and X-ray magnetic
circular dichroism experiments reveal that this enhancement is due to an
additional interface-generated spin current of the nonmagnetic interface and/or
improved spin transparency achieved by suppressing the proximity-induced moment
in the Pt layer. Our results demonstrate that interface engineering affords an
effective approach to improve spin-orbit torque and thereby magnetization
switching efficiency.",1803.06961v1
2019-11-28,Spin-torque memristors based on perpendicular magnetic tunnel junctions with a hybrid chiral texture,"Spin-torque memristors were proposed in 2009, which could provide fast,
low-power and infinite memristive behavior for large-density non-volatile
memory and neuromorphic computing. However, the strict requirements of
combining high magnetoresistance, stable intermediate states and spin-polarized
current switching in a single device pose difficulties in physical
implementation. Here, we experimentally demonstrate a nanoscale spin-torque
memristor based on a perpendicular-anisotropy magnetic tunnel junction with a
CoFeB/W/CoFeB composite free layer structure. Its tunneling magnetoresistance
is higher than 200%, and memristive behavior can be realized by spin-transfer
torque switching. Memristive states are maintained by robust domain wall
pinning around clusters of W atoms, where nanoscale vertical chiral spin
textures could be formed through the competition between opposing
Dzyaloshinskii-Moriya interactions and the fluctuating interlayer coupling
caused by the Ruderman-Kittel-Kasuya-Yosida interaction between the two CoFeB
free layers. Spike-timing-dependent plasticity is also demonstrated in this
device.",1911.12784v1
2017-05-29,"Contributions of Co and Fe orbitals to Perpendicular Magnetic Anisotropy of MgO/CoFeB Bilayers with Spin-Orbit-Torque-Related (Ta, W, IrMn, and Ti) Underlayers","We study the perpendicular magnetic anisotropy (PMA) of the CoFeB/MgO
bilayers in contact with W, Ta, IrMn and Ti which has been suggested as the
spin-orbit-torque-related underlayers. The saturation magnetization of the
CoFeB depends on the underlayer materials due to formation of a dead-layer,
affecting PMA strength of each film. The x-ray magnetic circular dichroism
measurement reveals that the interfacial intermixing suppresses only the
perpendicular orbital moment of Fe, while the intermixing simultaneously
suppresses both the perpendicular and in-plane orbital moments of Co.",1705.10064v1
2019-10-30,Ultra-low switching current density in all-amorphous W-Hf / CoFeB / TaOx films,"We study current-induced deterministic magnetization switching and domain
wall motion via polar Kerr microscopy in all-amorphous
W$_{66}$Hf$_{34}$/CoFeB/TaO$_\text{x}$ with perpendicular magnetic anisotropy
and large spin Hall angle. Investigations of magnetization switching as a
function of in-plane assist field and current pulse-width yield switching
current densities as low as $3\times 10^{9}$ A/m$^2$. We accredit this low
switching current density to a low depinning current density, which was
obtained from measurements of domain wall displacements upon current injection.
This correlation is verified by investigations of a Ta/CoFeB/MgO/Ta reference
sample, which showed critical current densities of at least one order of
magnitude larger, respectively.",1910.13837v3
2018-12-05,Voltage-induced strain clocking of nanomagnets with perpendicular magnetic anisotropies,"Nanomagnetic logic (NML) has attracted attention during the last two decades
due to its promise of high energy efficiency combined with non-volatility. Data
transmission in NML relies on Bennett clocking through dipole interaction
between neighboring nanomagnetic bits. This paper uses a fully coupled finite
element model to simulate Bennett clocking based on strain-mediated
multiferroic system for Ni, CoFeB and Terfenol-D with perpendicular magnetic
anisotropies. Simulation results demonstrate that Terfenol-D system has the
highest energy efficiency, which is 2 orders of magnitude more efficient than
Ni and CoFeB. However, the high efficiency is associated with switching
incoherency due to its large magnetostriction coefficient. It is also suggested
that the CoFeB clocking system is slower and has lower bit-density than in Ni
or Terfenol-D systems due to its large dipole coupling. Moreover, we
demonstrate that the precessional perpendicular switching and the Bennett
clocking can be achieved using the same strain-mediated multiferroic
architecture with different voltage pulsing. This study opens new possibilities
to an all-spin in-memory computing system.",1812.02268v1
2018-12-18,Enhancement of Spintronic Terahertz Emission via Annealing in Ferromagnetic Heterostructures,"We systematically investigate the influence of annealing effect on terahertz
(THz) generation from CoFeB based magnetic nanofilms driven by femtosecond
laser pulses. Three times enhancement of THz yields are achieved in W/CoFeB
through annealing effect, and double boosting is obtained in Pt/CoFeB. The
mechanism of annealing effect originates from the increase of hot electron mean
free path induced by crystallization, which is experimentally corroborated by
THz transmission measurement on time-domain spectroscopy. Comparison studies of
the thickness dependent THz efficiency after annealing are also implemented,
and we eventually conclude that annealing and thickness optimization are of
importance for scaling up THz intensity. Our observations not only deepen
understanding of the spintronic THz radiation mechanism but also provide normal
platform for high speed spintronic opto-electronic devices.",1812.07113v2
2020-04-07,Nonreciprocal Dzyaloshinskii-Moriya magnetoacoustic waves,"We study the interaction of surface acoustic waves with spin waves in
ultra-thin CoFeB/Pt bilayers. Due to the interfacial Dzyaloshinskii-Moriya
interaction (DMI), the spin wave dispersion is non-degenerate for oppositely
propagating spin waves in CoFeB/Pt. In combination with the additional
nonreciprocity of the magnetoacoustic coupling itself, highly nonreciprocal
acoustic wave transmission through the magnetic film is observed. We
systematically characterize the magnetoacoustic wave propagation in a thickness
series of CoFeB($d$)/Pt samples as a function of magnetic field magnitude and
direction, and at frequencies up to 7 GHz. We quantitatively model our results
to extract the strength of the DMI and magnetoacoustic driving fields.",2004.03535v2
2021-04-05,"Optical Damage Threshold and THz Generation Efficiency of (Fe,CoFeB)/(Ta,Pt) Spintronic Emitters","THz pulses are generated from femtosecond pulse-excited
ferromagnetic/nonmagnetic spintronic heterostructures via inverse spin Hall
effect. The contribution from ultrafast demagnetization/remagnetization is
extremely weak, in the comparison. The highest possible THz signal strength
from spintronic THz emitters is limited by the optical damage threshold of the
corresponding heterostructures. The THz generation efficiency does not saturate
with the excitation fluence even up till the damage threshold. Bilayer (Fe,
CoFeB)/(Pt, Ta) based FM/NM spintronic heterostructures have been studied for
an optimized performance for THz generation when pumped by sub-50 fs amplified
laser pulses at 800 nm. Among them, CoFeB/Pt is the best combination for an
efficient THz source. The optimized FM/NM spintronic heterostructure on a
quartz substrate, having alpha-phase Ta as the nonmagnetic layer, show the
highest damage threshold as compared to those with Pt, irrespective of their
generation efficiency. The damage threshold of the Fe/Ta heterostructure on
quartz substrate is ~85 GW/cm2.",2104.02028v1
2023-02-14,Direct evidence of terahertz emission arising from anomalous Hall effect,"A detailed understanding of the different mechanisms being responsible for
terahertz (THz) emission in ferromagnetic (FM) materials will aid in designing
efficient THz emitters. In this report, we present direct evidence of THz
emission from single layer Co$_{0.4}$Fe$_{0.4}$B$_{0.2}$ (CoFeB) FM thin films.
The dominant mechanism being responsible for the THz emission is the anomalous
Hall effect (AHE), which is an effect of a net backflow current in the FM layer
created by the spin-polarized current reflected at the interfaces of the FM
layer. The THz emission from the AHE-based CoFeB emitter is optimized by
varying its thickness, orientation, and pump fluence of the laser beam. Results
from electrical transport measurements show that skew scattering of charge
carriers is responsible for the THz emission in the CoFeB AHE-based THz
emitter.",2302.07398v2
2023-03-21,Inducing or suppressing the anisotropy in multilayers based on CoFeB,"Controlling the uniaxial magnetic anisotropy is of practical interest to a
wide variety of applications. We study Co$_{40}$Fe$_{40}$B$_{20}$ single films
grown on various crystalline orientations of LiNbO$_3$ substrates and on
oxidized silicon. We identify the annealing conditions that are appropriate to
induce or suppress uniaxial anisotropy. Anisotropy fields can be increased by
annealing up to 11 mT when using substrates with anisotropic surfaces. They can
be decreased to below 1 mT when using isotropic surfaces. In the first case,
the observed increase of the anisotropy originates from the biaxial strain in
the film caused by the anisotropic thermal contraction of the substrate when
back at room temperature after strain relaxation during annealing. In the
second case, anisotropy is progressively removed by applying successive
orthogonal fields that are assumed to progressively suppress any chemical
ordering within the magnetic film. The method can be applied to CoFeB/Ru/CoFeB
synthetic antiferromagnets but the tuning of the anisotropy comes with a
decrease of the interlayer exchange coupling and a drastic change of the
exchange stiffness.",2303.11718v1
2023-06-19,Spin transport and magnetic proximity effect in CoFeB/normal metal/Pt trilayers,"We present a study of the damping and spin pumping properties of CoFeB/X/Pt
systems with $\rm X=Al,Cr$ and $\rm Ta$. We show that the total damping of the
CoFeB/Pt systems is strongly reduced when an interlayer is introduced
independently of the material. Using a model that considers spin relaxation, we
identify the origin of this contribution in the magnetically polarized Pt
formed by the magnetic proximity effect (MPE), which is suppressed by the
introduction of the interlayer. The induced ferromagnetic order in the Pt layer
is confirmed by transverse magneto-optical Kerr spectroscopy at the M$_{2,3}$
and N$_7$ absorption edges as an element-sensitive probe. We discuss the impact
of the MPE on parameter extraction in the spin transport model.",2306.11009v2
2023-11-09,Magnon-phonon coupling of synthetic antiferromagnets in a surface acoustic wave cavity resonator,"We use a surface acoustic wave (SAW) cavity resonator to study the coupling
of acoustic magnons in a synthetic antiferromagnet (SAF) and the phonons
carried by SAWs. The SAF is composed of a CoFeB/Ru/CoFeB trilayer and the
scattering matrix of the SAW resonator is studied to assess the coupling. We
find that the spectral linewidth of the SAW resonator is modulated when the
frequency of the excited magnons approaches the SAW resonance frequency.
Moreover, the linewidth modulation varies with the magnitude and orientation of
the external magnetic field. Such change in the spectral linewidth can be well
reproduced using macrospin-like model calculations. From the model analyses, we
estimate the magnon-phonon coupling strength to be $\sim$15.6 MHz at a SAW
resonance frequency of 1.8 GHz: the corresponding magnomechanical cooperativity
is $\sim$0.66. As the spectral shape hardly changes in a CoFeB single layer
reference sample under the same experimental condition, these results show that
SAF provides an ideal platform to study magnon-phonon coupling in a SAW cavity
resonator.",2311.05275v1
2024-01-15,Multiple Frequency Steps in Synthetic Antiferromagnet Based Double Spin Josephson Junctions Using CoFeB and Fe3Sn,"Superconducting quantum interference device (SQUID) which is made of two
parallel Josephson junctions has applications in magnetometry. A similar
spin-based device is proposed here where spin superfluid in ferromagnet (FM)
mimics the superconducting state. Two materials CoFeB and Fe3Sn are used for
spin superfluid-based SQUID like device where easy plane anisotropy in CoFeB
can be engineered and Fe3Sn has inherent easy plane anisotropy. Frequency
varies in spin based proposed devices. Frequency increases and again decreases
with the increase in both applied magnetic field and applied spin current. The
proposed device can be used as nano oscillator and detector. The frequency in
the proposed device shows multiple frequency steps which can be used for
neuromorphic applications.",2401.08005v1
2008-01-09,Current-induced magnetization switching in MgO barrier magnetic tunnel junctions with CoFeB based synthetic ferrimagnetic free layers,"We investigated the effect of using a synthetic ferrimagnetic (SyF) free
layer in MgO-based magnetic tunnel junctions (MTJs) on current-induced
magnetization switching (CIMS), particularly for application to spin-transfer
torque random access memory (SPRAM). The employed SyF free layer had a
Co40Fe40B20/ Ru/ Co40Fe40B20 and Co20Fe60B20/Ru/Co20Fe60B20 structures, and the
MTJs(100x(150-300) nm^2) were annealed at 300oC. The use of SyF free layer
resulted in low intrinsic critical current density (Jc0) without degrading the
thermal-stability factor (E/kBT, where E, kB, and T are the energy potential,
the Boltzmann constant, and temperature,respectively). When the two CoFeB
layers of a strongly antiferromagnetically coupled SyF free layer had the same
thickness, Jc0 was reduced to 2-4x10^6 A/cm^2. This low Jc0 may be due to the
decreased effective volume under the large spin accumulation at the CoFeB/Ru.
The E/kBT was over 60, resulting in a retention time of over ten years and
suppression of the write current dispersion for SPRAM. The use of the SyF free
layer also resulted in a bistable (parallel/antiparallel) magnetization
configuration at zero field, enabling the realization of CIMS without the need
to apply external fields to compensate for the offset field.",0801.1355v1
2012-07-11,Layer thickness dependence of the current induced effective field vector in Ta|CoFeB|MgO,"The role of current induced effective magnetic field in ultrathin magnetic
heterostructures is increasingly gaining interest since it can provide
efficient ways of manipulating magnetization electrically. Two effects, known
as the Rashba spin orbit field and the spin Hall spin torque, have been
reported to be responsible for the generation of the effective field. However,
quantitative understanding of the effective field, including its direction with
respect to the current flow, is lacking. Here we show vector measurements of
the current induced effective field in Ta|CoFeB|MgO heterostructrures. The
effective field shows significant dependence on the Ta and CoFeB layers'
thickness. In particular, 1 nm thickness variation of the Ta layer can result
in nearly two orders of magnitude difference in the effective field. Moreover,
its sign changes when the Ta layer thickness is reduced, indicating that there
are two competing effects that contribute to the effective field. The relative
size of the effective field vector components, directed transverse and parallel
to the current flow, varies as the Ta thickness is changed. Our results
illustrate the profound characteristics of just a few atomic layer thick metals
and their influence on magnetization dynamics.",1207.2521v1
2013-05-12,Spin Pumping and Inverse Spin Hall Effect in Germanium,"We have measured the inverse spin Hall effect (ISHE) in \textit{n}-Ge at room
temperature. The spin current in germanium was generated by spin pumping from a
CoFeB/MgO magnetic tunnel junction in order to prevent the impedance mismatch
issue. A clear electromotive force was measured in Ge at the ferromagnetic
resonance of CoFeB. The same study was then carried out on several test
samples, in particular we have investigated the influence of the MgO tunnel
barrier and sample annealing on the ISHE signal. First, the reference CoFeB/MgO
bilayer grown on SiO$_{2}$ exhibits a clear electromotive force due to
anisotropic magnetoresistance and anomalous Hall effect which is dominated by
an asymmetric contribution with respect to the resonance field. We also found
that the MgO tunnel barrier is essential to observe ISHE in Ge and that sample
annealing systematically lead to an increase of the signal. We propose a
theoretical model based on the presence of localized states at the interface
between the MgO tunnel barrier and Ge to account for these observations.
Finally, all of our results are fully consistent with the observation of ISHE
in heavily doped $n$-Ge and we could estimate the spin Hall angle at room
temperature to be $\approx$0.001.",1305.2602v1
2017-03-10,Room-temperature perpendicular magnetization switching through giant spin-orbit torque from sputtered BixSe(1-x) topological insulator material,"The spin-orbit torque (SOT) arising from materials with large spin-orbit
coupling promises a path for ultra-low power and fast magnetic-based storage
and computational devices. We investigated the SOT from magnetron-sputtered
BixSe(1-x) thin films in BixSe(1-x)/CoFeB heterostructures by using a dc planar
Hall method. Remarkably, the spin Hall angle (SHA) was found to be as large as
18.83, which is the largest ever reported at room temperature (RT). Moreover,
switching of a perpendicular CoFeB multilayer using SOT from the BixSe(1-x) has
been observed with the lowest-ever switching current density reported in a
bilayer system: 2.3 * 105 A/cm2 at RT. The giant SHA, smooth surface, ease of
growth of the films on silicon substrate, successful growth and switching of a
perpendicular CoFeB multilayer on BixSe(1-x) film opens a path for use of
BixSe(1-x) topological insulator (TI) material as a spin-current generator in
SOT-based memory and logic devices.",1703.03822v1
2019-07-08,Bloch-to-Néel domain wall transition evinced through morphology of magnetic bubble expansion in Ta/CoFeB/MgO layers,"Ta/CoFeB/MgO trilayers with perpendicular magnetic anisotropy are often
characterised by vanishing or modest values of interfacial
Dzyaloshinskii-Moriya interaction (DMI), which results in purely Bloch or mixed
Bloch-N\'eel domain walls (DWs). Here we investigate the creep evolution of the
overall magnetic bubble morphology in these systems under the combined presence
of in-plane and out-of-plane magnetic fields and we show that He$^+$ ion
irradiation induces a transition of the internal DW structure towards a fully
N\'eel spin texture. This transition can be correlated to a simultaneous
increase in DMI strength and reduction in saturation magnetisation -- which are
a direct consequence of the effects of ion irradiation on the bottom and top
CoFeB interfaces, respectively. The threshold irradiation dose above which DWs
acquire a pure N\'eel character is experimentally found to be between 12
$\times$ 10$^{18}$ He$^+$/m$^2$ and 16 $\times$ 10$^{18}$ He$^+$/m$^2$,
matching estimations from the one dimensional DW model based on material
parameters. Our results indicate that evaluating the global bubble shape during
its expansion can be an effective tool to sense the internal bubble DW
structure. Furthermore, we show that ion irradiation can be used to achieve
post-growth engineering of a desired DW spin texture.",1907.03708v1
2019-07-14,Current-Induced magnetization switching by the high spin Hall conductivity $α$-W,"The spin Hall effect originating from 5d heavy transition metal thin films
such as Pt, Ta, and W is able to generate efficient spin-orbit torques that can
switch adjacent magnetic layers. This mechanism can serve as an alternative to
conventional spin-transfer torque for controlling next-generation magnetic
memories. Among all 5d transition metals, W in its resistive amorphous phase
typically shows the largest spin-orbit torque efficiency ~ 0.20-0.50. In
contrast, its conductive and crystalline $\alpha$ phase possesses a
significantly smaller efficiency ~ 0.03 and no spin-orbit torque switching has
yet been realized using $\alpha$-W thin films as the spin Hall source. In this
work, through a comprehensive study of high quality W/CoFeB/MgO and the
reversed MgO/CoFeB/W magnetic heterostructures, we show that although
amorphous-W has a greater spin-orbit torque efficiency, the spin Hall
conductivity of $\alpha$-W
($|\sigma_{\operatorname{SH}}^{\alpha\operatorname{-W}}|=3.71\times10^{5}\operatorname{\Omega}^{-1}\operatorname{m}^{-1}$)
is ~3.5 times larger than that of amorphous-W
($|\sigma_{\operatorname{SH}}^{\operatorname{amorphous-W}}|=1.05\times10^{5}\operatorname{\Omega}^{-1}\operatorname{m}^{-1}$).
Moreover, we demonstrate spin-orbit torque driven magnetization switching using
a MgO/CoFeB/$\alpha$-W heterostructure. Our findings suggest that the
conductive and high spin Hall conductivity $\alpha$-W can be a potential
candidate for future low power consumption spin-orbit torque memory
applications.",1907.06192v1
2019-10-02,A Low Temperature Functioning CoFeB/MgO Based Perpendicular Magnetic Tunnel Junction for Cryogenic Nonvolatile Random Access Memory,"We investigated the low temperature performance of CoFeB/MgO based
perpendicular magnetic tunnel junctions (pMTJs) by characterizing their
quasi-static switching voltage, high speed pulse write error rate and endurance
down to 9 K. pMTJ devices exhibited high magnetoresistance (>120%) and reliable
(error rate<10-4) bi-directional switching with 2 to 200 ns voltage pulses. The
endurance of the devices at 9 K surpassed that at 300 K by three orders of
magnitude under the same write conditions, functioning for more than 10^12
cycles with 10 ns write pulses. The critical switching voltage at 9 K was
observed to increase by 33% to 93%, depending on pulse duration, compared to
that at 350 K. Ferromagnetic resonance and magnetization measurements on
blanket pMTJ film stacks suggest that the increased switching voltage is
associated with an increase in effective magnetic anisotropy and magnetization
of free layer with decreasing temperature. Our work demonstrates that CoFeB/MgO
based pMTJs have great potential to enable cryogenic MRAM and that their low
temperature magnetization and effective magnetic anisotropy can be further
optimized to lower operating power and improve endurance.",1910.01149v1
2018-02-22,Correlation of tunnel magnetoresistance with the magnetic properties in perpendicular CoFeB-based junctions with exchange bias,"We investigate the dependence of magnetic properties on the post-annealing
temperature/time, the thickness of soft ferromagnetic electrode and Ta dusting
layer in the pinned electrode as well as their correlation with the tunnel
magnetoresistance ratio, in a series of perpendicular magnetic tunnel junctions
of materials sequence
Ta/Pd/IrMn/CoFe/Ta$(\textit{x})$/CoFeB/MgO$(\textit{y})$/CoFeB$(\textit{z})$/Ta/Pd.
We obtain a large perpendicular exchange bias of 79.6$\,$kA/m for $x=0.3\,$nm.
For stacks with $z=1.05\,$nm, the magnetic properties of the soft electrode
resemble the characteristics of superparamagnetism. For stacks with
$x=0.4\,$nm, $y=2\,$nm, and $z=1.20\,$nm, the exchange bias presents a
significant decrease at post annealing temperature
$T_\textrm{ann}=330\,^{\circ}$C for 60 min, while the interlayer exchange
coupling and the saturation magnetization per unit area sharply decay at
$T_\textrm{ann}=340\,^{\circ}$C for 60 min. Simultaneously, the tunnel
magnetoresistance ratio shows a peak of $65.5\%$ after being annealed at
$T_\textrm{ann}=300\,^{\circ}$C for 60 min, with a significant reduction down
to $10\%$ for higher annealing temperatures
($T_\textrm{ann}\geq330\,^{\circ}$C) and down to $14\%$ for longer annealing
times ($T_\textrm{ann}=300\,^{\circ}$C for 90 min). We attribute the large
decrease of tunnel magnetoresistance ratio to the loss of exchange bias in the
pinned electrode.",1802.08002v2
2020-04-07,Laser-induced magnetization precession in individual magnetoelastic domains of a multiferroic CoFeB/BaTiO$_3$ composite,"Using a magneto-optical pump-probe technique with micrometer spatial
resolution we show that magnetization precession can be launched in individual
magnetic domains imprinted in a Co$_{40}$Fe$_{40}$B$_{20}$ (CoFeB) layer by
elastic coupling to ferroelectric domains in a BaTiO$_{3}$ substrate. The
dependence of the precession parameters on external magnetic field strength and
orientation reveal that by laser-induced ultrafast partial quenching of the
magnetoelastic coupling parameter of CoFeB by $\approx$27% along with 10%
ultrafast demagnetization trigger the magnetization precession. The relation
between the laser-induced reduction of the magnetoelastic coupling and the
demagnetization is approximated by the $n(n+1)/2$-law with n$\approx$2. This
correspondence confirms the thermal origin of the laser-induced anisotropy
change. Based on the analysis and modeling of the excited precession we find
signatures of laser-induced precessional switching, which occurs when the
magnetic field is applied along the hard magnetization axis and its value is
close to the effective magnetoelastic anisotropy field. The precession
excitation process in an individual magnetoelastic domain is found to be
unaffected by neighboring domains. This makes laser-induced changes of
magnetoelastic anisotropy a promising tool for driving magnetization dynamics
and switching in composite multiferroics with spatial selectivity.",2004.03566v5
2018-04-11,Thermographic measurements of spin-current-induced temperature modulation in metallic bilayers,"Spin-to-heat current conversion effects have been investigated in bilayer
films consisting of a paramagnetic metal (PM; Pt, W, or Ta) and a ferromagnetic
metal (FM; CoFeB or permalloy). When a charge current is applied to the PM/FM
bilayer film, a spin current is generated across the PM/FM interface owing to
the spin Hall effect in PM. The spin current was found to exhibit cooling and
heating features depending on the sign of the spin Hall angle of PM, where the
spin-current-induced contribution is estimated by subtracting the contribution
of the anomalous Ettingshausen effect in FM monolayer films. We also found that
the magnitude of the spin-current-induced temperature modulation in the
Pt/CoFeB film is greater than but comparable to that in the Pt/permalloy film,
although the spin dependence of the Peltier coefficient for CoFeB is expected
to be greater than that for permalloy. We discuss the origin of the observed
behaviors with the aid of model calculations; the signals in the PM/FM films
may contain the contributions not only from the electron-driven spin-dependent
Peltier effect but also from the magnon-driven spin Peltier effect.",1804.04068v1
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
2019-12-16,Spin-current manipulation of photoinduced magnetization dynamics in heavy metal / ferromagnet double layer based nanostructures,"Spin currents offer a way to control static and dynamic magnetic properties,
and therefore they are crucial for next-generation MRAM devices or spin-torque
oscillators. Manipulating the dynamics is especially interesting within the
context of photo-magnonics. In typical $3d$ transition metal ferromagnets like
CoFeB, the lifetime of light-induced magnetization dynamics is restricted to
about 1 ns, which e.g. strongly limits the opportunities to exploit the wave
nature in a magnonic crystal filtering device. Here, we investigate the
potential of spin-currents to increase the spin wave lifetime in a functional
bilayer system, consisting of a heavy metal (8 nm of $\beta$-Tantalum
(Platinum)) and 5 nm CoFeB. Due to the spin Hall effect, the heavy metal layer
generates a transverse spin current when a lateral charge current passes
through the strip. Using time-resolved all-optical pump-probe spectroscopy, we
investigate how this spin current affects the magnetization dynamics in the
adjacent CoFeB layer. We observed a linear spin current manipulation of the
effective Gilbert damping parameter for the Kittel mode from which we were able
to determine the system's spin Hall angles. Furthermore, we measured a strong
influence of the spin current on a high-frequency mode. We interpret this mode
an an exchange dominated higher order spin-wave resonance. Thus we infer a
strong dependence of the exchange constant on the spin current.",1912.07728v1
2019-12-20,Atomistic investigation of the temperature and size dependence of the energy barrier of CoFeB/MgO nanodots,"The balance between low power consumption and high efficiency in memory
devices is a major limiting factor in the development of new technologies.
Magnetic random access memories (MRAM) based on CoFeB/MgO magnetic tunnel
junctions (MTJs) have been proposed as candidates to replace the current
technology due to their non-volatility, high thermal stability and efficient
operational performance. Understanding the size and temperature dependence of
the energy barrier and the nature of the transition mechanism across the
barrier between stable configurations is a key issue in the development of
MRAM. Here we use an atomistic spin model to study the energy barrier to
reversal in CoFeB/MgO nanodots to determine the effects of size, temperature
and external field. We find that for practical device sizes in the 10-50 nm
range the energy barrier has a complex behaviour characteristic of a transition
from a coherent to domain wall driven reversal process. Such a transition
region is not accessible to simple analytical estimates of the energy barrier
preventing a unique theoretical calculation of the thermal stability. The
atomistic simulations of the energy barrier give good agreement with
experimental measurements for similar systems which are at the state of the art
and can provide guidance to experiments identifying suitable materials and MTJ
stacks with the desired thermal stability.",1912.09761v1
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
2020-12-10,Pulse-width and Temperature Dependence of Memristive Spin-Orbit Torque Switching,"It is crucial that magnetic memory devices formed from magnetic
heterostructures possess sizable spin-orbit torque (SOT) efficiency and high
thermal stability to realize both efficient SOT control and robust storage of
such memory devices. However, most previous studies on various types of
magnetic heterostructures have focused on only their SOT efficiencies, whereas
the thermal stabilities therein have been largely ignored. In this work, we
study the temperature-dependent SOT and stability properties of two types of
W-based heterostructures, namely W/CoFeB/MgO (standard) and CoFeB/W/CoFeB/MgO
(field-free), from 25 ^{\circ}C (298 K) to 80 ^{\circ}C (353 K). Via
temperature-dependent SOT characterization, the SOT efficacies for both systems
are found to be invariant within the range of studied temperatures.
Temperature-dependent current-induced SOT switching measurements further show
that the critical switching current densities decrease with respect to the
ambient temperature; thermal stability factors ({\Delta}) are also found to
degrade as temperature increases for both standard and field-free systems. The
memristive SOT switching behaviors in both systems with various pulse-widths
and temperatures are also examined. Our results suggest that although the SOT
efficacy is robust against thermal effects, the reduction of {\Delta} at
elevated temperatures could be detrimental to standard memory as well as
neuromorphic (memristive) device applications.",2012.05531v1
2021-02-06,Spin pumping and inverse spin Hall effect in CoFeB/IrMn heterostructures,"High spin to charge conversion efficiency is the requirement for the
spintronics devices which is governed by spin pumping and inverse spin Hall
effect (ISHE). In last one decade, ISHE and spin pumping are heavily
investigated in ferromagnet/ heavy metal (HM) heterostructures. Recently
antiferromagnetic (AFM) materials are found to be good replacement of HMs
because AFMs exhibit terahertz spin dynamics, high spin-orbit coupling, and
absence of stray field. In this context we have performed the ISHE in CoFeB/
IrMn heterostructures. Spin pumping study is carried out for
$Co_{40}Fe_{40}B_{20} (12\ nm)/ Cu (3\ nm)/ Ir_{50}Mn_{50} (t\ nm)/ AlO_{x} (3\
nm)$ samples where \textit{t} value varies from 0 to 10 nm. Damping of all the
samples are higher than the single layer CoFeB which indicates that spin
pumping due to IrMn is the underneath mechanism. Further the spin pumping in
the samples are confirmed by angle dependent ISHE measurements. We have also
disentangled other spin rectifications effects and found that the spin pumping
is dominant in all the samples. From the ISHE analysis the real part of spin
mixing conductance (\textit{$g_{r}^{\uparrow \downarrow}$}) is found to be
0.704 $\pm$ 0.003 $\times$ $10^{18}$ $m^{-2}$.",2102.03624v2
2021-06-10,Spin-Orbit Torque Engineering in β-W/CoFeB Heterostructures via Ta and V Alloying at Interfaces,"Spin-orbit torque manifested as an accumulated spin-polarized moment at
nonmagnetic normal metal, and ferromagnet interfaces is a promising
magnetization switching mechanism for spintronic devices. To fully exploit this
in practice, materials with a high spin Hall angle, i.e., a charge-to-spin
conversion efficiency, are indispensable. To date, very few approaches have
been made to devise new nonmagnetic metal alloys. Moreover, new materials need
to be compatible with semiconductor processing. Here we introduce W-Ta and W-V
alloys and deploy them at the interface between $\beta$-W/CoFeB layers. First,
spin Hall conductivities of W-Ta and W-V structures with various compositions
are carried out by first-principles band calculations, which predict the spin
Hall conductivity of the W-V alloy is improved from $-0.82 \times 10^3$ S/cm
that of W to $-1.98 \times 10^3$ S/cm. Subsequently, heterostructure
fabrication and spin-orbit torque properties are characterized experimentally.
By alloying $\beta$-W with V at a concentration of 20 at%, we observe a large
enhancement of the absolute value of spin Hall conductivity of up to $-(2.77
\pm 0.31) \times 10^3$ S/cm. By employing X-ray diffraction and scanning
transmission electron microscopy, we further explain the enhancement of
spin-orbit torque efficiency is stemmed from W-V alloy between W and CoFeB.",2106.05460v1
2022-11-04,Exchange energies in CoFeB/Ru/CoFeB Synthetic Antiferromagnets,"The interlayer exchange coupling confers specific properties to Synthetic
Antiferromagnets that make them suitable for several applications of
spintronics. The efficient use of this magnetic configuration requires an
in-depth understanding of the magnetic properties and their correlation with
the material structure. Here we establish a reliable procedure to quantify the
interlayer exchange coupling and the intralayer exchange stiffness in synthetic
antiferromagnets; we apply it to the ultrasmooth and amorphous
Co$_{40}$Fe$_{40}$B$_{20}$ (5-40 nm)/Ru/ Co$_{40}$Fe$_{40}$B$_{20}$ material
platform. The complex interplay between the two exchange interactions results
in a gradient of the magnetization orientation across the thickness of the
stack which alters the hysteresis and the spin wave eigenmodes of the stack in
a non trivial way. We measured the field-dependence of the frequencies of the
first four spin waves confined within the thickness of the stack. We modeled
these frequencies and the corresponding thickness profiles of these spin waves
using micromagnetic simulations. The comparison with the experimental results
allows to deduce the magnetic parameters that best account for the sample
behavior. The exchange stiffness is established to be 16 $\pm$ 2 pJ/m,
independently of the Co$_{40}$Fe$_{40}$B$_{20}$ thickness. The interlayer
exchange coupling starts from -1.7 mJ/m$^2$ for the thinnest layers and it can
be maintained above -1.3 mJ/m$^2$ for CoFeB layers as thick as 40 nm. The
comparison of our method with earlier characterizations using the sole
saturation fields argues for a need to revisit the tabulated values of
interlayer exchange coupling in thick synthetic antiferromagnets.",2211.02497v1
2023-06-29,Ultrafast THz probing of nonlocal orbital current in transverse multilayer metallic heterostructures,"THz generation from femtosecond photoexcited spintronic heterostructures has
recently become a versatile tool for investigating ultrafast spin-transport and
transient charge-current in a non-contact and non-invasive manner. The same
from the orbital effects is still in the primitive stage. Here, we
experimentally demonstrate orbital-to-charge current conversion in metallic
heterostructures, consisting of a ferromagnetic layer adjacent to either a
light or a heavy metal layer, through detection of the emitted THz pulses.
Temperature-dependent experiments help to disentangle the orbital and spin
components that are manifested in the respective Hall-conductivities,
contributing to THz emission. NiFe/Nb shows the strongest inverse orbital Hall
effect with an experimentally extracted value of effective Hall-conductivity,
\sigma_SOH^int^eff ~ 195 {\Omega}^(-1){cm}^(-1), while CoFeB/Pt shows maximum
contribution from the inverse spin Hall effect. In addition, we observe nearly
ten-fold enhancement in the THz emission due to pronounced orbital-transport in
W-insertion heavy metal layer in CoFeB/W/Ta heterostructure as compared to the
CoFeB/Ta bilayer counterpart.",2306.17027v2
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
2019-11-08,Room Temperature Spin to Charge Conversion in Amorphous Topological Insulating Gd-Alloyed BixSe1-x/CoFeB Bilayers,"Disordered topological insulator (TI) films have gained intense interest by
benefiting from both the TIs exotic transport properties and the advantage of
mass production by sputtering. Here, we report on the clear evidence of
spin-charge conversion (SCC) in amorphous Gd-alloyed BixSe1-x (BSG)/CoFeB
bilayers fabricated by sputtering, which could be related to the amorphous TI
surface states. Two methods have been employed to study SCC in BSG/CoFeB(5 nm)
bilayers with different BSG thicknesses. Firstly, spin pumping is used to
generate a spin current in CoFeB and to detect SCC by inverse Edelstein effect.
The maximum SCC efficiency (SCE) is measured as large as 0.035 nm in a 6 nm
thick BSG sample, which shows a strong decay when tBSG increases due to the
increase of BSG surface roughness. The second method is the THz time-domain
spectroscopy, which reveals a small tBSG dependence of SCE, validating the
occurrence of a pure interface state related SCC. Furthermore, our
angle-resolved photoemission spectroscopy data show dispersive two-dimensional
surface states that cross the bulk gap until to the Fermi level, strengthening
the possibility of SCC due to the amorphous TI states. Our studies provide a
new experimental direction towards the search for topological systems in the
amorphous solids.",1911.03323v12
2022-08-10,Enhancement of Spin-Charge Conversion Efficiency for Co$_{3}$Sn$_{2}$S$_{2}$ across Transition from Paramagnetic to Ferromagnetic Phase,"Co$_{3}$Sn$_{2}$S$_{2}$ (CSS) is one of the shandite compounds and becomes a
magnetic Weyl semimetal candidate below the ferromagnetic phase transition
temperature ($\textit{T}_\textrm{C}$). In this paper, we investigate the
temperature ($\textit{T}$) dependence of conversion between charge current and
spin current for the CSS thin film by measuring the spin-torque ferromagnetic
resonance (ST-FMR) for the trilayer consisting of CSS / Cu / CoFeB. Above
$\textit{T}_\textrm{C}$ ~ 170 K, the CSS / Cu / CoFeB trilayer exhibits the
clear ST-FMR signal coming from the spin Hall effect in the paramagnetic CSS
and the anisotropic magnetoresistance (AMR) of CoFeB. Below
$\textit{T}_\textrm{C}$, on the other hand, it is found that the ST-FMR signal
involves the dc voltages ($\textit{V}_\textrm{dc}$) not only through the AMR
but also through the giant magnetoresistance (GMR). Thus, the resistance
changes coming from both AMR and GMR should be taken into account to correctly
understand the characteristic field angular dependence of
$\textit{V}_\textrm{dc}$. The spin Hall torque generated from the ferromagnetic
CSS, which possesses the same symmetry as that for spin Hall effect, dominantly
acts on the magnetization of CoFeB. A definite increase in the spin-charge
conversion efficiency ($\xi$) is observed at $\textit{T}$ <
$\textit{T}_\textrm{C}$, indicating that the phase transition to the
ferromagnetic CSS promotes the highly efficient spin-charge conversion. In
addition, our theoretical calculation shows the increase in spin Hall
conductivity with the emergence of magnetic moment at $\textit{T}$ <
$\textit{T}_\textrm{C}$, which is consistent with the experimental observation.",2208.05394v1
2023-11-10,Observation by SANS and PNR of pure Néel-type domain wall profiles and skyrmion suppression below room temperature in magnetic [Pt/CoFeB/Ru]$_{10}$ multilayers,"We report investigations of the magnetic textures in periodic [Pt(1
nm)/(CoFeB(0.8 nm)/Ru(1.4 nm)]$_{10}$ multilayers using polarised neutron
reflectometry (PNR) and small-angle neutron scattering (SANS). The multilayers
are known to host skyrmions stabilized by Dzyaloshinskii-Moriya interactions
induced by broken inversion symmetry and spin-orbit coupling at the asymmetric
interfaces. From depth-dependent PNR measurements, we observe well-defined
structural features, and obtain the layer-resolved magnetization profiles. The
in-plane magnetization of the CoFeB layers calculated from fitting of the PNR
profiles is found to be in excellent agreement with magnetometry data. Using
SANS as a bulk probe of the entire multilayer, we observe long-period magnetic
stripe domains and skyrmion ensembles with full orientational disorder at room
temperature. No sign of skyrmions is found below 250\,K, which we suggest is
due to an increase of a effective magnetic anisotropy in the CoFeB layer on
cooling that suppresses skyrmion stability. Using polarised SANS at room
temperature, we prove the existence of pure N\'eel-type windings in both stripe
domain and skyrmion regimes. No Bloch-type winding admixture, i.e. an
indication for hybrid windings, is detected within the measurement sensitivity,
in good agreement with expectations according to our micromagnetic modelling of
the multilayers. Our findings using neutron techniques offer valuable
microscopic insights into the rich magnetic behavior of skyrmion-hosting
multilayers, which are essential for the advancement of future skyrmion-based
spintronic devices.",2311.05959v2
2007-12-17,Spin tunneling in junctions with disordered ferromagnets,"We provide compelling evidence to establish that, contrary to one's
elementary guess, the tunneling spin polarization (TSP) of amorphous CoFeB is
larger than that of highly textured fcc CoFeB. First principles atomic and
electronic structure calculations reveal striking agreement between the
measured TSP and the predicted s-electron spin polarization. Given the
disordered structure of the ternary alloy, not only do these results strongly
endorse our communal understanding of tunneling through AlOx, but they also
portray the key concepts that demand primary consideration in such complex
systems.",0712.2722v1
2011-03-22,Inductive determination of the optimum tunnel barrier thickness in magnetic tunnelling junction stacks for spin torque memory applications,"We use pulsed inductive microwave magnetometry to study the precessional
magnetization dynamics of the free layer in CoFeB/MgO/CoFeB based magnetic
tunnelling junction stacks with varying MgO barrier thickness. From the field
dependence of the precession frequency we are able to derive the uniaxial
anisotropy energy and the exchange coupling between the free and the pinned
layer. Furthermore the field dependence of the effective damping parameter is
derived. Below a certain threshold barrier thickness we observe an increased
effective damping for antiparallel orientation of free and pinned layer which
would inhibit reversible low current density spin torque magnetization
reversal. Such inductive measurements, in combination with wafer probe station
based magneto transport experiments, allow a fast determination of the optimum
tunnel barrier thickness range for spin torque memory applications in a
lithography free process.",1103.4248v1
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
2012-05-13,Voltage-Induced Ferromagnetic Resonance in Magnetic Tunnel Junctions,"We demonstrate excitation of ferromagnetic resonance in CoFeB/MgO/CoFeB
magnetic tunnel junctions (MTJs) by the combined action of voltage-controlled
magnetic anisotropy (VCMA) and spin transfer torque (ST). Our measurements
reveal that GHz-frequency VCMA torque and ST in low-resistance MTJs have
similar magnitudes, and thus that both torques are equally important for
understanding high-frequency voltage-driven magnetization dynamics in MTJs. As
an example, we show that VCMA can increase the sensitivity of an MTJ-based
microwave signal detector to the sensitivity level of semiconductor Schottky
diodes.",1205.2835v2
2012-08-08,Spin transfer torque devices utilizing the giant spin Hall effect of tungsten,"We report a giant spin Hall effect (SHE) in {\beta}-W thin films. Using spin
torque induced ferromagnetic resonance with a {\beta}-W/CoFeB bilayer
microstrip we determine the spin Hall angle to be |\theta|=0.30\pm0.02, large
enough for an in-plane current to efficiently reverse the orientation of an
in-plane magnetized CoFeB free layer of a nanoscale magnetic tunnel junction
adjacent to a thin {\beta}-W layer. From switching data obtained with such
3-terminal devices we independently determine |\theta|=0.33\pm0.06. We also
report variation of the spin Hall switching efficiency with W layers of
different resistivities and hence of variable ({\alpha} and {\beta}) phase
composition.",1208.1711v1
2013-03-19,X-ray absorption spectroscopy and magnetic circular dichroism studies of L1_0-Mn-Ga thin films,"Tetragonally distorted \(\rm{Mn}_{3-x}\rm{Ga}_x\) thin films with \(0.1< x <
2\) show a strong perpendicular magnetic anisotropy and low magnetization and
thus have the potential to serve as electrodes in spin transfer torque magnetic
random access memory. Because a direct capping of these films with MgO is
problematic due to oxide formation, we examined the influence of a CoFeB
interlayer, and of two different deposition methods for the MgO barrier on the
formation of interfacial MnO for \(\rm{Mn}_{62}\rm{Ga}_{38}\) by element
specific X-ray absorption spectroscopy (XAS) and magnetic circular dichroism
(XMCD). A highly textured L1\(_0\) crystal structure of the Mn-Ga films was
verified by X-ray diffraction (XRD) measurements. For samples with e-beam
evaporated MgO barrier no evidence for MnO was found, whereas in samples with
magnetron sputtered MgO MnO was detected, even for the thickest interlayer
thickness. Both XAS and XMCD measurements showed an increasing interfacial MnO
amount with decreasing CoFeB interlayer thickness. Additional element specific
full hysteresis loops determined an out-of-plane magnetization axis for the Mn
and Co, respectively.",1303.4648v2
2013-05-27,Magnetization reversal in sub-100nm magnetic tunnel junctions with ultrathin MgO barrier biased along hard axis,"We report on room temperature magnetoresistance and low frequency noise in
sub-100nm elliptic CoFeB/MgO/CoFeB magnetic tunnel junctions with ultrathin
(0.9nm) barriers. For magnetic fields applied along the hard axis, we observe
current induced magnetization switching between the antiparallel and parallel
alignments at DC current densities as low as 4*106A/cm2. We attribute the low
value of the critical current to the influence of localized reductions in the
tunnel barrier, which affects the current distribution. The analysis of random
telegraph noise, which appears in the field interval near a magnetization
switch, provides an estimate to the dimension of the pseudo pinholes that
trigger the magnetization switching via local spin torque. Micromagnetic
simulations qualitatively and quantitatively reproduce the main experimental
observations.",1305.6209v1
2013-05-29,Enhanced interface perpendicular magnetic anisotropy in Ta|CoFeB|MgO using nitrogen doped Ta underlayers,"We show that the magnetic characteristics of Ta|CoFeB|MgO magnetic
heterostructures are strongly influenced by doping the Ta underlayer with
nitrogen. In particular, the saturation magnetization drops upon doping the Ta
underlayer, suggesting that the doped underlayer acts as a boron diffusion
barrier. In addition, the thickness of the magnetic dead layer decreases with
increasing nitrogen doping. Surprisingly, the interface magnetic anisotropy
increases to ~1.8 erg/cm2 when an optimum amount of nitrogen is introduced into
the Ta underlayer. These results show that nitrogen doped Ta serves as a good
underlayer for Spintronics applications including magnetic tunnel junctions and
domain wall devices.",1305.6660v1
2013-08-08,Interface control of the magnetic chirality in TaN|CoFeB|MgO heterosctructures,"Recent advances in the understanding of spin orbital effects in ultrathin
magnetic heterostructures have opened new paradigms to control magnetic moments
electrically. The Dzyaloshinskii-Moriya interaction (DMI) is said to play a key
role in forming a Neel-type domain wall that can be driven by the spin Hall
torque, a torque resulting from the spin current generated in a neighboring
non-magnetic layer via the spin Hall effect. Here we show that the sign of the
DMI, which determines the direction to which a domain wall moves with current,
can be changed by modifying the adjacent non-magnetic layer. We find that the
sense of rotation of a domain wall spiral is reversed when the Ta underlayer is
doped with nitrogen in Ta|CoFeB|MgO heterostructures. The spin Hall angle of
the Ta and nitrogen doped Ta underlayers carry the same sign, suggesting that
the sign of the DMI is defined at the interface. Depending on the sense of
rotation, spin transfer torque and spin Hall torque can either compete or
assist each other, thus influencing the efficiency of moving domain walls with
current.",1308.1751v1
2013-10-18,Quantitative characterization of the spin orbit torque using harmonic Hall voltage measurements,"Solid understanding of current induced torques is key to the development of
current and voltage controlled magnetization dynamics in ultrathin magnetic
heterostructures. To evaluate the size and direction of such torques, or
effective fields, a number of methods have been employed. Here we examine the
adiabatic (low frequency) harmonic Hall voltage measurement that has been used
to study the effective field. We derive an analytical formula for the harmonic
Hall voltages to evaluate the effective field for both out of plane and
in-plane magnetized systems. The formula agrees with numerical calculations
based on a macrospin model. Two different in-plane magnetized films,
Pt|CoFeB|MgO and CuIr|CoFeB|MgO are studied using the formula developed. The
effective field obtained for the latter system shows relatively good agreement
with that estimated using a spin torque switching phase diagram measurements
reported previously. Our results illustrate the versatile applicability of
harmonic Hall voltage measurement for studying current induced torques in
magnetic heterostructures.",1310.4879v1
2013-12-03,Depth dependent magnetization profiles of hybrid exchange springs,"We report on the magnetization depth profile of a hybrid exchange spring
system in which a Co/Pd multilayer with perpendicular anisotropy is coupled to
a CoFeB thin film with in-plane anisotropy. The competition between these two
orthogonal anisotropies promotes a strong depth dependence of the magnetization
orientation. The angle of the magnetization vector is sensitive both to the
strength of the individual anisotropies and to the local exchange constant, and
is thus tunable by changing the thickness of the CoFeB layer and by
substituting Ni for Pd in one layer of the Co/Pd stack. The resulting magnetic
depth profiles are directly probed by element specific x-ray magnetic circular
dichroism (XMCD) of the Co, Fe, and Ni layers located at different average
depths. The experimental results are corroborated by micromagnetic simulations.",1312.0878v1
2014-04-04,Angular and temperature dependence of current induced spin-orbit effective fields in Ta/CoFeB/MgO nanowires,"Current induced spin-orbit effective magnetic fields in
metal/ferromagnet/oxide trilayers provide a new way to manipulate the
magnetization, which is an alternative to the conventional current induced spin
transfer torque arising from noncollinear magnetization. Ta/CoFeB/MgO
structures are expected to be useful for non-volatile memories and logic
devices due to its perpendicular anisotropy and large current induced
spin-orbit effective fields. However many aspects such as the angular and
temperature dependent phenomena of the effective fields are little understood.
Here, we evaluate the angular and temperature dependence of the current-induced
spin-orbit effective fields considering contributions from both the anomalous
and planar Hall effects. The longitudinal and transverse components of
effective fields are found to have strong angular dependence on the
magnetization direction at 300 K. The transverse field decreases significantly
with decreasing temperature, whereas the longitudinal field shows weaker
temperature dependence. Our results reveal important features and provide an
opportunity for a more comprehensive understanding of current induced
spin-orbit effective fields.",1404.1130v1
2014-04-08,Ferromagnetic resonance spin pumping in CoFeB with highly resistive non-magnetic electrodes,"The relative contribution of spin pumping and spin rectification from the
ferromagnetic resonance of CoFeB/non-magnetic bilayers was investigated as a
function of non-magnetic electrode resistance. Samples with highly resistive
electrodes of Ta or Ti exhibit a stronger spin rectification signal, which may
result in over-(or under-)estimation of the spin Hall angle of the materials,
while those with low resistive electrodes of Pt or Pd show the domination of
the inverse spin Hall effect from spin pumping. By comparison with samples of
single FM layer and an inverted structure, we provide a proper analysis method
to extract spin pumping contribution.",1404.1993v1
2014-05-02,Spin-orbit torque-driven magnetization switching and thermal effects studied in Ta\CoFeB\MgO nanowires,"We demonstrate magnetization switching in out-of-plane magnetized
Ta\CoFeB\MgO nanowires by current pulse injection along the nanowires, both
with and without a constant and uniform magnetic field collinear to the current
direction. We deduce that an effective torque arising from spin-orbit effects
in the multilayer drives the switching mechanism. While the generation of a
component of the magnetization along the current direction is crucial for the
switching to occur, we observe that even without a longitudinal field thermally
generated magnetization fluctuations can lead to switching. Analysis using a
generalized N\'eel-Brown model enables key parameters of the thermally induced
spin-orbit torques switching process to be estimated, such as the attempt
frequency and the effective energy barrier.",1405.0452v1
2014-05-08,Controlling boron redistribution in CoFeB/MgO magnetic tunnel junctions during annealing by variation of cap layer materials and MgO deposition methods,"Magnetic tunnel junctions with crystalline MgO tunnel barrier and amorphous
CoFeB electrodes received much attention due to their high tunnel magneto
resistance ratio at room temperature. One important parameter for achieving
high tunnel magneto resistance ratios is to control the boron diffusion from
the electrodes especially during post growth annealing. By high resolution
transmission electron microscopy and electron energy loss spectroscopy
techniques we show that the cap layer material adjacent to the electrodes and
the MgO deposition method are crucial to control boron redistribution. It is
pointed out, that Ta cap layers acts as sinks for boron during annealing in
contrast to Ru layers. Furthermore, radio frequency sputtered MgO tunneling
barriers contain a rather high concentraion of boron in trigonal
[BO$_3$]$^{3-}$ - environment after annealing in contrast to electron beam
evaporated MgO which is virtually free from any boron. Our data further
indicate that neither boron nor oxygen-vacancy-related gap states in the bulk
of MgO barriers affect spin polarized transport for tunnel magneto resistance
ratios at the level of 200%.",1405.1907v1
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
2014-11-18,Towards wafer scale inductive characterization of spin transfer torque critical current density of magnetic tunnel junction stacks,"We explore the prospects of wafer scale inductive probing of the critical
current density $j^{c0}$ for spin transfer torque switching of a
CoFeB/MgO/CoFeB magnetic tunnel junction with varying MgO thickness. From
inductive measurements magnetostatic parameters and the effective damping are
derived and $j^{c0}$ is calculated based on spin transfer torque equations. The
inductive values compare well to the values derived from current induced
switching measurements on individual nanopillars. Using a wafer scale inductive
probe head could in the future enable wafer probe station based metrology of
$j^{c0}$.",1411.4868v1
2015-04-25,Magnetic phase transitions in Ta/CoFeB/MgO multilayers,"We study thin films and magnetic tunnel junction nanopillars based on
Ta/Co$_{20}$Fe$_{60}$B$_{20}$/MgO multilayers by electrical transport and
magnetometry measurements. These measurements suggest that an ultrathin
magnetic oxide layer forms at the Co$_{20}$Fe$_{60}$B$_{20}$/MgO interface. At
approximately 160 K, the oxide undergoes a phase transition from an insulating
antiferromagnet at low temperatures to a conductive weak ferromagnet at high
temperatures. This interfacial magnetic oxide is expected to have significant
impact on the magnetic properties of CoFeB-based multilayers used in spin
torque memories.",1504.06716v1
2015-05-21,Resistance minimum and electrical conduction mechanism in polycrystalline CoFeB thin films,"The temperature dependent resistance $R$($T$) of polycrystalline
ferromagnetic CoFeB thin films of varying thickness are analyzed considering
various electrical scattering processes. We observe a resistance minimum in
$R$($T$) curves below $\simeq$ 29 K, which can be explained as an effect of
intergranular Coulomb interaction in a granular system. The structural and
Coulomb interaction related scattering processes contribute more as the film
thickness decreases implying the role of disorder and granularity. Although the
magnetic contribution to the resistance is the weakest compared to these two,
it is the only thickness independent process. On the contrary, the negative
coefficient of resistance can be explained by electron interaction effect in
disordered amorphous films.",1505.05711v2
2015-08-04,Scanning Kerr microscopy of current induced switching in Ta/CoFeB/MgO films with perpendicular magnetic anisotropy,"Ta/CoFeB/MgO trilayers with perpendicular magnetic anisotropy are expected to
play a key role in the next generation of current and electric field switched
memory and logic devices. In this study we use Kerr microscopy alongside
electrical transport measurement to gain insight into the underlying switching
mechanisms of such devices. We find switching to be a stochastic, domain wall
driven process, the speed of which is strongly dependent on the switching
current. Kerr imaging shows domain nucleation at the edge of the device which
modelling reveals is likely assisted by the perpendicular component of the
Oersted field. Further domain growth, leading to magnetisation reversal may be
assisted by spin torques.",1508.00833v2
2015-10-16,Enhanced orbital magnetic moments in magnetic heterostructures with interface perpendicular magnetic anisotropy,"We have studied the magnetic layer thickness dependence of the orbital
magnetic moment in magnetic heterostructures to identify contributions from
interfaces. Three different heterostructures, Ta/CoFeB/MgO, Pt/Co/AlO$_x$ and
Pt/Co/Pt, which possess significant interface contribution to the perpendicular
magnetic anisotropy, are studied as model systems. X-ray magnetic circular
dichroism spectroscopy is used to evaluate the relative orbital moment, i.e.
the ratio of the orbital to spin moments, of the magnetic elements constituting
the heterostructures. We find that the relative orbital moment of Co in
Pt/Co/Pt remains constant against its thickness whereas the moment increases
with decreasing Co layer thickness for Pt/Co/AlO$_x$, suggesting that a
non-zero interface orbital moment exists for the latter system. For
Ta/CoFeB/MgO, a non-zero interface orbital moment is found only for Fe. X-ray
absorption spectra shows that a particular oxidized Co state in Pt/Co/AlO$_x$,
absent in other heterosturctures, may give rise to the interface orbital moment
in this system. These results show element specific contributions to the
interface orbital magnetic moments in ultrathin magnetic heterostructures.",1510.04756v1
2015-12-02,Dynamic cantilever magnetometry of individual CoFeB nanotubes,"We investigate single CoFeB nanotubes with hexagonal cross-section using
dynamic cantilever magnetometry (DCM). We develop both an analytical model
based on the Stoner-Wohlfarth approximation and a broadly applicable numerical
framework for analyzing DCM measurements of magnetic nanostructures.
Magnetometry data show the presence of a uniformly magnetized configuration at
high external fields with $\mu_0 M_s =1.3 \pm 0.1$ T and non-uniform
configurations at low fields. In this low-field regime, comparison between
numerical simulations and DCM measurements supports the existence of
flux-closure configurations. Crucially, evidence of such configurations is only
apparent because of the sensitivity of DCM to single nanotubes, whereas
conventional measurements of ensembles are often obscured by sample-to-sample
inhomogeneities in size, shape, and orientation",1512.00621v1
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,THz-driven ultrafast spin-lattice scattering in amorphous metallic ferromagnets,"We use single-cycle THz fields and the femtosecond magneto-optical Kerr
effect to respectively excite and probe the magnetization dynamics in two
thin-film ferromagnets with different lattice structure: crystalline Fe and
amorphous CoFeB. We observe Landau-Lifshitz-torque magnetization dynamics of
comparable magnitude in both systems, but only the amorphous sample shows
ultrafast demagnetization caused by the spin-lattice depolarization of the
THz-induced ultrafast spin current. Quantitative modelling shows that such
spin-lattice scattering events occur on similar time scales than the
conventional spin conserving electronic scattering ($\sim30$ fs). This is
significantly faster that optical laser-induced demagnetization. THz
conductivity measurements point towards the influence of lattice disorder in
amorphous CoFeB as the driving force for enhanced spin-lattice scattering.",1604.04077v1
2016-04-26,Comparison of laser induced and intrinsic tunnel magneto-Seebeck effect in CoFeB/MgAl$_2$O$_4$ and CoFeB/MgO magnetic tunnel junctions,"We present a comparison of the tunnel magneto-Seebeck effect for laser
induced and intrinsic heating. Therefore,
Co$_{40}$Fe$_{40}$B$_{20}$/MgAl$_2$O$_4$ and Co$_{25}$Fe$_{55}$B$_{20}$/MgO
magnetic tunnel junctions have been prepared. The TMS ratio of 3\,\% in case of
the MAO MTJ agrees well with ratios found for other barrier materials, while
the TMS ratio of 23\,\% of the MgO MTJ emphasizes the influence of the CoFe
composition. We find results using the intrinsic method that differ in sign and
magnitude in comparison to the results of the laser heating. The intrinsic
contributions can alternatively be explained by the Brinkman model and the
given junction properties. Especially, we are able to demonstrate that the
symmetric contribution is solely influenced by the barrier asymmetry. Thus, we
conclude that the symmetry analysis used for the intrinsic method is not
suitable to unambiguously identify an intrinsic tunnel magneto-Seebeck effect.",1604.07569v2
2016-05-23,Direct measurement of interfacial Dzyaloshinskii-Moriya interaction in X/CoFeB/MgO heterostructures with a scanning-NV magnetometer,"The Dzyaloshinskii-Moriya Interaction (DMI) has recently attracted
considerable interest owing to its fundamental role in the stabilization of
chiral spin textures in ultrathin ferromagnets, which are interesting
candidates for future spintronic technologies. Here we employ a scanning
nano-magnetometer based on a single nitrogen-vacancy (NV) defect in diamond to
locally probe the strength of the interfacial DMI in CoFeB/MgO ultrathin films
grown on different heavy metal underlayers X=Ta,TaN, and W. By measuring the
stray field emanating from DWs in micron-long wires of such materials, we
observe deviations from the Bloch profile for TaN and W underlayers that are
consistent with a positive DMI value favoring right-handed chiral spin
structures. Moreover, our measurements suggest that the DMI constant might vary
locally within a single sample, illustrating the importance of local probes for
the study of magnetic order at the nanoscale.",1605.07044v1
2016-07-21,Effect of annealing on the interfacial Dzyaloshinskii-Moriya interaction in Ta/CoFeB/MgO trilayers,"The interfacial Dzyaloshinskii-Moriya interaction (DMI) has been shown to
stabilize homochiral N\'eel-type domain walls in thin films with perpendicular
magnetic anisotropy and as a result permit them to be propagated by a spin Hall
torque. In this study, we demonstrate that in Ta/Co$_{20}$Fe$_{60}$B$_{20}$/MgO
the DMI may be influenced by annealing. We find that the DMI peaks at
$D=0.057\pm0.003$ mJ/m$^{2}$ at an annealing temperature of 230 $^{\circ}$C.
DMI fields were measured using a purely field-driven creep regime domain
expansion technique. The DMI field and the anisotropy field follow a similar
trend as a function of annealing temperature. We infer that the behavior of the
DMI and the anisotropy are related to interfacial crystal ordering and B
expulsion out of the CoFeB layer as the annealing temperature is increased.",1607.06405v1
2017-07-26,Reorientable Spin Direction for Spin Current Produced by the Anomalous Hall Effect,"We show experimentally that the spin direction of the spin current generated
by spin-orbit interactions within a ferromagnetic layer can be reoriented by
turning the magnetization direction of this layer. We do this by measuring the
field-like component of spin-orbit torque generated by an exchange-biased FeGd
thin film and acting on a nearby CoFeB layer. The relative angle of the CoFeB
and FeGd magnetic moments is varied by applying an external magnetic field. We
find that the resulting torque is in good agreement with predictions that the
spin current generated by the anomalous Hall effect from the FeGd layer depends
on the FeGd magnetization direction $\hat{m}_{FeGd}$ according to
$\vec{\sigma}\propto\left ( \hat{y}\cdot \hat{m}_{FeGd} \right
)\hat{m}_{FeGd}$, where $\hat{y}$ is the in-plane direction perpendicular to
the applied charge current. Because of this angular dependence, the spin-orbit
torque arising from the anomalous Hall effect can be non-zero in a sample
geometry for which the spin Hall torque generated by non-magnetic materials is
identically zero.",1707.08631v1
2017-11-30,Domain Wall Motion Driven by Laplace Pressure in CoFeB-MgO Nanodots with Perpendicular Anisotropy,"We have studied the magnetization reversal of CoFeB-MgO nanodots with
perpendicular anisotropy for size ranging from w=400 nm to 1 {\mu}m. Contrary
to previous experiments, the switching field distribution is shifted toward
lower magnetic fields as the size of the elements is reduced with a mean
switching field varying as 1/w. We show that this mechanism can be explained by
the nucleation of a pinned magnetic domain wall (DW) at the edges of the
nanodots where damages are introduced by the patterning process. As the surface
tension (Laplace pressure) applied on the DW increases when reducing the size
of the nanodots, we demonstrate that the depinning field to reverse the entire
elements varies as 1/w. These results suggest that the presence of DWs has to
be considered in the switching process of nanoscale elements and open a path
toward scalable spintronic devices.",1711.11334v2
2018-03-08,CMOS compatible W/CoFeB/MgO spin Hall nano-oscillators with wide frequency tunability,"We demonstrate low-operational-current W/Co$_{20}$Fe$_{60}$B$_{20}$/MgO spin
Hall nano-oscillators (SHNOs) on highly resistive silicon (HiR-Si) substrates.
Thanks to a record high spin Hall angle of the $\beta$-phase W ($\theta_{SH}$ =
-0.53), a very low threshold current density of 3.3 $\times$ 10$^{7}$ A/cm$^2$
can be achieved. Together with their very wide frequency tunability (7-28 GHz),
promoted by a moderate perpendicular magnetic anisotropy, this makes
HiR-Si/W/CoFeB based SHNOs potential candidates for wide-band microwave signal
generation. Their CMOS compatibility offers a promising route towards the
integration of spintronic microwave devices with other on-chip semiconductor
microwave components.",1803.03032v1
2018-06-13,Enhanced Spin-Orbit Torques and Magnetization Switching through Interface Engineering,"The origin of spin-orbit torques generated from the conversion of
charge-to-spin currents is of considerable debate. Solid understanding of the
physics behind is key to the development of current and voltage controlled
switching dynamics in ultrathin heterostuctures. The field free switching
observed recently (Phys. Rev. Lett. 120, 117703 (2018)) in a Pt/W/CoFeB
structure has intensified such a debate. Here we derive a formula to evaluate a
perpendicular effective field generated when the current flows through the
heterostructure, considering the large resistivity difference between the two
normal metal layers and the chemical potential gradient created at the
interface. Together with recent X-ray photoelectron spectroscopy findings at
the interface of a Pd/CoFeB structure, we conclude that a new torque generated
may play a key role in the field free switching. The model and mechanism
proposed agrees with previous reports on spin current injection and field free
switching using different interface engineering methods.",1806.05163v1
2018-06-20,Domain wall resistance in CoFeB-based heterostructures with interface Dzyaloshinskii-Moriya interaction,"We have studied the domain wall resistance in W/Ta/CoFeB/MgO
heterostructures. The Ta layer thickness is varied to control the type of
domain walls via changes in the interfacial Dzyaloshinskii Moriya interaction.
We find a nearly constant domain wall resistance against the Ta layer
thickness. Adding contributions from the anisotropic magnetoresistance, spin
Hall magnetoresistance and anomalous Hall effect describe well the domain wall
resistance of the thick Ta layer films. However, a discrepancy remains for the
thin Ta layer films wherein chiral N\'eel-like domain walls are found. These
results show the difficulty of studying the domain wall type from resistance
measurements.",1806.07750v1
2018-11-13,Spin-orbit torques acting upon a perpendicularly-magnetized Py layer,"We show that Py, a commonly-used soft ferromagnetic material with weak
anisotropy, can become perpendicularly-magnetized while depositing on Ta buffer
layer with Hf or Zr insertion layers (ILs) and MgO capping layer. By using two
different approaches, namely harmonic voltage measurement and hysteresis loop
shift measurement, the dampinglike spin-orbit torque (DL-SOT) efficiencies from
Ta/IL/Py/IL/MgO magnetic heterostructures with perpendicular magnetic
anisotropy are characterized. We find that though Ta has a significant spin
Hall effect, the DL-SOT efficiencies are small in systems with the Ta/Py
interface compared to that obtained from the control sample with the
traditional Ta/CoFeB interface. Our results indicate that the spin transparency
for the Ta/Py interface is much less than that for the Ta/CoFeB interface,
which might be related to the variation of spin mixing conductance for
different interfaces.",1811.05164v1
2017-04-01,Thermally nucleated magnetic reversal in CoFeB/MgO nanodots,"Power consumption is the main limitation in the development of new high
performance random access memory for portable electronic devices. Magnetic RAM
(MRAM) with CoFeB/MgO based magnetic tunnel junctions (MTJs) is a promising
candidate for reducing the power consumption given its non-volatile nature
while achieveing high performance. The dynamic properties and switching
mechanisms of MTJs are critical to understanding device operation and to enable
scaling of devices below 30 nm in diameter. Here we show that the magnetic
reversal mechanism is incoherent and that the switching is thermally nucleated
at device operating temperatures. Moreover, we find an intrinsic thermal
switching field distribution arising on the sub-nanosecond timescale even in
the absence of size and anisotropy distributions or material defects. These
features represent the characteristic signature of the dynamic properties in
MTJs and give an intrinsic limit to reversal reliability in small magnetic
nanodevices.",1704.00106v1
2017-05-13,"Tuning the perpendicular magnetic anisotropy, spin Hall switching current density and domain wall velocity by submonolayer insertion in Ta / CoFeB / MgO heterostructures","By submonolayer insertion of Au, Pt, or Pd into Ta / CoFeB / MgO / Ta
heterostructures we tune the perpendicular magnetic anisotropy and the coercive
field of the ferromagnetic layer. We demonstrate that this has a major
influence on the spin Hall switching current density and its dependence on the
external magnetic field. Despite a rather small effective spin Hall angle of
$\theta_\mathrm{SH} \approx -0.07$, we obtain switching current densities as
low as $2 \times 10^{10}$ A/m$^2$ with a 2 \AA{} Au interlayer. We find that
the Dzyaloshinskii-Moriya interaction parameter $D$ is reduced with Au or Pd
interlayers, and the perpendicular anisotropy field is reduced by an order of
magnitude with the Pd interlayer. The dependence of the switching current
density on the current pulse width is quantitatively explained with a domain
wall nucleation and propagation model. Interface engineering is thus found to
be a suitable route to tailor the current-induced magnetization switching
properties of magnetic heterostructures.",1705.04800v1
2019-10-17,Planar Hall driven torque in a FM/NM/FM system,"An important goal of spintronics is to covert a charge current into a spin
current with a controlled spin polarization that can exert torques on an
adjacent magnetic layer. Here we demonstrate such torques in a two ferromagnet
system. A CoNi multilayer is used as a spin current source in a sample with
structure CoNi/Au/CoFeB. Spin torque ferromagnetic resonance is used to measure
the torque on the CoFeB layer. The response as a function of the applied field
angle and current is consistent with the symmetry expected for a torques
produced by the planar Hall effect originating in CoNi. We find the strength of
this effect to be comparable to that of the spin Hall effect in platinum,
indicating that the planar Hall effect holds potential as a spin current source
with a controllable polarization direction.",1910.08039v1
2019-06-14,Mechanisms of FMR line broadening in CoFeB-LiNbO$_3$ granular films in the vicinity of metal-insulator transition,"Metal-insulator (CoFeB)$_x$(LiNbO$_3$)$_{100-x}$ nanocomposite films with
different content of the ferromagnetic (FM) phase $x$ are investigated by
ferromagnetic resonance (FMR) technique. A strong change of the FMR line shape
is observed in the vicinity of metal-insulator transition (MIT) of the film,
where the hopping-type conductivity $\sigma$ modifies to the regime of a strong
intergranular tunnelling, characterized by a logarithmic dependence $\sigma(T)$
at high temperatures. It is shown that below MIT, the FMR linewidth is mainly
determined by the inhomogeneous distribution of the local anisotropy axes in
the film plane. Above MIT, the contribution of this inhomogeneity to the line
broadening decreases. At the same time, two-magnon magnetic relaxation
processes begin to play a significant role in the formation of the linewidth.
The observed behaviour indicates the critical role of interparticle exchange in
the tunnelling regime above MIT of the nanocomposite.",1906.06320v2
2020-04-03,Modulation of field-like spin orbit torque in heavy metal / ferromagnet heterostructure,"Recent studies rediscovered the crucial role of field-like spin orbit torque
(SOT) in nanosecond-timescale SOT dynamics. However, there is not yet an
effective way to control its relative amplitude. Here, we experimentally
modulate the field-like SOT in W/CoFeB/MgO trilayers through tuning the
interfacial spin accumulation. By performing spin Hall magnetoresistance
measurement, we find that the CoFeB with enhanced spin dephasing, either
generated from larger layer thickness or from proper annealing, can distinctly
boost the spin absorption and enhance the interfacial spin mixing conductance
G_r. While the damping-like torque efficiency increases with G_r, the
field-like torque efficiency turns out to decrease with it. The results suggest
that the interfacial spin accumulation, which largely contributes to a
field-like torque, is reduced by higher interfacial spin transparency. Our work
shows a new path to further improve the performance of SOT-based magnetic
devices.",2004.01357v1
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
2020-05-11,Improving thermal stability of MnN/CoFeB exchange bias systems by optimizing the Ta buffer layer,"We investigated the influence of the Ta buffer layer on the thermal stability
of polycrystalline Ta/ MnN/ CoFeB exchange bias systems, showing high exchange
bias of about 1800 Oe at room temperature. The thermal stability of those
trilayer systems is limited by nitrogen diffusion that occurs during annealing
processes. Most of the nitrogen diffuses into the Ta buffer layer, which is
necessary for good crystal growth of MnN and thus a crucial component of the
exchange bias system. In order to improve the thermal stability, we prepared
exchange bias stacks where we varied the Ta thickness to look for an optimum
value that guarantees stable and high exchange over a broad temperature range.
Our findings show that thin layers of 2-5 nm Ta indeed support stable exchange
bias up to annealing temperatures of more than $550^{\circ}$C. Furthermore, we
found that the introduction of a TaN$_{\text{x}}$ layer between MnN and Ta,
acting as a barrier, can prevent nitrogen diffusion. However, our results show
that those measures, even though being beneficial in terms of thermal
stability, often lead to decreased crystallinity and thus lower the exchange
bias.",2005.05166v1
2017-02-24,Giant perpendicular exchange bias with antiferromagnetic MnN,"We investigated an out-of-plane exchange bias system that is based on the
antiferromagnet MnN. Polycrystalline, highly textured film stacks of Ta / MnN /
CoFeB / MgO / Ta were grown on SiO$_x$ by (reactive) magnetron sputtering and
studied by x-ray diffraction and Kerr magnetometry. Nontrivial modifications of
the exchange bias and the perpendicular magnetic anisotropy were observed both
as functions of film thicknesses as well as field cooling temperatures. In
optimized film stacks, a giant perpendicular exchange bias of 3600 Oe and a
coercive field of 350 Oe were observed at room temperature. The effective
interfacial exchange energy is estimated to be $J_\mathrm{eff} = 0.24$ mJ/m$^2$
and the effective uniaxial anisotropy constant of the antiferromagnet is
$K_\mathrm{eff} = 24$ kJ/m$^3$. The maximum effective perpendicular anisotropy
field of the CoFeB layer is $H_\mathrm{ani} = 3400$ Oe. These values are larger
than any previously reported values. These results possibly open a route to
magnetically stable, exchange biased perpendicularly magnetized spin valves.",1702.07538v2
2019-05-22,Reduced Exchange Interactions in Magnetic Tunnel Junction Free Layers with Insertion Layers,"Perpendicularly magnetized CoFeB layers with ultra-thin non-magnetic
insertion layers are very widely used as the electrodes in magnetic tunnel
junctions for spin transfer magnetic random access memory devices. Exchange
interactions play a critical role in determining the thermal stability of
magnetic states in such devices and their spin torque switching efficiency.
Here the exchange constant of free layers incorporated in full magnetic tunnel
junction layer stacks, specifically CoFeB free layers with W insertion layers
is determined by magnetization measurements in a broad temperature range. A
significant finding is that the exchange constant decreases significantly and
abruptly with W insertion layer thickness. The perpendicular magnetic
anisotropy shows the opposite trend; it initially increases with W insertion
layer thickness and shows a broad maximum for approximately one monolayer (0.3
nm) of W. These results highlight the interdependencies of magnetic
characteristics required to optimize the performance of magnetic tunnel
junction devices.",1905.09329v1
2019-08-06,Superparamagnetic dwell times and tuning of switching rates in perpendicular CoFeB/MgO/CoFeB tunnel junctions,"Thin electrodes of magnetic tunnel junctions can show superparamagnetism at
surprisingly low temperature. We analysed their thermally induced switching for
varying temperature, magnetic and electric field. Although the dwell times
follow an Arrhenius law, they are orders of magnitude too small compared to a
model of single domain activation. Including entropic effects removes this
inconsistency and leads to a magnetic activation volume much smaller than that
of the electrode. Comparing data for varying barrier thickness then allows to
separate the impact of Zeman energy, spin-transfer-torque and voltage induced
anisotropy change on the dwell times. Based on these results, we demonstrate a
tuning of the switching rates by combining magnetic and electric fields, which
opens a path for their application in noisy neural networks.",1908.02139v3
2019-09-06,Effect of Tantalum spacer thickness and deposition conditions on the properties of MgO/CoFeB/Ta/CoFeB/MgO free layers,"To get stable perpendicularly magnetized tunnel junctions at small device
dimensions, composite free layers that comprise two MgO/FeCoB interfaces as
sources of interface anisotropy are generally used. Proper cristallisation and
annealing robustness is typically ensured by the insertion of a spacer layer of
the early transition metal series within the FeCoB layer. We study the
influence of the spacer thickness and growth condition on the switching metrics
of tunnel junctions thermally annealed at 400$^\circ$C for the case of 1-4
\r{A} Ta spacers. Thick Ta spacer results in a large anisotropies indicative of
a better defined top FeCoB/MgO interface, but this is achieved at the
systematic expense of a stronger damping. For the best anisotropy-damping
compromise, junctions of diameter 22 nm can still be stable and spin-torque
switched. Coercivity and inhomogeneous linewidth broadening, likely arising
from roughness at the FeCoB/Ta interface, can be reduced if a sacrificial Mg
layer is inserted before the Ta spacer deposition.",1909.02741v1
2020-02-16,Electric-field control of spin-orbit torques in perpendicularly magnetized W/CoFeB/MgO films,"Controlling magnetism by electric fields offers a highly attractive
perspective for designing future generations of energy-efficient information
technologies. Here, we demonstrate that the magnitude of current-induced
spin-orbit torques in thin perpendicularly magnetized CoFeB films can be tuned
and even increased by electric field generated piezoelectric strain. Using
theoretical calculations, we uncover that the subtle interplay of spin-orbit
coupling, crystal symmetry, and orbital polarization is at the core of the
observed strain dependence of spin-orbit torques. Our results open a path to
integrating two energy efficient spin manipulation approaches, the electric
field-induced strain and the current-induced magnetization switching, thereby
enabling novel device concepts.",2002.06578v1
2020-10-31,Effect of spin glass frustration on exchange bias in NiMn/CoFeB bilayers,"Exchange bias in ferromagnetic/antiferromagnetic systems can be explained in
terms of various interfacial phenomena. Among these spin glass frustration can
affect the magnetic properties in exchange bias systems. Here we have studied a
NiMn/CoFeB exchange bias system in which spin glass frustration seems to play a
crucial role. In order to account the effect of spin glass frustration on
magnetic properties, we have performed the temperature and cooling field
dependence of exchange bias. We have observed the decrease of exchange bias
field (HEB) with cooling field (HFC) whereas there is not significant effect on
coercive field (HC). Exponential decay of HEB and HC is found in these exchange
bias systems. Further, training effect measurements have been performed to
study the spin relaxation mechanism. We have fitted the training effect data
with frozen and rotatable spin relaxation model. We have investigated the ratio
of relaxation rate of interfacial rotatable and frozen spins in this study. The
training effect data are also fitted with various other models. Further, we
observed the shifting of peak temperature towards higher temperature with
frequency from the ac susceptibility data.",2011.00188v1
2021-01-18,Magnetization switching induced by spin-orbit torque from Co2MnGa magnetic Weyl semimetal thin films,"This study reports the magnetization switching induced by spin-orbit torque
(SOT) from the spin current generated in Co2MnGa magnetic Weyl semimetal (WSM)
thin films. We deposited epitaxial Co2MnGa thin films with highly B2-ordered
structure on MgO(001) substrates. The SOT was characterized by harmonic Hall
measurements in a Co2MnGa/Ti/CoFeB heterostructure and a relatively large spin
Hall efficiency of -7.8% was obtained.The SOT-induced magnetization switching
of the perpendicularly magnetized CoFeB layer was further demonstrated using
the structure. The symmetry of second harmonic signals, thickness dependence of
spin Hall efficiency, and shift of anomalous Hall loops under applied currents
were also investigated. This study not only contributes to the understanding of
the mechanisms of spin-current generation from magnetic-WSM-based
heterostructures, but also paves a way for the applications of magnetic WSMs in
spintronic devices.",2101.06881v1
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
2021-05-06,Interfacial and bulk spin Hall contributions to field-like spin-orbit torque generated by Iridium,"We present measurements of spin orbit torques generated by Ir as a function
of film thickness in sputtered Ir/CoFeB and Ir/Co samples. We find that Ir
provides a damping-like component of spin orbit torque with a maximum spin
torque conductivity 1.4e5 in SI unit and a maximum spin-torque efficiency of
0.04, which is sufficient to drive switching in an 0.8 nm film of CoFeB with
perpendicular magnetic anisotropy. We also observe a surprisingly large field
like spin orbit torque. Measurements as a function of Ir thickness indicate a
substantial contribution to the FLT from an interface mechanism so that in the
ultrathin limit there is a non-zero FLT with a maximum torque conductivity
-5.0E4 in the SI unit. When the Ir film thickness becomes comparable to or
greater than its spin diffusion length, 1.6 nm, there is also a smaller bulk
contribution to the fieldlike torque.",2105.02787v1
2021-06-04,Driving skyrmions with low threshold current density in Pt/CoFeB thin film,"Magnetic skyrmions are topologically stable spin swirling particle like
entities which are appealing for next generation spintronic devices. The
expected low critical current density for the motion of skyrmions makes them
potential candidates for future energy efficient electronic devices. Several
heavy metal/ferromagnetic (HM/FM) systems have been explored in the past decade
to achieve faster skyrmion velocity at low current densities. In this context,
we have studied Pt/CoFeB/MgO heterostructures in which skyrmions have been
stabilized at room temperature (RT). It has been observed that the shape of the
skyrmions are perturbed even by the small stray field arising from low moment
magnetic tips while performing the magnetic force microscopy (MFM), indicating
presence of low pinning landscape in the samples. This hypothesis is indeed
confirmed by the low threshold current density to drive the skyrmions in our
sample, at velocities of few 10m/s.",2106.02407v3
2021-08-16,Highly fcc-textured Pt-Al alloy films grown on MgO(001) showing enhanced spin Hall efficiency,"We report on a systematic comparative study of the spin Hall efficiency
between highly face-centered cubic (fcc)-textured Pt-Al alloy films grown on
MgO(001) and poorly-crystallized Pt-Al alloy films grown on SiO$_2$. Using
CoFeB as the detector, we show that for Al compositions centering around $x =
25$, mainly L1$_{2}$ ordered Pt$_{100-x}$Al$_x$ alloy films grown on MgO
exhibit outstanding charge-spin conversion efficiency. For
Pt$_{78}$Al$_{22}$/CoFeB bilayer on MgO, we obtain damping-like spin Hall
efficiency as high as $\xi_\textrm{DL} \sim +0.20$ and expect up to seven-fold
reduction of power consumption compared to the polycrystalline bilayer of the
same Al composition on SiO$_2$. This work demonstrates that improving the
crystallinity of fcc Pt-based alloys is a crucial step for achieving large spin
Hall efficiency and low power consumption in this material class.",2108.06927v1
2021-08-23,Reconfigurable 3D magnonic crystal: tunable and localized spin-wave excitations in CoFeB meander-shaped film,"In this work, we study experimentally by broadband ferromagnetic resonance
measurements, the dependence of the spin-wave excitation spectra on the
magnetic applied field in CoFeB meander-shaped films. Two different
orientations of the external magnetic field were explored, namely parallel or
perpendicular to the lattice cores. The interpretation of the field dependence
of the frequency and spatial profiles of major spin-wave modes were obtained by
micromagnetic simulations. We show that the vertical segments lead to the
easy-axis type of magnetic anisotropy and support the in-phase and out-of-phase
spin-wave precession amplitude in the vertical segments. The latter could
potentially be used for the design of tunable metasurfaces or in magnetic
memories based on meandering 3D magnetic films.",2108.10232v1
2021-11-16,Energy-efficient W$_{\text{100-x}}$Ta$_{\text{x}}$/CoFeB/MgO spin Hall nano-oscillators,"We investigate a W-Ta alloying route to reduce the auto-oscillation current
densities and the power consumption of nano-constriction based spin Hall nano
oscillators. Using spin-torque ferromagnetic resonance (ST-FMR) measurements on
microbars of W$_{\text{100-x}}$Ta$_{\text{x}}$(5 nm)/CoFeB(t)/MgO stacks with t
= 1.4, 1.8, and 2.0 nm, we measure a substantial improvement in both the
spin-orbit torque efficiency and the spin Hall conductivity. We demonstrate a
34\% reduction in threshold auto-oscillation current density, which translates
into a 64\% reduction in power consumption as compared to pure W-based SHNOs.
Our work demonstrates the promising aspects of W-Ta alloying for the
energy-efficient operation of emerging spintronic devices.",2111.08627v1
2021-12-23,Electric-field-induced parametric excitation of exchange magnons in a CoFeB/MgO junction,"Electric-field controlled magnetization dynamics is an important integrant in
low power spintronic devices. In this letter, we demonstrate electric-field
induced parametric excitation for CoFeB/MgO junctions by using interfacial
in-plane magnetic anisotropy. When the in-plane magnetic anisotropy and the
external magnetic field are parallel to each other, magnons are efficiently
excited by using electric-field induced parametric excitation. Its wavelength
and wavenumber are tuned by changing input power and frequency of the applied
voltage. A generalized phenomenological model is developed to explain the
underlying role of the electric-field torque. Electrical excitation with no
Joule heating offers a good opportunity for developing magnonic devices and
exploring various nonlinear dynamics in magnetic systems.",2112.12308v2
2022-03-17,Symmetry effects on the static and dynamic properties of coupled magnetic oscillators,"The effect of symmetry on the resonance spectra of antiferromagnetically
coupled oscillators has attracted new interest with the discovery of
symmetry-breaking induced anti-crossings. Here, we experimentally characterise
the resonance spectrum of a synthetic antiferromagnet Pt/CoFeB/Ru/CoFeB/Pt,
where we are able to independently tune the effective magnetisation of the two
coupled magnets. To model our results we apply the mathematical methods of
group theory to the solutions of the Landau Lifshitz Gilbert equation. This
general approach, usually applied to quantum mechanical systems, allows us to
identify the main features of the resonance spectrum in terms of symmetry
breaking and to make a direct comparison with crystal antiferromagnets.",2204.01490v1
2022-04-25,Ab-initio study of magneto-ionic mechanisms in ferromagnet/oxide multilayers,"The application of gate voltages in heavy metal/ferromagnet/Oxide multilayer
stacks has been identified as one possible candidate to manipulate their
anisotropy at will. However, this method has proven to show a wide variety of
behaviours in terms of reversibility, depending on the nature of the
metal/oxide interface and its degree of oxidation. In order to shed light on
the microscopic mechanism governing the complex magneto-ionic behaviour in
$\text{Ta/CoFeB/}\text{HfO}_2$, we perform ab-initio simulations on various
setups comprising $\text{Fe/O, Fe/HfO}_2$ interfaces with different oxygen atom
interfacial geometries. After the determination of the more stable interfacial
configurations, we calculate the magnetic anisotropy energy on the different
unit cell configurations and formulate a possible mechanism that well describes
the recent experimental observations in $\text{Ta/CoFeB/}\text{HfO}_2$.",2204.11699v1
2022-06-21,Extrinsic tunnel Hall effect in CoFeB/MgO/Pt junctions,"The Hall effect that occurs when current flows through a CoFeB/MgO/Pt tunnel
junction is investigated. It is shown that the transverse voltage in Pt
electrode is nonlinear on a DC voltage applied to the tunnel junction. It has
both linear (odd) and quadratic (even) parts. The linear part contains
well-known contributions of the anomalous Hall effect in the ferromagnetic
electrode, inverse spin-hall effect in platinum and others. The quadratic part
is a phenomenon caused by the spin-orbit scattering of electrons in an external
electric field induced by a voltage applied to the barrier. This field reaches
values of $10^9$ V/m which is close to internal atomic fields. The magnitude of
both effects decreases as thickness of Pt electrode is increased due to
shunting effects.",2206.10264v1
2023-06-08,Unidirectionality of spin waves in Synthetic Antiferromagnets,"We study the frequency non-reciprocity of the spin waves in symmetric
CoFeB/Ru/CoFeB synthetic antiferromagnets stacks set in the scissors state by
in-plane applied fields. Using a combination of Brillouin Light Scattering and
propagating spin wave spectroscopy experiments, we show that the acoustical
spin waves in synthetic antiferromagnets possess a unique feature if their
wavevector is parallel to the applied field: the frequency non-reciprocity due
to layer-to-layer dipolar interactions can be so large that the acoustical spin
waves transfer energy in a unidirectional manner for a wide and bipolar
interval of wavevectors. Analytical modeling and full micromagnetic
calculations are conducted to account for the dispersion relations of the
optical and acoustical spin waves for arbitrary field orientations. Our
formalism provides a simple and direct method to understand and design devices
harnessing unidirectional propagation of spin waves in synthetic
antiferromagnets.",2306.05259v2
2024-03-13,Theoretical limits of magnetic detection of structural surface defects at the nanometer scale,"We present a theoretical study on the magnetic signals of structural surface
defects like cracks or indents combined with rough surfaces or subsurface
inclusions of soft ferromagnetic metals like body-centered cubic Fe or
amorphous CoFeB. We discuss limits of early detection of small surface defects
on the basis of calculated magnetic stray fields few tens of nm above the
surface. The considered surface imperfections have extensions of a few nm which
correspond to low multiples of the magnetic exchange lengths of Fe or CoFeB.
The detection of such small inhomogeneities requires that the sensor is about
as close to the surface as the size of the inhomogeneity is. Furthermore, the
step width of a scanning sensor must be of the same size as well. Both these
requirements may be fulfilled for instance by scanning microscopy with diamond
nitrogen-vacancy-center quantum sensors.",2403.08412v1
2005-10-18,"Domain imaging, MOKE and magnetoresistance studies of CoFeB films for MRAM applications","We present a detailed study on domain imaging, Kerr effect magnetometry
(MOKE) and magnetoresistance (MR), for a series of 20 nm
Co$_{73.8}$Fe$_{16.2}$B$_{10}$ thin films, both as-deposited (amorphous) and
annealed (crystalline). By considering the two different (orthogonal) in-plane
magnetization components, obtained by MOKE measurements, we were able to study
the uniaxial anisotropy induced during CoFeB-deposition and to discriminate the
magnetization processes under a magnetic field parallel and perpendicular to
such axis. MOKE magnetic imaging enabled us to observe the dominant
magnetization processes, namely domain wall motion and moment rotation. These
processes were correlated with the behavior of the magnetoresistance, which
depends both on short-range spin disorder electron scattering and on the angle
between the electrical current and the spontaneous magnetization
($\emph{\textbf{M}}_{S}$). A simple numerical treatment based on
Stoner-Wolfarth model enables us to satisfactorily predict the magnetization
behaviour observed in these films. A comparison between the results in
Co$_{73.8}$Fe$_{16.2}$B$_{10}$ films and the previous ones obtained in annealed
Co$_{80}$Fe$_{20}$ films, show that the introduction of boron in CoFe reduces
significatively the coercive and saturation fields along the easy axis (e.g.
$H_{c}$ from $\sim$ 2 down to $\sim$ 0.5 kAm$^{-1}$). Also, the magnetization
along the hard axis saturates at lower fields. We conclude that amorphous and
nanocrystalline CoFeB films show low coercive fields and abrupt switching, as
well as absence of short range spin disorder effects after switching when
compared with Co$_{80}$Fe$_{20}$.",0510479v2
2014-05-05,On/off switching of bit readout in bias-enhanced tunnel magneto-Seebeck effect,"Thermoelectric effects in magnetic tunnel junctions are currently an
attractive research topic. Here, we demonstrate that the tunnel magneto-Seebeck
effect (TMS) in CoFeB/MgO/CoFeB tunnel junctions can be switched on to a logic
1 state and off to 0 by simply changing the magnetic state of the CoFeB
electrodes. We enable this new functionality of magnetic tunnel junctions by
combining a thermal gradient and an electric field. This new technique unveils
the bias-enhanced tunnel magneto-Seebeck effect, which can serve as the basis
for logic devices or memories in a green information technology with a pure
thermal write and read process. Furthermore, the thermally generated voltages
that are referred to as the Seebeck effect are well known to sensitively depend
on the electronic structure and therefore have been valued in solid-state
physics for nearly one hundred years. Here, we lift Seebeck's historic
discovery from 1821 to a new level of current spintronics. Our results show
that the signal crosses zero and can be adjusted by tuning a bias voltage that
is applied between the electrodes of the junction; hence, the name of the
effect is bias-enhanced tunnel magneto-Seebeck effect (bTMS). Via the spin- and
energy-dependent transmission of electrons in the junction, the bTMS effect can
be configured using the bias voltage with much higher control than the tunnel
magnetoresistance (TMR) and even completely suppressed for only one magnetic
configuration, which is either parallel (P) or anti-parallel (AP). This option
allows a readout contrast for the magnetic information of -3000% at room
temperature while maintaining a large signal for one magnetic orientation. This
contrast is much larger than the value that can be obtained using the TMR
effect. Moreover, our measurements are a step towards the experimental
realization of high TMS ratios, which are predicted for specific Co-Fe
compositions.",1405.1064v1
2020-05-08,Nanoscale Spin Injector Driven by a Microwave Voltage,"We propose an electrically driven spin injector into normal metals and
semiconductors, which is based on a magnetic tunnel junction (MTJ) subjected to
a microwave voltage. Efficient functioning of such an injector is provided by
electrically induced magnetization precession in the ""free"" layer of MTJ, which
generates the spin pumping into a metallic or semiconducting overlayer. We
theoretically describe the spin and charge dynamics in the
CoFeB/MgO/CoFeB/Au(GaAs) heterostructures. First, the magnedynamics in the free
CoFeB layer is quantified with the account of a spin-transfer torque and a
voltage-controlled magnetic anisotropy. By numerically solving the
magnetodynamics equation, we determine dependences of the precession amplitude
on the frequency $f$ and magnitude $V_\mathrm{max}$ of the ac voltage applied
to the MTJ. It is found that the frequency dependence changes drastically above
the threshold amplitude $V_\mathrm{max} \approx 200$mV, exhibiting a break at
the resonance frequency $f_\mathrm{res}$ due to nonlinear effects. The results
obtained for the magnetization dynamics are used to describe the spin injection
and pumping into the Au and GaAs overlayers. Since the generated spin current
creates additional charge current owing to the inverse spin Hall effect, we
also calculate distribution of the electric potential in the thick Au
overlayer. The calculations show that the arising transverse voltage becomes
experimentally measurable at $f = f_\mathrm{res}$. Finally, we evaluate the
spin accumulation in a long n$^+$-GaAs bar coupled to the MTJ and determine its
temporal variation and spatial distribution along the bar. It is found that the
spin accumulation under resonant excitation is large enough for experimental
detection even at micrometer distances from the MTJ. This result demonstrates
high efficiency of the described nanoscale spin injector.",2005.03896v1
2019-12-08,Thermal Conductivity of Oxide Tunnel Barriers in Magnetic Tunnel Junctions Measured by Ultrafast Thermoreflectance and Magneto-optic Kerr Effect Thermometry,"Spin-dependent charge transport in magnetic tunnel junctions (MTJs) can be
manipulated by a temperature gradient, which can be utilized for spintronic and
spin caloritronic applications. Evaluation of the thermally induced phenomena
requires knowledge of the temperature differences across the oxide tunnel
barrier adjacent to the ferromagnetic (FM) leads. However, it is challenging to
accurately measure thermal properties of an oxide tunnel barrier consisting of
only a few atomic layers. In this work, we experimentally interrogate the
temperature evolutions in Ru/oxide/FM/seed/MgO (oxide=MgO, MgAl2O4; FM=Co,
CoFeB; seed=Pt, Ta) structures having perpendicular magnetic anisotropy using
ultrafast thermometry. The Ru layer is optically thick and heated by ultrafast
laser pulses; the subsequent temperature changes are monitored using
thermoreflectance of Ru and magneto-optic Kerr effect (MOKE) of the FM layers.
We independently measure the response times of Co and CoFeB magnetism using
quadratic MOKE and obtain {\tau}em=0.2 ps for Co and 2 ps for CoFeB. These time
scales are much shorter than the time scale of heat transport through the oxide
tunnel barrier, which occurs at 10-3000 ps. We determine effective thermal
conductivities of MgO and MgAl2O4 tunnel barriers in the range of 0.4-0.6 W m-1
K-1, comparable to an estimate of the series conductance of the Ru/oxide and
oxide/FM interfaces and an order of magnitude smaller than the thermal
conductivity of MgO thin films. We find that the electron-phonon thermal
conductance near the tunnel barrier is only a factor of 5-12 larger than the
thermal conductance of the oxide tunnel barrier. Therefore, the drop in the
electronic temperature is approximately 20-30% larger than the drop in the
phonon temperature across the tunnel barrier.",1912.03588v1
2021-06-18,Large perpendicular magnetic anisotropy in Ta/CoFeB/MgO on full coverage monolayer MoS2 and first principle study of its electronic structure,"Perpendicularly magnetized spin injector with high Curie temperature is a
prerequisite for developing spin optoelectronic devices on 2D materials working
at room temperature (RT) with zero applied magnetic field. Here, we report the
growth of Ta/CoFeB/MgO structures with a large perpendicular magnetic
anisotropy (PMA) on full coverage monolayer (ML) MoS2. A large perpendicular
interface anisotropy energy of 0.975mJ/m2 has been obtained at the CoFeB/MgO
interface, comparable to that observed in magnetic tunnel junction systems. It
is found that the insertion of MgO between the ferromagnetic metal (FM) and the
2D material can effectively prevent the diffusion of the FM atoms into the 2D
material. Moreover, the MoS2 ML favors a MgO(001) texture and plays a critical
role to establish the large PMA. First principle calculations on a similar
Fe/MgO/MoS2 structure reveal that the MgO thickness can modify the MoS2 band
structure, from an indirect bandgap with 7ML-MgO to a direct bandgap with
3ML-MgO. Proximity effect induced by Fe results in a splitting of 10meV in the
valence band at the {\Gamma} point for the 3ML-MgO structure while it is
negligible for the 7ML-MgO structure. These results pave the way to develop RT
spin optoelectronic devices on 2D transition-metal dichalcogenide materials.",2106.10317v1
2022-10-20,Ultrafast behavior of induced and intrinsic magnetic moments in CoFeB/Pt bilayers probed by element-specific measurements in the extreme ultraviolet spectral range,"The ultrafast and element-specific response of magnetic systems containing
ferromagnetic 3d transition metals and 4d/5d heavy metals is of interest both
from a fundamental as well as an applied research perspective. However, to date
no consensus about the main microscopic processes describing the interplay
between intrinsic 3d and induced 4d/5d magnetic moments upon femtosecond laser
excitation exist. In this work, we study the ultrafast response of CoFeB/Pt
bilayers by probing element-specific, core-to-valence band transitions in the
extreme ultraviolet spectral range using high harmonic radiation. We show that
the combination of magnetic scattering simulations and analysis of the energy-
and time-dependent magnetic asymmetries allows to accurately disentangle the
element-specific response in spite of overlapping Co and Fe M$_{2,3}$ as well
as Pt O$_{2,3}$ and N$_7$ resonances. We find a considerably smaller
demagnetization time constant as well as much larger demagnetization amplitudes
of the induced moment of Pt compared to the intrinsic moment of CoFeB. Our
results are in agreement with enhanced spin-flip probabilities due to the high
spin-orbit coupling localized at the heavy metal Pt, as well as with the
recently formulated hypothesis that a laser generated, incoherent magnon
population within the ferromagnetic film leads to an overproportional reduction
of the induced magnetic moment of Pt.",2210.11390v3
2009-08-17,The superferromagnetic state in the ensemble of oriented Stoner-Wohlfarth particles: a coercivity due to phase stability,"It is observed experimentally that the coercive field has an anomalous
angular dependence at temperatures above the blocking temperature in physically
nonpercolated granular films CoFeB-SiO$_{2}$ with anisotropic granules oriented
in the same direction. It is shown that the anomaly is determined by the
singularity of an angular dependence of the critical field causing the absolute
loss of phase stability of the superferromagnetic state of an ensemble of
interacting superparamagnetic granules.",0908.2286v1
2014-04-04,Electrical detection of microwave assisted magnetization reversal by spin pumping,"Microwave assisted magnetization reversal has been investigated in a bilayer
system of Pt/ferromagnet by detecting a change in the polarity of the spin
pumping signal. The reversal process is studied in two material systems,
Pt/CoFeB and Pt/NiFe, for different aspect ratios. The onset of the switching
behavior is indicated by a sharp transition in the spin pumping voltage. At a
threshold value of the external field, the switching process changes from
partial to full reversal with increasing microwave power. The proposed method
provides a simple way to detect microwave assisted magnetization reversal.",1404.1133v1
2014-04-04,Thermally assisted domain wall nucleation in perpendicular anisotropy trilayer nanowires,"We study thermally assisted domain wall generation in perpendicular magnetic
anisotropy CoFeB trilayer nanowires by the effect of Joule heating. The
anomalous Hall effect is utilized to detect magnetization reversal in order to
study the domain wall generation. We observe a statistical distribution in the
switching process which is consistent with the thermal activation process. Our
results show that the proposed method provides an efficient way for generating
domain walls in perpendicular magnetic nanowires at predefined locations.",1404.1135v1
2014-05-14,Generation of magnonic spin wave traps,"Spatially resolved measurements of the magnetization dynamics induced by an
intense laser pump-pulse reveal that the frequencies of resulting spin wave
modes depend strongly on the distance to the pump center. This can be
attributed to a laser generated temperature profile. On a CoFeB thin film
magnonic crystal, Damon-Eshbach modes are expected to propagate away from the
point of excitation. The experiments show that this propagation is frustrated
by the strong temperature gradient",1405.3470v1
2017-01-06,Imaging magnetic vortex configurations in ferromagnetic nanotubes,"We image the remnant magnetization configurations of CoFeB and permalloy
nanotubes (NTs) using x-ray magnetic circular dichroism photo-emission electron
microscopy. The images provide direct evidence for flux-closure configurations,
including a global vortex state, in which magnetization points
circumferentially around the NT axis. Furthermore, micromagnetic simulations
predict and measurements confirm that vortex states can be programmed as the
equilibrium remnant magnetization configurations by reducing the NT aspect
ratio.",1701.01685v1
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
2019-12-24,Large spin Hall angle and spin mixing conductance in highly resistive antiferromagnetic Mn2Au,"Antiferromagnetic (AFM) materials recently have shown interest in the
research in spintronics due to its zero stray magnetic field, high anisotropy,
and spin orbit coupling. In this context, the bi-metallic AFM Mn2Au has drawn
attention because it exhibits unique properties and its Neel temperature is
very high. Here, we report spin pumping and inverse spin Hall effect
investigations in Mn2Au and CoFeB bilayer system using ferromagnetic resonance.
We found large spin Hall angle {\theta}_SH = 0.22",1912.11522v2
2008-09-20,Anomalous low-temperature magnetoelastic properties of nanogranular (CoFeB)$_{x}$-(SiO$_{2}$)$_{1-x}$,"We report magnetostatic measurements for granulated films
(CoFeB)$_{x}$-(SiO$_{2}$)$_{1-x}$ with fabrication induced intraplanar
anisotropy. The measurements have been performed in the film plane in the wide
temperature interval 4.5$\div$300 K. They demonstrate that above films have
low-temperature anomaly below the percolation threshold for conductivity. The
essence of the above peculiarity is that below 100 K the temperature dependence
of coercive field for magnetization along easy direction deviates strongly from
Neel-Brown law. At temperature lowering, the sharp increase of coercivity is
observed, accompanied by the appearance of coercive field for magnetization
along hard direction in the film plane. We establish that observed effect is
related to the properties of individual ferromagnetic granules. The effect
weakens as granules merge into conglomerates at $x$ higher then percolation
threshold and disappears completely at $x>1$. We explain the above effect as a
consequence of the difference in thermal expansion coefficients of granule and
cover material. At temperature lowering this difference weakens the envelopment
of an individual granule by the cover matrix material, thus permitting to
realize the spontaneous magnetostriction of a granule. The latter induces an
additional anisotropy with new easy axis of a granule magnetization along the
external magnetic field direction. Our explanation is tested and corroborated
by the ferromagnetic resonance measurements in the films at $T$ = 300 K and $T$
= 77 K.",0809.3499v1
2013-09-27,Structural and magnetic properties of Cr-diluted CoFeB,"The crystallization process and the magnetization of Cr diluted CoFeB was
investigated in both ribbon samples and thin film samples with Cr content up to
30 at. %. A primary crystallization of bcc phase from an amorphous precursor in
ribbon samples was observed when the annealing temperature rose to between 421
oC and 456 oC, followed by boron segregation at temperatures between 518 oC and
573 oC. The two onset crystallization temperatures showed strong dependences on
both Cr and B concentrations. The impact of Cr concentration on the magnetic
properties including a reduced saturation magnetization and an enhanced
coercive field was also observed. The magnetizations of both ribbon samples and
thin film samples were well fitted using the generalized Slater-Pauling curve
with modified moments for B (-0.94 {\mu}B) and Cr (-3.6 {\mu}B). Possible
origins of the enhanced coercive field were also discussed. We also achieved a
damping parameter in CoFeCrB thin films at the same level as Co40Fe40B20, much
lower than the value reported for CoFeCrB films previously. The results suggest
a possible advantage of CoFeCrB in reducing the critical switching current
density in Spin Transfer Torque Random Access Memory (STT-RAM).",1309.7331v1
2014-01-15,Interface control of the magnetic chirality in CoFeB|MgO heterosctructures with heavy metal underlayers,"Recent advances in the understanding of spin orbital effects in ultrathin
magnetic heterostructures have opened new paradigms to control magnetic moments
electrically. The Dzyaloshinskii-Moriya interaction (DMI) is said to play a key
role in forming a Neel-type domain wall that can be driven by the spin Hall
torque, a torque resulting from the spin current generated in a neighboring
non-magnetic layer via the spin Hall effect. Here we show that the strength and
sign of the DMI can be changed by modifying the adjacent heavy metal underlayer
(X) in perpendicularly magnetized X|CoFeB|MgO heterstructures. Albeit the same
spin Hall angle, a domain wall moves along or against the electron flow
depending on the underlayer. We find that the sense of rotation of a domain
wall spiral11 is reversed when the underlayer is changed from Hf to W and the
strength of DMI varies as the number of 5d electrons of the heavy metal layer
changes. The DMI can even be tuned by adding nitrogen to the underlayer, thus
allowing interface engineering of the magnetic texture in ultrathin magnetic
heterostructures.",1401.3568v1
2014-02-05,Fieldlike and antidamping spin-orbit torques in as-grown and annealed Ta/CoFeB/MgO layers,"We present a comprehensive study of the current-induced spin-orbit torques in
perpendicularly magnetized Ta/CoFeB/MgO layers. The samples were annealed in
steps up to 300 degrees C and characterized using x-ray absorption
spectroscopy, transmission electron microscopy, resistivity, and Hall effect
measurements. By performing adiabatic harmonic Hall voltage measurements, we
show that the transverse (field-like) and longitudinal (antidamping-like)
spin-orbit torques are composed of constant and magnetization-dependent
contributions, both of which vary strongly with annealing. Such variations
correlate with changes of the saturation magnetization and magnetic anisotropy
and are assigned to chemical and structural modifications of the layers. The
relative variation of the constant and anisotropic torque terms as a function
of annealing temperature is opposite for the field-like and antidamping
torques. Measurements of the switching probability using sub-{\mu}s current
pulses show that the critical current increases with the magnetic anisotropy of
the layers, whereas the switching efficiency, measured as the ratio of magnetic
anisotropy energy and pulse energy, decreases. The optimal annealing
temperature to achieve maximum magnetic anisotropy, saturation magnetization,
and switching efficiency is determined to be between 240 degrees and 270
degrees C.",1402.0986v2
2014-02-26,Anomalous temperature dependence of current induced torques in CoFeB|MgO heterostructures with Ta based underlayers,"We have studied the underlayer thickness and temperature dependences of the
current induced effective field in CoFeB|MgO heterostructures with Ta based
underlayers. The underlayer thickness at which the effective field saturates is
found to be different between the two orthogonal components of the effective
field, i.e. the damping-like term tends to saturate at smaller underlayer
thickness than the field-like term. For large underlayer thickness films in
which the effective field saturates, we find that the temperature significantly
influences the size of the effective field. A striking difference is found in
the temperature dependence of the two components: the damping-like term
decreases whereas the field-like term increases with increasing temperature.
Using a simple spin diffusion-spin transfer model, we find that all of these
results can be accounted for provided the real and imaginary parts of an
effective spin mixing conductance are negative. These results imply that either
spin transport in this system is different from conventional metallic
interfaces or effects other spin diffusion into the magnetic layer need to be
taken account in order to model the system accurately.",1402.6388v1
2014-10-28,Current driven asymmetric magnetization switching in perpendicularly magnetized CoFeB/MgO heterostructures,"The flow of in-plane current through ultrathin magnetic heterostructures can
cause magnetization switching or domain wall nucleation owing to bulk and
interfacial effects. Within the magnetic layer, the current can create magnetic
instabilities via spin transfer torques (STT). At interface(s), spin current
generated from the spin Hall effect in a neighboring layer can exert torques,
referred to as the spin Hall torques, on the magnetic moments. Here, we study
current induced magnetization switching in perpendicularly magnetized CoFeB/MgO
heterostructures with a heavy metal (HM) underlayer. Depending on the thickness
of the HM underlayer, we find distinct differences in the inplane field
dependence of the threshold switching current. The STT is likely responsible
for the magnetization reversal for the thinner underlayer films whereas the
spin Hall torques cause the switching for thicker underlayer films. For the
latter, we find differences in the switching current for positive and negative
currents and initial magnetization directions. We find that the growth process
during the film deposition introduces an anisotropy that breaks the symmetry of
the system and causes the asymmetric switching. The presence of such symmetry
breaking anisotropy enables deterministic magnetization switching at zero
external fields.",1410.7473v2
2015-06-12,Origin of interfacial perpendicular magnetic anisotropy in MgO/CoFe/metallic capping layer structures,"Spin-transfer-torque magnetic random access memory (STT-MRAM) attracts
extensive attentions due to its non-volatility, high density and low power
consumption. The core device in STT-MRAM is CoFeB/MgO-based magnetic tunnel
junction (MTJ), which possesses a high tunnel magnetoresistance ratio as well
as a large value of perpendicular magnetic anisotropy (PMA). It has been
experimentally proven that a capping layer coating on CoFeB layer is essential
to obtain a strong PMA. However, the physical mechanism of such effect remains
unclear. In this paper, we investigate the origin of the PMA in
MgO/CoFe/metallic capping layer structures by using a first-principles
computation scheme. The trend of PMA variation with different capping materials
agrees well with experimental results. We find that interfacial PMA in the
three-layer structures comes from both the MgO/CoFe and CoFe/capping layer
interfaces, which can be analyzed separately. Furthermore, the PMAs in the
CoFe/capping layer interfaces are analyzed through resolving the magnetic
anisotropy energy by layer and orbital. The variation of PMA with different
capping materials is attributed to the different hybridizations of both d and p
orbitals via spin-orbital coupling. This work can significantly benefit the
research and development of nanoscale STT-MRAM.",1506.04078v4
2016-05-07,Giant interfacial perpendicular magnetic anisotropy in MgO/CoFe/capping layer structures,"Magnetic tunnel junction (MTJ) based on CoFeB/MgO/CoFeB structures is of
great interest due to its application in the spin-transfer-torque magnetic
random access memory (STT-MRAM). Large interfacial perpendicular magnetic
anisotropy (PMA) is required to achieve high thermal stability. Here we use
first-principles calculations to investigate the magnetic anisotropy energy
(MAE) of MgO/CoFe/capping layer structures, where the capping materials include
5d metals Hf, Ta, Re, Os, Ir, Pt, Au and 6p metals Tl, Pb, Bi. We demonstrate
that it is feasible to enhance PMA by using proper capping materials.
Relatively large PMA is found in the structures with capping materials of Hf,
Ta, Os, Ir and Pb. More importantly, the MgO/CoFe/Bi structure gives rise to
giant PMA (6.09 mJ/m2), which is about three times larger than that of the
MgO/CoFe/Ta structure. The origin of the MAE is elucidated by examining the
contributions to MAE from each atomic layer and orbital. These findings provide
a comprehensive understanding of the PMA and point towards the possibility to
achieve advanced-node STT-MRAM with high thermal stability.",1605.02247v5
2017-08-14,Current-induced magnetization switching in atom-thick tungsten engineered perpendicular magnetic tunnel junctions with large tunnel magnetoresistance,"Perpendicular magnetic tunnel junctions based on MgO/CoFeB structures are of
particular interest for magnetic random-access memories because of their
excellent thermal stability, scaling potential, and power dissipation. However,
the major challenge of current-induced switching in the nanopillars with both a
large tunnel magnetoresistance ratio and a low junction resistance is still to
be met. Here, we report spin transfer torque switching in nano-scale
perpendicular magnetic tunnel junctions with a magnetoresistance ratio up to
249% and a resistance area product as low as 7.0 {\Omega}.{\mu}m2, which
consists of atom-thick W layers and double MgO/CoFeB interfaces. The efficient
resonant tunnelling transmission induced by the atom-thick W layers could
contribute to the larger magnetoresistance ratio than conventional structures
with Ta layers, in addition to the robustness of W layers against high
temperature diffusion during annealing. The switching critical current density
could be lower than 3.0 MA.cm-2 for devices with a 45 nm radius.",1708.04111v3
2018-01-11,Efficient Charge-Spin Conversion and Magnetization Switching though Rashba Effect at Topological Insulator/Ag Interface,"We report the observation of efficient charge-to-spin conversion in the
three-dimensional topological insulator (TI) Bi2Se3 and Ag bilayer by the
spin-torque ferromagnetic resonance technique. The spin orbit torque ratio in
the Bi2Se3/Ag/CoFeB heterostructure shows a significant enhancement as the Ag
thickness increases to ~2 nm and reaches a value of 0.5 for 5 nm Ag, which is
~3 times higher than that of Bi2Se3/CoFeB at room temperature. The observation
reveals the interfacial effect of Bi2Se3/Ag exceeds that of the topological
surface states (TSS) in the Bi2Se3 layer and plays a dominant role in the
charge-to-spin conversion in the Bi2Se3/Ag/CoFeB system. Based on the
first-principles calculations, we attribute our observation to the large
Rashba-splitting bands which wrap the TSS band and has the same net spin
polarization direction as TSS of Bi2Se3. Subsequently, we demonstrate for the
first time the Rashba induced magnetization switching in Bi2Se3/Ag/Py with a
low current density of 5.8 X 10^5 A/cm2.",1801.03689v1
2020-01-13,Large and Robust Charge-to-Spin Conversion in Sputtered Conductive WTex with Disorder,"Topological materials with large spin-orbit coupling and immunity to
disorder-induced symmetry breaking show great promise for efficiently
converting charge to spin. Here, we report that long-range disordered sputtered
WTex thin films exhibit local chemical and structural order as those of Weyl
semimetal WTe2 and conduction behavior that is consistent with semi-metallic
Weyl fermion. We find large charge-to-spin conversion properties and electrical
conductivity in thermally annealed sputtered WTex films that are comparable
with those in crystalline WTe2 flakes. Besides, the strength of unidirectional
spin Hall magnetoresistance in annealed WTex/Mo/CoFeB heterostructure is 5 to
20 times larger than typical SOT layer/ferromagnet heterostructures reported at
room temperature. We further demonstrate room temperature damping-like
SOT-driven magnetization switching of in-plane magnetized CoFeB. These large
charge-to-spin conversion properties that are robust in the presence of
long-range disorder and thermal annealing pave the way for industrial
application of a new class of sputtered semimetals.",2001.04054v2
2021-07-16,Large Unidirectional Magnetoresistance in Metallic Heterostructures in the Spin Transfer Torque Regime,"A large unidirectional magnetoresistance (UMR) ratio of UMR/$R_{xx}\sim$
$0.36\%$ is found in W/CoFeB metallic bilayer heterostructures at room
temperature. Three different regimes in terms of the current dependence of UMR
ratio are identified: A spin-dependent-scattering mechanism regime at small
current densities $J \sim$ $10$$^{9}$A/m$^{2}$ (UMR ratio $\propto$ $J$), a
spin-magnon-interaction mechanism regime at intermediate $J \sim$
$10$$^{10}$A/m$^{2}$ (UMR ratio $\propto$ $J$$^{3}$), and a spin-transfer
torque (STT) regime at $J \sim$ $10$$^{11}$A/m$^{2}$ (UMR ratio independent of
$J$). We verify the direct correlation between this large UMR and the transfer
of spin angular momentum from the W layer to the CoFeB layer by both
field-dependent and current-dependent UMR characterizations. Numerical
simulations further confirm that the large STT-UMR stems from the tilting of
the magnetization affected by the spin Hall effect-induced spin-transfer
torques. An alternative approach to estimate damping-like spin-torque
efficiencies from magnetic heterostructures is also proposed.",2107.07780v1
2017-05-18,Magnetization dynamics and its scattering mechanism in thin CoFeB films with interfacial anisotropy,"Studies of magnetization dynamics have incessantly facilitated the discovery
of fundamentally novel physical phenomena, making steady headway in the
development of magnetic and spintronics devices. The dynamics can be induced
and detected electrically, offering new functionalities in advanced electronics
at the nanoscale. However, its scattering mechanism is still disputed.
Understanding the mechanism in thin films is especially important, because most
spintronics devices are made from stacks of multilayers with nanometer
thickness. The stacks are known to possess interfacial magnetic anisotropy, a
central property for applications, whose influence on the dynamics remains
unknown. Here, we investigate the impact of interfacial anisotropy by adopting
CoFeB/MgO as a model system. Through systematic and complementary measurements
of ferromagnetic resonance (FMR), on a series of thin films, we identify
narrower FMR linewidths at higher temperatures. We explicitly rule out the
temperature dependence of intrinsic damping as a possible cause, and it is also
not expected from existing extrinsic scattering mechanisms for ferromagnets. We
ascribe this observation to motional narrowing, an old concept so far neglected
in the analyses of FMR spectra. The effect is confirmed to originate from
interfacial anisotropy, impacting the practical technology of spin-based
nanodevices up to room temperature.",1705.06624v1
2019-01-26,Low spin-polarization in the heavy metal\ferromagnet structures detected through the domain wall motion by synchronized magnetic field and current,"CoFeB is a very soft material, in which Domain Wall (DW) can be moved easily
under a weak magnetic field. However, it is very difficult to move DWs in
Ta\CoFeB\MgO nanowires with interfacial perpendicular magnetic anisotropy
through a spin-polarized current, and this limits the perspectives of racetrack
memory driven by the current-in-plane mechanism. To investigate this
phenomenon, we performed experiments of DW velocity measurement by applying a
magnetic field and a current simultaneously. Working in the precessional
regime, we have been able to see a very important effect of the spin-polarized
current, which allows evaluating the polarization rate of the charge carriers.
An unexpected quite low spin polarization rate down to 0.26 have been obtained,
which can explain the low efficiency of DW motion induced by the spin-polarized
current. Possible reasons for this low rate are analyzed, such as the spin
relaxation in the Ta layer.",1901.09256v1
2018-12-06,Optimization of high performance spintronic terahertz sources,"To achieve high efficiency and good performance of spintronic terahertz
sources, we propose and corroborate a remnant magnetization method to radiate
continuously and stably terahertz pulses from W/CoFeB/Pt magnetic nanofilms
without carrying magnets on the transmitters driven by femtosecond laser
pulses. We systematically investigate the influences of the pumping central
wavelength and find out the optimal wavelength for a fixed sample thickness. We
also optimize the incidence angle of the pumping laser and find the emission
efficiency is enhanced under oblique incidence. Combing the aforementioned
optimizations, we finally obtain comparable radiation efficiency and broadband
spectra in W/CoFeB/Pt heterostructures compared with that from 1 mm thick ZnTe
nonlinear crystals via optical rectification under the same pumping conditions
of 100 fs pulse duration from a Ti:sapphire laser oscillator, which was not
previously demonstrated under such pulse duration. We believe our observations
not only benefit for a deep insight into the physics of femtosecond spin
dynamics, but also help develop novel and cost-effective ultrabroadband
spintronic terahertz emitters.",1812.02286v2
2020-08-04,Ultrathin perpendicular free layers for lowering the switching current in STT-MRAM,"The critical current density $J_{c0}$ required for switching the
magnetization of the free layer (FL) in a spin-transfer torque magnetic random
access memory (STT-MRAM) cell is proportional to the product of the damping
parameter, saturation magnetization and thickness of the free layer, $\alpha
M_S t_F$. Conventional FLs have the structure CoFeB/nonmagnetic spacer/CoFeB.
By reducing the spacer thickness, W in our case, and also splitting the single
W layer into two layers of sub-monolayer thickness, we have reduced $t_F$ while
minimizing $\alpha$ and maximizing $M_S$, ultimately leading to lower $J_{c0}$
while maintaining high thermal stability. Bottom-pinned MRAM cells with device
diameter in the range of 55-130 nm were fabricated, and $J_{c0}$ is lowest for
the thinnest (1.2 nm) FLs, down to 4 MA/cm$^2$ for 65 nm devices, $\sim$30%
lower than 1.7 nm FLs. The thermal stability factor $\Delta_{\mathrm{dw}}$, as
high as 150 for the smallest device size, was determined using a domain wall
reversal model from field switching probability measurements. With high
$\Delta_{\mathrm{dw}}$ and lowest $J_{c0}$, the thinnest FLs have the highest
spin-transfer torque efficiency.",2008.01343v1
2021-03-08,Anisotropic magnon-magnon coupling in synthetic antiferromagnets,"The magnon-magnon coupling in synthetic antiferromagnets advances it as
hybrid magnonic systems to explore the quantum information technologies. To
induce the magnon-magnon coupling, the parity symmetry between two
magnetization needs to be broken. Here we experimentally demonstrate a
convenient method to break the parity symmetry by the asymmetric thickness of
two magnetic layers and thus introduce a magnon-magnon coupling in Ir-based
synthetic antiferromagnets CoFeB(10 nm)/Ir(tIr=0.6 nm, 1.2 nm)/CoFeB(13 nm).
Remarkably, we find that the weakly uniaxial anisotropy field (~ 20 Oe) makes
the magnon-magnon coupling anisotropic. The coupling strength presented by a
characteristic anticrossing gap varies in the range between 0.54 GHz and 0.90
GHz for tIr =0.6 nm, and between nearly zero to 1.4 GHz for tIr = 1.2 nm,
respectively. Our results demonstrate a feasible way to induce the
magnon-magnon coupling by an asymmetric structure and tune the coupling
strength by varying the direction of in-plane magnetic field. The magnon-magnon
coupling in this highly tunable material system could open exciting
perspectives for exploring quantum-mechanical coupling phenomena.",2103.04512v2
2017-02-01,Continuous Tuning the Magnitude and Direction of Spin-Orbit Torque Using Bilayer Heavy Metals,"Spin-orbit torques (SOTs) have opened a new path to switch the magnetization
in perpendicularly magnetized films and are of great interest due to their
potential applications in novel data storage technology, such as the magnetic
random access memory (MRAM). The effective manipulation of SOT has thus become
an important step towards these applications. Here, current induced spin-orbit
effective fields and magnetization switching are investigated in
Pt/Ta/CoFeB/MgO structures with bilayer heavy metals. With a fixed thickness (1
nm) of the Ta layer, the magnitude and sign of current induced spin-orbit
effective fields can be continuously tuned by changing the Pt layer thickness,
consistent with the current induced magnetization switching data. The ratio of
longitudinal to transverse spin-orbit effective fields is found to be
determined by the Ta/CoFeB interface and can be continuously tuned by changing
the Pt layer thickness. The Dzyaloshinskii-Moriya interaction (DMI) is found to
be weak and shows an insignificant variation with the Pt thickness. The results
demonstrate an effective method to tune SOTs utilizing bilayer heavy metals
without affecting the DMI, a desirable feature which will be useful for the
design of SOT-based devices.",1702.00147v1
2017-06-19,Investigation of the Dzyaloshinskii-Moriya interaction and room temperature skyrmions in W/CoFeB/MgO thin films and microwires,"Recent studies have shown that material structures, which lack structural
inversion symmetry and have high spin-orbit coupling can exhibit chiral
magnetic textures and skyrmions which could be a key component for next
generation storage devices. The Dzyaloshinskii-Moriya Interaction (DMI) that
stabilizes skyrmions is an anti-symmetric exchange interaction favoring
non-collinear orientation of neighboring spins. It has been shown that material
systems with high DMI can lead to very efficient domain wall and skyrmion
motion by spin-orbit torques. To engineer such devices, it is important to
quantify the DMI for a given material system. Here we extract the DMI at the
Heavy Metal (HM) /Ferromagnet (FM) interface using two complementary
measurement schemes namely asymmetric domain wall motion and the magnetic
stripe annihilation. By using the two different measurement schemes, we find
for W(5 nm)/Co20Fe60B20(0.6 nm)/MgO(2 nm) the DMI to be 0.68 +/- 0.05 mJ/m2 and
0.73 +/- 0.5 mJ/m2, respectively. Furthermore, we show that this DMI stabilizes
skyrmions at room temperature and that there is a strong dependence of the DMI
on the relative composition of the CoFeB alloy. Finally we optimize the layers
and the interfaces using different growth conditions and demonstrate that a
higher deposition rate leads to a more uniform film with reduced pinning and
skyrmions that can be manipulated by Spin-Orbit Torques.",1706.05987v1
2018-04-30,Field-Free Switching of Perpendicular Magnetic Tunnel Junction via Voltage-Gated Spin Hall Effect for Low-Power Spintronic Memory,"Spin Hall effect (SHE) and voltage-controlled magnetic anisotropy (VCMA) are
two promising methods for low-power electrical manipulation of magnetization.
Recently, magnetic field-free switching of perpendicular magnetization through
SHE has been reported with the aid of an exchange bias from an
antiferromagnetic IrMn layer. In this letter, we experimentally demonstrate
that the IrMn/CoFeB/MgO structure exhibits a VCMA effect of 39 fJ/Vm, which is
comparable to that of the Ta/CoFeB/MgO structure. Magnetization dynamics under
a combination of the SHE and VCMA are modeled and simulated. It is found that,
by applying a voltage of 1.5 V, the critical SHE switching current can be
decreased by 10 times owing to the VCMA effect, leading to low-power
operations. Furthermore, a high-density spintronic memory structure can be
built with multiple magnetic tunnel junctions (MTJs) located on a single IrMn
strip. Through hybrid CMOS/MTJ simulations, we demonstrate that fast-speed
write operations can be achieved with power consumption of only 8.5 fJ/bit.
These findings reveal the possibility to realize high-density and low-power
spintronic memory manipulated by voltage-gated SHE.",1804.11025v1
2020-06-04,Defect-Correlated Skyrmions and Controllable Generation in Perpendicularly Magnetized CoFeB Ultrathin Films,"Skyrmions have attracted significant interest due to their topological spin
structures and fascinating physical features. The skyrmion phase arises in
materials with Dzyaloshinskii-Moriya (DM) interaction at interfaces or in
volume of non-centrosymmetric materials. However, although skyrmions were
generated experimentally, one critical intrinsic relationship between
fabrication, microstructures, magnetization and the existence of skyrmions
remains to be established. Here, two series of CoFeB ultrathin films with
controlled atomic scale structures are employed to reveal this relationship. By
inverting the growth order, the amount of defects can be artificially tuned,
and skyrmions are shown to be preferentially formed at defect sites. The stable
region and the density of the skyrmions can be efficiently controlled in the
return magnetization loops by utilizing first-order reversal curves to reach
various metastable states. These findings establish the general and intrinsic
relationship from sample preparation to skyrmion generation, offering an
universal method to control skyrmion density.",2006.02864v1
2020-07-27,Magnonic band structure in vertical meander-shaped CoFeB thin films,"The dispersion of spin waves in vertical meander-shaped CoFeB thin films
consisting of segments located at 90{\deg} angles with respect to each other is
investigated by Brillouin light scattering spectroscopy. We reveal the periodic
character of several dispersive branches as well as alternating frequency
ranges where spin waves are allowed or forbidden to propagate. Noteworthy is
the presence of the frequency band gaps between each couple of successive modes
only for wave numbers k=n$\pi$/a, where n is an even integer number and a is
the size of the meander unit cell, whereas the spectra show propagating modes
in the orthogonal film segments for the other wavenumbers. The micromagnetic
simulations and analytical calculations allow us to understand and explain the
results in terms of the mode spatial localization and symmetry. The obtained
results demonstrate the wave propagation in three dimensions opening the path
for multi-level magnonic architectures for signal processing.",2007.13707v2
2020-09-14,Memristive control of mutual SHNO synchronization for neuromorphic computing,"Synchronization of large spin Hall nano-oscillators (SHNO) arrays is an
appealing approach toward ultra-fast non-conventional computing based on
nanoscale coupled oscillator networks. However, for large arrays, interfacing
to the network, tuning its individual oscillators, their coupling, and
providing built-in memory units for training purposes, remain substantial
challenges. Here, we address all these challenges using memristive gating of
W/CoFeB/MgO/AlOx based SHNOs. In its high resistance state (HRS), the memristor
modulates the perpendicular magnetic anisotropy (PMA) at the CoFeB/MgO
interface purely by the applied electric field. In its low resistance state
(LRS), and depending on the voltage polarity, the memristor adds/subtracts
current to/from the SHNO drive. The operation in both the HRS and LRS affects
the SHNO auto-oscillation mode and frequency, which can be tuned up to 28
MHz/V. This tuning allows us to reversibly turn on/off mutual synchronization
in chains of four SHNOs. We also demonstrate two individually controlled
memristors to tailor both the coupling strength and the frequency of the
synchronized state. Memristor gating is therefore an efficient approach to
input, tune, and store the state of the SHNO array for any non-conventional
computing paradigm, all in one platform.",2009.06594v1
2021-02-15,Controlling Domain-Wall Nucleation in Ta/CoFeB/MgO Nanomagnets via Local Ga+ Ion Irradiation,"Comprehensive control of the domain wall nucleation process is crucial for
spin-based emerging technologies ranging from random-access and storage-class
memories over domain-wall logic concepts to nanomagnetic logic. In this work,
focused Ga+ ion-irradiation is investigated as an effective means to control
domain-wall nucleation in Ta/CoFeB/MgO nanostructures. We show that analogously
to He+ irradiation, it is not only possible to reduce the perpendicular
magnetic anisotropy but also to increase it significantly, enabling new,
bidirectional manipulation schemes. First, the irradiation effects are assessed
on film level, sketching an overview of the dose-dependent changes in the
magnetic energy landscape. Subsequent time-domain nucleation characteristics of
irradiated nanostructures reveal substantial increases in the anisotropy fields
but surprisingly small effects on the measured energy barriers, indicating
shrinking nucleation volumes. Spatial control of the domain wall nucleation
point is achieved by employing focused irradiation of pre-irradiated magnets,
with the diameter of the introduced circular defect controlling the coercivity.
Special attention is given to the nucleation mechanisms, changing from a
Stoner-Wohlfarth particle's coherent rotation to depinning from an anisotropy
gradient. Dynamic micromagnetic simulations and related measurements are used
in addition to model and analyze this depinning-dominated magnetization
reversal.",2102.07540v1
2021-06-12,Spin pumping and inverse spin Hall effect in CoFeB/C$_{60}$ bilayers,"Pure spin current based research is mostly focused on ferromagnet (FM)/heavy
metal (HM) system. Because of the high spin orbit coupling (SOC) these HMs
exhibit short spin diffusion length and therefore possess challenges for device
application. Low SOC (elements of light weight) and large spin diffusion length
make the organic semiconductors (OSCs) suitable for future spintronic
applications. From theoretical model it is explained that, due to $\pi$ -
$\sigma$ hybridization the curvature of the C$_{60}$ molecules may increase the
SOC strength. Here, we have investigated spin pumping and inverse spin hall
effect (ISHE) in CoFeB/C$_{60}$ bilayer system using coplanar wave guide based
ferromagnetic resonance (CPW-FMR) set-up. We have performed angle dependent
ISHE measurement to disentangle the spin rectification effects for example
anisotropic magnetoresistance, anomalous Hall effect etc. Further, effective
spin mixing conductance (g$_{eff}^{\uparrow\downarrow}$) and spin Hall angle
($\theta_{SH}$) for C$_{60}$ have been reported here. The evaluated value for
$\theta_{SH}$ is 0.055.",2106.06829v2
2022-06-29,Tailoring the switching efficiency of magnetic tunnel junctions by the fieldlike spin-orbit torque,"Current-induced spin-orbit torques provide a versatile tool for switching
magnetic devices. In perpendicular magnets, the dampinglike component of the
torque is the main driver of magnetization reversal. The degree to which the
fieldlike torque assists the switching is a matter of debate. Here we study the
switching of magnetic tunnel junctions with a CoFeB free layer and either W or
Ta underlayers, which have a ratio of fieldlike to dampinglike torque of 0.3
and 1, respectively. We show that the fieldlike torque can either assist or
hinder the switching of CoFeB when the static in-plane magnetic field required
to define the polarity of spin-orbit torque switching has a component
transverse to the current. In particular, the non-collinear alignment of the
field and current can be exploited to increase the switching efficiency and
reliability compared to the standard collinear alignment. By probing individual
switching events in real-time, we also show that the combination of transverse
magnetic field and fieldlike torque can accelerate or decelerate the reversal
onset. We validate our observations using micromagnetic simulations and
extrapolate the results to materials with different torque ratios. Finally, we
propose device geometries that leverage the fieldlike torque for density
increase in memory applications and synaptic weight generation.",2206.14587v2
2022-08-17,Magnetic domain scanning imaging using phase-sensitive THz-pulse detection,"In our study, we determine the alignment of magnetic domains in a CoFeB layer
using THz radiation. We generate THz-pulses by fs-laser-pulses in magnetized
CoFeB/Pt heterostructures, based on spin currents. An LT-GaAs Auston switch
detects the radiation phase-sensitively and allows to determine the
magnetization alignment. Our scanning technique with motorized stages with step
sizes in the sub-micrometer range, allows to image two dimensional magnetic
structures. Theoretically the resolution is restricted to half of the
wavelength if focusing optics in the far-field limit are used. By applying
near-field imaging, the spatial resolution is enhanced to the single digit
micrometer range. For this purpose, spintronic emitters in diverse geometric
shapes, e.g. circles, triangles, squares, and sizes are prepared to observe the
formation of magnetization patterns. The alignment of the emitted THz radiation
can be influenced by applying unidirectional external magnetic fields. We
demonstrate how magnetic domains with opposite alignment and different shapes
divided by domain walls are created by demagnetizing the patterns using minor
loops and imaged using phase sensitive THz radiation detection. For analysis,
the data is compared to Kerr microscope images. The possibility to combine this
method with THz range spectroscopic information of magnetic texture or
antiferromagnets in direct vicinity to the spintronic emitter, makes this
detection method interesting for much wider applications probing THz excitation
in spin systems with high resolution beyond the Abbe diffraction limit, limited
solely by the laser excitation area.",2208.08219v1
2022-08-19,Revealing nanoscale disorder in W/CoFeB/MgO ultra-thin films using domain wall motion,"Disorder in ultra-thin magnetic films can significantly hinder domain wall
motion. One of the main issues on the path towards efficient domain wall based
devices remains the characterization of the pinning landscape at the nanoscale.
In this paper, we study domain wall motion in W/CoFeB/MgO thin films with
perpendicular magnetic anisotropy crystallized by annealing at 400$^{\circ}$C
and a process based on He$^{+}$ irradiation combined with elevated
temperatures. Magnetic properties are similar for the whole series of samples,
while the magnetic domain wall mobility is critically improved in the
irradiated samples. By using an analytical model to extract nanoscale pinning
parameters, we reveal important variations in the disorder of the crystallized
samples. This work offers a unique opportunity to selectively analyze the
effects of disorder on the domain wall dynamics, without the contribution of
changes in the magnetic properties. Our results highlight the importance of
evaluating the nanoscale pinning parameters of the material when designing
devices based on domain wall motion, which in return can be a powerful tool to
probe the disorder in ultra-thin magnetic films.",2208.09280v1
2022-09-05,Nonlocal detection of interlayer three-magnon coupling,"A leading nonlinear effect in magnonics is the interaction that splits a
high-frequency magnon into two low-frequency ones with conserved linear
momentum. Here, we report experimental observation of nonlocal three-magnon
scattering between spatially separated magnetic systems, viz. a CoFeB nanowire
and an yttrium iron garnet (YIG) thin film. Above a certain threshold power of
an applied microwave field, a CoFeB Kittel magnon splits into a pair of
counter-propagating YIG magnons that induce voltage signals in Pt electrodes on
each side, in excellent agreement with model calculations based on the
interlayer dipolar interaction. The excited YIG magnon pairs reside mainly in
the first excited (n=1) perpdendicular standing spin-wave mode. With increasing
power, the n=1 magnons successively scatter into nodeless (n=0) magnons through
a four-magnon process. Our results help to assess non-local scattering
processes in magnonic circuits that may enable quantum entanglement between
distant magnons for quantum information applications.",2209.01875v1
2022-10-04,Strain Coupled Domains in BaTiO3(111)-CoFeB Heterostructures,"Domain pattern transfer from ferroelectric to ferromagnetic materials is a
critical step for the electric field control of magnetism and has the potential
to provide new schemes for low-power data storage and computing devices. Here
we investigate domain coupling in BaTiO$_3$(111)/CoFeB heterostructures by
direct imaging in a wide-field Kerr microscope. The magnetic easy axis is found
to locally change direction as a result of the underlying ferroelectric domains
and their polarisation. By plotting the remanent magnetisation as a function of
angle in the plane of the CoFeB layer, we find that the magnetic easy axes in
adjacent domains are angled at 60$^\circ$ or 120$^\circ$, corresponding to the
angle of rotation of the polarisation from one ferroelectric domain to the
next, and that the magnetic domain walls may be charged or uncharged depending
on the magnetic field history. Micromagnetic simulations show that the
properties of the domain walls vary depending on the magnetoelastic easy axis
configuration and the charged or uncharged nature of the wall. The
configuration where the easy axis alternates by 60$^\circ$ and a charged wall
is initialised exhibits the largest change in domain wall width from 192 nm to
119 nm as a function of in-plane magnetic field. Domain wall width tuning
provides an additional degree of freedom for devices that seek to manipulate
magnetic domain walls using strain coupling to ferroelectrics.",2210.01511v1
2023-03-10,Field-driven collapsing dynamics of skyrmions in magnetic multilayers,"Magnetic skyrmions are fascinating topological particle-like textures
promoted by a trade-off among interfacial properties (perpendicular anisotropy
and Dzyaloshinskii-Moriya interaction (DMI)) and dipolar interactions.
Depending on the dominant interaction, complex spin textures, including pure
N\'eel and hybrid skyrmions have been observed in multilayer heterostructures.
A quantification of these different spin textures typically requires a
depth-reoslved magnetic imaging or scattering techniques. In the present work,
we will show qualitatively different collapsing dynamics for pure N\'eel and
hybrid skyrmions induced by a perpendicular magnetic field in two
representative systems, [Pt/Co/Ir]15 and [Ta/CoFeB/MgO]15 multilayers.
Skyrmions in the former stack undergo two morphological transitions, upon
reversing the perpendicular field direction. Skyrmions in [Ta/CoFeB/MgO]15
multilayers exhibit a continuous transition, which is mainly linked to a
reversible change of the skyrmion size. A full micromagnetic phase diagram is
presented to identify these two collapsing mechanisms as a function of material
parameters. Since the two distinct collapsing dynamics rely on the detailed
layer-dependent spin structures of skyrmions, they could be used as potential
fingerprints for identifying the skyrmion type in magnetic multilayers. Our
work suggests the employment of pure and hybrid skyrmions for specific
applications, due to the strong correlation between the skyrmion dynamics and
3-dimentional spin profiles.",2303.05988v1
2023-07-05,Comparative Analysis of THz Signal Emission from SiO$_2$/CoFeB/Metal Heterostructures: Wideband and High-Frequency THz Signal Advantage of PtBi-based Emitter,"Spintronic THz emitters have attracted much attention due to their desirable
properties, such as affordability, ultra-wideband capability, high efficiency,
and tunable polarization. In this study, we investigate the characteristics of
THz signals, including their frequency, bandwidth, and amplitude, emitted from
a series of heterostructures with ferromagnetic (FM) and nonmagnetic (NM)
materials. The FM layer consists of a wedge-shaped CoFeB layer with a thickness
of 0 to 5 nm, while the NM materials include various metals such as Pt, Au, W,
Ru, Pt$_{\%92}$Bi$_{\%8}$, and Ag$_{\%90}$Bi$_{\%10}$ alloys. Our experiments
show that the emitter with Pt-NM layer has the highest amplitude of the emitted
THz signal. However, the PtBi-based emitter exhibits a higher central THz peak
and wider bandwidth, making it a promising candidate for broadband THz
emitters. These results pave the way for further exploration of the specific
compositions of Pt$_{1-x}$Bi$_{x}$ for THz emitter design, especially with the
goal of generating higher frequency and wider bandwidth THz signals. These
advances hold significant potential for applications in various fields such as
high-resolution imaging, spectroscopy, communications, medical diagnostics, and
more.",2307.02232v1
2023-09-06,Mid-infrared optical properties of non-magnetic-metal/CoFeB/MgO heterostructures,"We report on the optical characterization of non-magnetic metal/ferromagnetic
(Co$_{20}$Fe$_{60}$B$_{20}$)/MgO heterostructures and interfaces by using mid
infrared spectroscopic ellipsometry at room temperature. We extracted for the
mid-infrared range the dielectric function of Co$_{20}$Fe$_{60}$B$_{20}$, that
is lacking in literature, from a multisample analysis. From the optical
modelling of the heterostructures we detected and determined the dielectric
tensor properties of a two-dimensional gas (2DEG) forming at the non-magnetic
metal and the CoFeB interface. These properties comprise independent Drude
parameters for the in-plane and out-of plane tensor components, with the latter
having an epsilon-near-zero frequency within our working spectral range. A
feature assigned to spin-orbit coupling (SOC) is identified. Furthermore, it is
found that both, the interfacial properties, 2DEG Drude parameters and SOC
strength, and the apparent dielectric function of the MgO layer depend on the
type of the underlying nonmagnetic metal, namely, Pt, W, or Cu. The results
reported here should be useful in tailoring novel phenomena in such types of
heterostructures by assessing their optical response noninvasively,
complementing existing characterization tools such as angle-resolved
photoemission spectroscopy, and those related to electron/spin transport.",2309.02981v1
2023-12-10,"Magnetoelectric Coupling in Pb(Zr,Ti)O3/CoFeB Nanoscale Waveguides Studied by Propagating Spin-Wave Spectroscopy","This study introduces a method for the characterization of the
magnetoelectric coupling in nanoscale Pb(Zr,Ti)O3/CoFeB thin film composites
based on propagating spin-wave spectroscopy. Finite element simulations of the
strain distribution in the devices indicated that the magnetoelastic effective
field in the CoFeB waveguides was maximized in the Damon - Eshbach
configuration. All-electrical broadband propagating spin-wave transmission
measurements were conducted on Pb(Zr,Ti)O3/CoFeB magnetoelectric waveguides
with lateral dimensions down to 700 nm. The results demonstrated that the
spin-wave resonance frequency can be modulated by applying a bias voltage to
Pb(Zr,Ti)O3. The modulation is hysteretic due to the ferroelastic behavior of
Pb(Zr,Ti)O3. An analytical model was then used to correlate the change in
resonance frequency to the induced magnetoelastic field in the magnetostrictive
CoFeB waveguide. We observe a hysteresis magnetoelastic field strength with
values as large as 5.61 mT, and a non-linear magnetoelectric coupling
coefficient with a maximum value of 1.69 mT/V.",2312.05819v1
2018-01-12,Anomalous Nernst effect on the nanometer scale: Exploring three-dimensional temperature gradients in magnetic tunnel junctions,"Localized laser heating creates temperature gradients in all directions and
thus leads to three-dimensional electron flux in metallic materials.
Temperature gradients in combination with material magnetization generate
thermomagnetic voltages. The interplay between these direction-dependent
temperature gradients and the magnetization along with their control enable to
manipulate the generated voltages, e.g. in magnetic nanodevices. We identify
the anomalous Nernst effect (ANE) generated on a nanometer length scale by
micrometer sized temperature gradients in magnetic tunnel junctions (MTJs). In
a systematic study, we extract the ANE by analyzing the influence of in-plane
temperature gradients on the tunnel magneto-Seebeck effect (TMS) in three
dimensional devices. To investigate these effects, we utilize in-plane
magnetized MTJs based on CoFeB electrodes with an MgO tunnel barrier. Due to
our measurement configuration, there is no necessity to disentangle the ANE
from the spin Seebeck effect in inverse spin-Hall measurements. The temperature
gradients are created by a tightly focused laser spot. The spatial extent of
the measured effects is defined by the MTJ size, while the spatial resolution
is given by the laser spot size and the step size of its lateral translation.
This method is highly sensitive to low voltages and yields an ANE coefficient
of $K_N\approx 1.6\cdot 10^{-8}\,\mathrm{V/TK}$ for CoFeB. In general, TMS
investigations in MTJs are motivated by the usage of otherwise wasted heat in
magnetic memory devices for read/write operations. Here, the additionally
generated ANE effect allows to expand the MTJs' functionality from simple
memory storage to nonvolatile logic devices and opens new application fields
such as direction dependent temperature sensing with the potential for further
downscaling.",1801.04186v1
2021-03-14,Perpendicular magnetic anisotropy in ultra-thin Cu$_2$Sb-type (Mn-Cr)AlGe films onto thermally oxidized silicon substrates,"Perpendicularly magnetized films showing small saturation magnetization,
$M_\mathrm{s}$, are essential for spin-transfer-torque writing type
magnetoresistive random access memories, STT-MRAMs. An intermetallic compound,
{(Mn-Cr)AlGe} of the Cu$_2$Sb-type crystal structure was investigated, in this
study, as a material showing the low $M_\mathrm{s}$ ($\sim 300$ kA/m) and
high-perpendicular magnetic anisotropy, $K_\mathrm{u}$. The layer thickness
dependence of $K_\mathrm{u}$ and effects of Mg-insertion layers at top and
bottom (Mn-Cr)AlGe$|$MgO interfaces were studied in film samples fabricated
onto thermally oxidized silicon substrates to realize high-$K_\mathrm{u}$ in
the thickness range of a few nanometer. Optimum Mg-insertion thicknesses were
1.4 and 3.0 nm for the bottom and the top interfaces, respectively, which were
relatively thick compared to results in similar insertion effect investigations
on magnetic tunnel junctions reported in previous studies. The cross-sectional
transmission electron microscope images revealed that the Mg-insertion layers
acted as barriers to interdiffusion of Al-atoms as well as oxidization from the
MgO layers. The values of $K_\mathrm{u}$ were about $7 \times 10^5$ and $2
\times 10^5$ J/m$^3$ at room temperature for 5 and 3 nm-thick (Mn-Cr)AlGe
films, respectively, with the optimum Mg-insertion thicknesses. The
$K_\mathrm{u}$ at a few nanometer thicknesses is comparable or higher than
those reported in perpendicularly magnetized CoFeB films which are
conventionally used in MRAMs, while the $M_\mathrm{s}$ value is one third or
less smaller than those of the CoFeB films. The developed (Mn-Cr)AlGe films are
promising from the viewpoint of not only the magnetic properties, but also the
compatibility to the silicon process in the film fabrication.",2103.07847v2
2019-12-08,Multifilamentary character of anticorrelated capacitive and resistive switching in memristive structures based on (CoFeB)x(LiNbO3)100-x nanocomposite,"Resistive and capacitive switching in capacitor metal/nanocomposite/metal
(M/NC/M) structures based on (CoFeB)x(LiNbO3)100-x NC fabricated by ion-beam
sputtering with metal content x $\approx$ 8-20 at. % is studied. The
peculiarity of the structure synthesis was the use of increased oxygen content
($\approx$ 2*10^-5 Torr) at the initial stage of the NC growth. The NC films,
along with metal nanogranules of 3-6 nm in size, contained a large number of
dispersed Co (Fe) atoms (up to ~10^22 cm^-3). Measurements were performed both
in DC and AC (frequency range 5-13 MHz) regimes. When switching structures from
high-resistance (Roff) to low-resistance (Ron) state, the effect of a strong
increase in their capacity was found, which reaches 8 times at x $\approx$ 15
at. % and the resistance ratio Roff/Ron $\approx$ 40. The effect is explained
by the synergetic combination of the multifilamentary character of resistive
switching (RS) and structural features of the samples associated, in
particular, with the formation of high-resistance and strongly polarizable
LiNbO3 layer near the bottom electrode of the structures. The proposed model is
confirmed by investigations of RS of two-layer nanoscale M/NC/LiNbO3/M
structures as well as by studies of the magnetization of M/NC/M structures in
the pristine state and after RS.",1912.03726v3
2023-05-13,Temperature dependence of magnetic anisotropy and domain wall tuning in BaTiO3(111)/CoFeB multiferroics,"Artificial multiferroics consist of two types of ferroic materials, typically
a ferroelectric and ferromagnet, often coupled interfacially by
magnetostriction induced by the lattice elongations in the ferroelectric. In
BaTiO3 the magnitude of strain induced by these elongations is heavily
temperature dependent, varying greatly between each of the polar crystal phases
and exerting a huge influence over the properties of a coupled magnetic film.
Here we demonstrate that temperature, and thus strain, is an effective means of
controlling the magnetic anisotropy in BaTiO3(111)/CoFeB heterostructures. We
investigate the three polar phases of BaTiO3: tetragonal (T) at room
temperature, orthorhombic (O) below 280 K and rhombohedral (R) below 190 K,
across a total range of 77 K to 420 K. We find two distinct responses; a
step-like change in the anisotropy across the low-temperature phase
transitions, and a sharp high-temperature reduction around the ferroelectric
Curie temperature, measured from hard axis hysteresis loops. Using our
measurements of this anisotropy strength we are then able to show by
micromagnetic simulation the behaviour of all possible magnetic domain wall
states and determine their scaling as a function of temperature. The most
significant changes occur in the head-to-head domain wall states, with a
maximum change of 210 nm predicted across the entire range effectively doubling
the size of the domain wall as compared to room temperature. Notably, similar
changes are seen for both high and low temperatures which suggest different
routes for potential control of magnetic anisotropy and elastically pinned
magnetic domain walls.",2305.07879v1
2006-07-21,Tunnel magnetoresistance and robust room temperature exchange bias with multiferroic BiFeO3 epitaxial thin films,"We report on the functionalization of multiferroic BiFeO3 epitaxial films for
spintronics. A first example is provided by the use of ultrathin layers of
BiFeO3 as tunnel barriers in magnetic tunnel junctions with La2/3Sr1/3MnO3 and
Co electrodes. In such structures, a positive tunnel magnetoresistance up to
30% is obtained at low temperature. A second example is the exploitation of the
antiferromagnetic spin structure of a BiFeO3 film to induce a sizeable (~60 Oe)
exchange bias on a ferromagnetic film of CoFeB, at room temperature.
Remarkably, the exchange bias effect is robust upon magnetic field cycling,
with no indications of training.",0607563v2
2006-08-25,Dependence of tunnel magnetoresistance on ferromagnetic electrode materials in MgO-barrier magnetic tunnel junctions,"We investigated the relationship between the tunnel magnetoresistance (TMR)
ratio and the electrode structure in MgO-barrier magnetic tunnel junctions
(MTJs). The TMR ratio in a MTJ with Co40Fe40B20 reference and free layers
reached 355% at the post-deposition annealing temperature of Ta=400 degree C.
When Co50Fe50 or Co90Fe10 is used for the reference layer material, no high TMR
ratio was observed. The key to have high TMR ratio is to have highly oriented
(001) MgO barrier/CoFeB crystalline electrodes. The highest TMR ratio obtained
so far is 450% at Ta = 450 degree C in a pseudo spin-valve MTJ.",0608551v1
2007-03-22,Spin-Torque Ferromagnetic Resonance Measurements of Damping in Nanomagnets,"We measure the magnetic damping parameter a in thin film CoFeB and permalloy
(Py) nanomagnets at room temperature using ferromagnetic resonance driven by
microwave frequency spin-transfer torque. We obtain $\alpha_{CoFeB} = 0.014 \pm
0.003$ and $\alpha_{Py}=0.010 \pm 0.002$, values comparable to measurements for
extended thin films, but significantly less than the effective damping
determined previously for similar nanomagnets by fits to time-domain studies of
large-angle magnetic excitations and magnetic reversal. The greater damping
found for the large amplitude nanomagnet dynamics is attributed to the
nonlinear excitation of non-uniform magnetic modes.",0703577v1
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
2007-10-10,Mechanisms of exchange bias with multiferroic BiFeO3 epitaxial thin films,"We have combined neutron scattering and piezoresponse force microscopy to
study the relation between the exchange bias observed in CoFeB/BiFeO3
heterostructures and the multiferroic domain structure of the BiFeO3 films. We
show that the exchange field scales with the inverse of the ferroelectric and
antiferromagnetic domain size, as expected from Malozemoff's model of exchange
bias extended to multiferroics. Accordingly, polarized neutron reflectometry
reveals the presence of uncompensated spins in the BiFeO3 film at the interface
with the CoFeB. In view of these results we discuss possible strategies to
switch the magnetization of a ferromagnet by an electric field using BiFeO3.",0710.2025v1
2009-10-22,MgO barrier-perpendicular magnetic tunnel junctions with CoFe/Pd multilayers and ferromagnetic insertion layers,"The authors studied an effect of ferromagnetic (Co20Fe60B20 or Fe) layer
insertion on tunnel magnetoresistance (TMR) properties of MgO-barrier magnetic
tunnel junctions (MTJs) with CoFe/Pd multilayer electrodes. TMR ratio in MTJs
with CoFeB/MgO/Fe stack reached 67% at an-nealing temperature (Ta) of 200
degree C and then decreased rapidly at Ta over 250 degree C. The degradation of
the TMR ratio may be related to crystallization of CoFe(B) into fcc(111) or
bcc(011) texture result-ing from diffusion of B into Pd layers. MTJs which were
in-situ annealed at 350oC just after depo-siting bottom CoFe/Pd multilayer
showed TMR ratio of 78% by post annealing at Ta =200 degree C.",0910.4204v1
2010-08-09,Detection of bottom ferromagnetic electrode oxidation in magnetic tunnel junctions by magnetometry measurements,"Surface oxidation of the bottom ferromagnetic (FM) electrode, one of the
major detrimental factors to the performance of a Magnetic Tunnel Junction
(MTJ), is difficult to avoid during the fabrication process of the MTJ's tunnel
barrier. Since Co rich alloys are commonly used for the FM electrodes in MTJs,
over-oxidation of the tunnel barrier results in the formation of a CoO
antiferromagnetic (AF) interface layer which couples with the bottom FM
electrode to form a typical AF/FM exchange bias (EB) system. In this work,
surface oxidation of the CoFe and CoFeB bottom electrodes was detected via
magnetometry measurements of exchange-bias characterizations including the EB
field, training effect, uncompensated spin density, and coercivity. Variations
of these parameters were found to be related to the surface oxidation of the
bottom electrode, among them the change of coercivity is most sensitive.
Annealed samples show evidence for an oxygen migration back to the MgO tunnel
barrier by annealing.",1008.1493v1
2011-03-16,Spin-Torque Diode Measurements of MgO-Based Magnetic Tunnel Junctions with Asymmetric Electrodes,"We present a detailed study of the spin-torque diode effect in
CoFeB/MgO/CoFe/NiFe magnetic tunnel junctions. From the evolution of the
resonance frequency with magnetic field at different angles, we clearly
identify the free-layer mode and find an excellent agreement with simulations
by taking into account several terms for magnetic anisotropy. Moreover, we
demonstrate the large contribution of the out-of-plane torque in our junctions
with asymmetric electrodes compared to the in-plane torque. Consequently, we
provide a way to enhance the sensitivity of these devices for the detection of
microwave frequency.",1103.3207v1
2013-04-09,Paramagnetic Fe_xTa_{1-x} alloys for engineering of perpendicularly magnetized tunnel junctions,"Exchange coupling between two magnetic layers through an interlayer is of
broad interest for numerous recent applications of nano-magnetic systems. In
this Letter we study ferromagnetic exchange coupling through amorphous
paramagnetic Fe-Ta alloys. We show that the exchange coupling depends
exponentially on spacer thickness and scales with the Fe-Ta susceptibility,
which can be tuned via the alloy composition and/or temperature. Such materials
are of high interest for the engineering of perpendicularly magnetized
CoFeB-MgO based tunnel junctions as it enables ferromagnetic coupling of
magnetic layers with differing crystalline lattices, suppresses dead layers and
can act as an inter-diffusion barrier during annealing.",1304.2763v1
2013-08-29,Control of Propagating Spin Waves via Spin Transfer Torque in a Metallic Bilayer Waveguide,"We investigate the effect of a direct current on propagating spin waves in a
CoFeB/Ta bilayer structure. Using the micro-Brillouin light scattering
technique, we observe that the spin wave amplitude may be attenuated or
amplified depending on the direction of the current and the applied magnetic
field. Our work suggests an effective approach for electrically controlling the
propagation of spin waves in a magnetic waveguide and may be useful in a number
of applications such as phase locked nano-oscillators and hybrid information
processing devices.",1308.6357v2
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
2014-02-18,Switching Properties in Magnetic Tunnel Junctions with Interfacial Perpendicular Anisotropy: Micromagnetic Study,"The role of universal memory can be successfully satisfied by magnetic tunnel
junctions (MTJs) where the writing mechanism is based on spin-transfer torque
(STT). An improvement in the switching properties (lower switching current
density maintaining the thermal stability) has been achieved in MTJs with
interfacial perpendicular anisotropy (IPA) at the interface between CoFeB and
MgO. In this paper, micromagnetic simulations point out the influence of IPA
and saturation magnetization (MS) on the properties of fast magnetization
reversal achieved in 5, 10 and 20 ns. Both cases of in-plane and out-of-plane
free layer are considered. In addition, the thermal effect is included for the
in-plane switching at 20 ns and a complete analysis of energy dissipation
during the switching is illustrated. This study can provide useful information
for the design of STT-based memories.",1402.4352v1
2014-07-23,Deterministic Domain Wall Motion Orthogonal To Current Flow Due To Spin Orbit Torque,"Deterministic control of domain walls orthogonal to the direction of current
flow is demonstrated by exploiting spin orbit torque in a perpendicularly
polarized Ta/CoFeB/MgO multilayer in presence of an in-plane magnetic field.
Notably, such orthogonal motion with respect to current flow is not possible
from traditional spin transfer torque driven domain wall propagation even in
presence of an external magnetic field. Reversing the polarity of either the
current flow or the in-plane field is found to reverse the direction of the
domain wall motion. From these measurements, which are unaffected by any
conventional spin transfer torque by symmetry, we estimate the spin orbit
torque efficiency of Ta to be 0.08.",1407.6137v1
2014-11-19,Universal domain wall dynamics under electric field in Ta/CoFeB/MgO devices with perpendicular anisotropy,"Electric field effects in ferromagnetic/oxide dielectric structures provide a
new route to control domain wall (DW) dynamics with low power dissipation.
However, electric field effects on DW velocities have only been observed so far
in the creep regime where DW velocities are low due to strong interactions with
pinning sites. Here, we show gate voltage modulation of DW velocities ranging
from the creep to the flow regime in Ta/Co40Fe40B20/MgO/TiO2 structures with
perpendicular magnetic anisotropy. We demonstrate a universal description of
the role of applied electric fields in the various pinning dependent regimes by
taking into account an effective magnetic field being linear with the electric
field. In addition, the electric field effect is found to change sign in the
Walker regime. Our work opens new opportunities for the study and optimization
of electric field effect at ferromagnetic metal/insulator interfaces.",1411.5267v2
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
2015-03-25,Spin Hall torque driven chiral domain walls in magnetic heterostructures,"The motion of magnetic domain walls in ultrathin magnetic heterostructures
driven by current via the spin Hall torque is described. We show results from
perpendicularly magnetized CoFeB|MgO heterostructures with various heavy metal
underlayers. The domain wall moves along or against the current flow depending
on the underlayer material. The direction to which the domain wall moves is
associated with the chirality of the domain wall spiral formed in these
heterostructures. The one-dimensional model is used to describe the
experimental results and extract parameters such as the Dzyaloshinskii-Moriya
exchange constant which is responsible for the formation of the domain wall
spiral. Fascinating effects arising from the control of interfaces in magnetic
heterostructures are described.",1503.07250v1
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
2015-08-18,Quasistatic and Pulsed Current-Induced Switching with Spin-Orbit Torques in Ultrathin Films with Perpendicular Magnetic Anisotropy,"Spin-orbit interaction derived spin torques provide a means of reversing the
magnetization of perpendicularly magnetized ultrathin films with currents that
flow in the plane of the layers. A basic and critical question for applications
is the speed and efficiency of switching with nanosecond current pulses. Here
we investigate and contrast the quasistatic (slowly swept current) and pulsed
current-induced switching characteristics of micron scale Hall crosses
consisting of very thin ($<1$ nm) perpendicularly magnetized CoFeB layers on
$\beta$-Ta. While complete magnetization reversal is found at a threshold
current density in the quasistatic case, short duration ($\leq 10$ ns) larger
amplitude pulses ($\simeq 10 \times$ the quasistatic threshold current) lead to
only partial magnetization reversal and domain formation. We associate the
partial reversal with the limited time for reversed domain expansion during the
pulse.",1508.04336v1
2015-08-24,Origin of robust interaction of spin waves with a single skyrmion in perpendicularly magnetized nanostripes,"We studied interactions between propagating spin waves (SWs) and a single
skyrmion in a perpendicularly magnetized CoFeB nanostripe where the magnetic
layer is interfaced with W and MgO. Micromagnetic numerical calculations
revealed that robust interactions between the incident SWs and the skyrmion
give rise to considerable forward skyrmion motions for specific SW frequencies
(e.g., here: fsw = 12 - 19 GHz). Additionally, it was found that there exists a
sufficiently low threshold field amplitude, e.g., 0.1 kOe for the fsw = 15 GHz
SWs. This frequency-dependent interaction originated from the robust coupling
of the SWs with the internal modes of the skyrmion, through the SWs' linear
momentum transfer torque acting on the skyrmion. This work provides for
all-magnetic control of skyrmion motions without electronic currents, and
facilitates further understanding of the interactions between magnons and
topological solitons in constricted geometries.",1508.05682v1
2015-12-11,Correlation between the spin Hall angle and the structural phases of early 5d transition metals,"We have studied the relationship between the structure and the spin Hall
angle of the early 5d transition metals in X/CoFeB/MgO (X=Hf, Ta, W, Re)
heterostructures. Spin Hall magnetoresistance (SMR) is used to characterize the
spin Hall angle of the heavy metals. Transmission electron microscopy images
show that all underlayers are amorphous-like when their thicknesses are small,
however, crystalline phases emerge as the thickness is increased for certain
elements. We find that the heavy metal layer thickness dependence of the SMR
reflects these changes in structure. The spin Hall angle largest
|\theta$_{SH}$| of Hf, Ta, W and Re (~0.11, 0.10, 0.23 and 0.07, respectively)
is found when the dominant phase is amorphous-like. We find that the
amorphous-like phase not only possesses large resistivity but also exhibits
sizeable spin Hall conductivity, which both contribute to the emergence of the
large spin Hall angle.",1512.03529v1
2016-02-24,Exchange stiffness in ultrathin perpendicularly-magnetized CoFeB layers determined using spin wave spectroscopy,"We measure the frequencies of spin waves in nm-thick perpendicularly
magnetized FeCoB systems, and model the frequencies to deduce the exchange
stiffness of this material in the ultrathin limit. For this, we embody the
layers in magnetic tunnel junctions patterned into circular nanopillars of
diameters ranging from 100 to 300 nm and we use magneto-resistance to determine
which rf-current frequencies are efficient in populating the spin wave modes.
Micromagnetic calculations indicate that the ultrathin nature of the layer and
the large wave vectors used ensure that the spin wave frequencies are
predominantly determined by the exchange stiffness, such that the number of
modes in a given frequency window can be used to estimate the exchange. For 1
nm layers the experimental data are consistent with an exchange stiffness A= 20
pJ/m, which is slightly lower that its bulk counterpart. The thickness
dependence of the exchange stiffness has strong implications for the numerous
situations that involve ultrathin films hosting strong magnetization gradients,
and the micromagnetic description thereof.",1602.07421v1
2016-04-15,Twist in the bias-dependence of spin-torques in magnetic tunnel junctions,"The spin-torque in magnetic tunnel junctions possesses two components that
both depend on the applied voltage. Here, we develop a new method for the
accurate extraction of this bias-dependence from experiments over large voltage
ranges. We study several junctions with different magnetic layer structures of
the top electrode. Our results obtained on junctions with symmetric CoFeB
electrodes agree well with theoretical calculations. The bias-dependences of
asymmetric samples, with top electrodes containing NiFe, however, are twisted
compared to the quadratic form generally assumed. Our measurements reveal the
complexity of spin-torque mechanisms at large bias.",1604.04517v1
2016-04-16,A broadband Ferromagnetic Resonance dipper probe for magnetic damping measurements from 4.2 K to 300 K,"A dipper probe for broadband Ferromagnetic Resonance (FMR) operating from 4.2
K to room temperature is described. The apparatus is based on a 2-port
transmitted microwave signal measurement with a grounded coplanar waveguide.
The waveguide generates a microwave field and records the sample response. A
3-stage dipper design is adopted for fast and stable temperature control. The
temperature variation due to FMR is in the milli-Kelvin range at liquid helium
temperature. We also designed a novel FMR probe head with a spring-loaded
sample holder. Improved signal-to-noise ratio and stability compared to a
common FMR head are achieved. Using a superconducting vector magnet we
demonstrate Gilbert damping measurements on two thin film samples using a
vector network analyzer with frequency up to 26 GHz: 1) A Permalloy film of 5
nm thickness and 2) a CoFeB film of 1.5 nm thickness. Experiments were
performed with the applied magnetic field parallel and perpendicular to the
film plane.",1604.04688v1
2016-05-18,Electrical control over perpendicular magnetization switching driven by spin-orbit torques,"Flexible control of magnetization switching by electrical manners is crucial
for applications of spin-orbitronics. Besides of a switching current that is
parallel to an applied field, a bias current that is normal to the switching
current is introduced to tune the magnitude of effective damping-like and
field-like torques and further to electrically control magnetization switching.
Symmetrical and asymmetrical control over the critical switching current by the
bias current with opposite polarities is both realized in Pt/Co/MgO and
$\alpha$-Ta/CoFeB/MgO systems, respectively. This research not only identifies
the influences of field-like and damping-like torques on switching process but
also demonstrates an electrical method to control it.",1605.05569v1
2016-09-14,Rashba-Edelstein Magnetoresistance in Metallic Heterostructure,"We report the observation of magnetoresistance originating from Rashba
spin-orbit coupling (SOC) in a metallic heterostructure: the Rashba-Edelstein
(RE) magnetoresistance. We show that the simultaneous action of the direct and
inverse RE effects in a Bi/Ag/CoFeB trilayer couples current-induced spin
accumulation to the electric resistance. The electric resistance changes with
the magnetic-field angle, reminiscent of the spin Hall magnetoresistance,
despite the fact that bulk SOC is not responsible for the magnetoresistance. We
further found that, even when the magnetization is saturated, the resistance
increases with increasing the magnetic-field strength, which is attributed to
the Hanle magnetoresistance in this system.",1609.04122v1
2016-11-03,Scalable synchronization of spin-Hall oscillators in out-of-plane field,"A strategy for a scalable synchronization of an array of spin-Hall
oscillators (SHOs) is illustrated. In detail, we present micromagnetic
simulations of two and five SHOs realized by means of couples of triangular
golden contacts on the top of a Pt/CoFeB/Ta trilayer. Results highlight that
the synchronization occurs for the whole current region that gives rise to the
excitation of self-oscillations. This is linked to the role of the
magnetodipolar coupling, which is the phenomenon driving the synchronization
when the distance between oscillators is not too large. Synchronization turns
out to be also robust against geometrical differences of the contacts,
simulated by considering variable distances between the tips ranging from 100nm
to 200nm. Besides, it entails an enlargement of the radiation pattern that can
be useful for the generation of spin-waves in magnonics applications.
Simulations performed to study the effect of the interfacial
Dzyaloshinskii-Moriya interaction show nonreciprocity in spatial propagation of
the synchronized spin-wave. The simplicity of the geometry and the robustness
of the achieved synchronization make this design of array of SHOs scalable for
a larger number of synchronized oscillators.",1611.01227v1
2017-01-23,Understanding stability diagram of perpendicular magnetic tunnel junctions,"Perpendicular magnetic tunnel junctions (MTJ) with a bottom pinned reference
layer and a composite free layer (FL) are investigated. Different thicknesses
of the FL were tested to obtain an optimal balance between tunneling
magnetoresistance (TMR) ratio and perpendicular magnetic anisotropy. After
annealing at 400 $^\circ$C, the TMR ratio for 1.5 nm thick CoFeB sublayer
reached 180 % at room temperature and 280 % at 20 K with an MgO tunnel barrier
thickness corresponding to the resistance area product RA = 10
Ohm$\mathrm{\mu}$m$^2$. The voltage vs. magnetic field stability diagrams
measured in pillar-shaped MTJs with 130 nm diameter indicate the competition
between spin transfer torque (STT), voltage controlled magnetic anisotropy
(VCMA) and temperature effects in the switching process. An extended stability
phase diagram model that takes into account all three parameters and the
effective damping measured independently using broadband ferromagnetic
resonance technique enabled the determination of both STT and VCMA coefficients
that are responsible for the FL magnetization switching.",1701.06411v1
2017-03-29,Ultrabroadband single-cycle terahertz pulses with peak fields of 300 kV cm$^{-1}$ from a metallic spintronic emitter,"To explore the capabilities of metallic spintronic thin-film stacks as a
source of intense and broadband terahertz electromagnetic fields, we excite a
W/CoFeB/Pt trilayer on a large-area glass substrate (diameter of 7.5 cm) by a
femtosecond laser pulse (energy 5.5 mJ, duration 40 fs, wavelength 800 nm).
After focusing, the emitted terahertz pulse is measured to have a duration of
230 fs, a peak field of 300 kV cm$^{-1}$ and an energy of 5 nJ. In particular,
the waveform exhibits a gapless spectrum extending from 1 to 10 THz at 10% of
amplitude maximum, thereby facilitating nonlinear control over matter in this
difficult-to-reach frequency range and on the sub-picosecond time scale.",1703.09970v1
2017-07-15,Stochastic Spin-Orbit Torque Devices as Elements for Bayesian Inference,"Probabilistic inference from real-time input data is becoming increasingly
popular and may be one of the potential pathways at enabling cognitive
intelligence. As a matter of fact, preliminary research has revealed that
stochastic functionalities also underlie the spiking behavior of neurons in
cortical microcircuits of the human brain. In tune with such observations,
neuromorphic and other unconventional computing platforms have recently started
adopting the usage of computational units that generate outputs
probabilistically, depending on the magnitude of the input stimulus. In this
work, we experimentally demonstrate a spintronic device that offers a direct
mapping to the functionality of such a controllable stochastic switching
element. We show that the probabilistic switching of Ta/CoFeB/MgO
heterostructures in presence of spin-orbit torque and thermal noise can be
harnessed to enable probabilistic inference in a plethora of unconventional
computing scenarios. This work can potentially pave the way for hardware that
directly mimics the computational units of Bayesian inference.",1707.04687v2
2017-09-08,Tunneling magnetoresistance of perpendicular CoFeB-based junctions with exchange bias,"Recently, magnetic tunnel junctions with perpendicular magnetized electrodes
combined with exchange bias films have attracted large interest. In this paper
we examine the tunnel magnetoresistance of
Ta/Pd/IrMn/Co-Fe/Ta/Co-Fe-B/MgO/Co-Fe-B/capping/Pd magnetic tunnel junctions in
dependence on the capping layer, i.e., Hf or Ta. In these stacks perpendicular
exchange bias fields of -500\,Oe along with perpendicular magnetic anisotropy
are combined. A tunnel magnetoresistance of $(47.2\pm 1.4)\%$ for the Hf-capped
sample was determined compared to the Ta one $(42.6\pm 0.7)\%$ at room
temperature. Interestingly, this observation is correlated to the higher boron
absorption of Hf compared to Ta which prevents the suppression of
$\Delta_{\textrm{1}}$ channel and leads to higher tunnel magnetoresistance
values. Furthermore, the temperature dependent coercivities of the soft
electrodes of both samples are mainly described by the Stoner-Wohlfarth model
including thermal fluctuations. Slight deviations at low temperatures can be
attributed to a torque on the soft electrode that is generated by the pinned
magnetic layer system.",1709.02607v1
2017-09-12,Scaling of Dzyaloshinskii Moriya interaction at heavy metal and ferromagnetic metal interfaces,"The Dzyaloshinskii Moriya Interaction (DMI) at the heavy metal (HM) and
ferromagnetic metal (FM) interface has been recognized as a key ingredient in
spintronic applications. Here we investigate the chemical trend of DMI on the
5d band filling (5d^3~5d^10) of the HM element in HM/CoFeB/MgO multilayer thin
films. DMI is quantitatively evaluated by measuring asymmetric spin wave
dispersion using Brillouin light scattering. Sign reversal and 20 times
modification of the DMI coefficient D have been measured as the 5d HM element
is varied. The chemical trend can be qualitatively understood by considering
the 5d and 3d bands alignment at the HM/FM interface and the subsequent orbital
hybridization around the Fermi level. Furthermore, a positive correlation is
observed between DMI and spin mixing conductance at the HM/FM interfaces. Our
results provide new insights into the interfacial DMI for designing future
spintronic devices.",1709.03961v1
2017-11-09,Room Temperature Giant Charge-to-Spin Conversion at SrTiO3/LaAlO3 Oxide Interface,"Two-dimensional electron gas (2DEG) formed at the interface between SrTiO3
(STO) and LaAlO3 (LAO) insulating layer is supposed to possess strong Rashba
spin-orbit coupling. To date, the inverse Edelstein effect (i.e. spin-to-charge
conversion) in the 2DEG layer is reported. However, the direct effect of
charge-to-spin conversion, an essential ingredient for spintronic devices in a
current induced spin-orbit torque scheme, has not been demonstrated yet. Here
we show, for the first time, a highly efficient spin generation with the
efficiency of ~6.3 in the STO/LAO/CoFeB structure at room temperature by using
spin torque ferromagnetic resonance. In addition, we suggest that the spin
transmission through the LAO layer at high temperature range is attributed to
the inelastic tunneling via localized states in the LAO band gap. Our findings
may lead to potential applications in the oxide insulator based spintronic
devices.",1711.03268v1
2017-12-05,Studies of CoFeB crystalline structure grown on PbSnTe topological insulator substrate,"Co40Fe40B20 layers were grown on the Pb0.71Sn0.29Te topological insulator
substrates by laser molecular beam epitaxy (LMBE) method, and the growth
conditions were studied. The possibility of growing epitaxial layers of a
ferromagnet on the surface of a topological insulator was demonstrated for the
first time. The Co40Fe40B20 layers obtained have a bcc crystal structure with a
crystalline (111) plane parallel to the (111) PbSnTe plane. The use of
three-dimensional mapping in the reciprocal space of reflection high electron
diffraction (RHEED) patterns made it possible to determine the epitaxial
relationship of main crystallographic axes between the film and the substrate
of topological insulator. Quenching of some reflections in diffraction pattern
allows confirmation of the substrate stoichiometry.",1712.01585v3
2017-12-23,Shape anisotropy revisited in single-digit nanometer magnetic tunnel junctions,"Nanoscale magnetic tunnel junction plays a pivotal role in magnetoresistive
random access memories. Successful implementation depends on a simultaneous
achievement of low switching current for the magnetization switching by
spin-transfer torque and high thermal stability, along with a continuous
reduction of junction size. Perpendicular-easy-axis CoFeB/MgO stacks possessing
interfacial anisotropy have paved the way down to 20-nm scale, below which a
new approach needs to be explored. Here we show magnetic tunnel junctions that
satisfy the requirements at ultrafine scale by revisiting shape anisotropy,
which is a classical part of magnetic anisotropy but has not been fully
utilized in the current perpendicular systems. Magnetization switching solely
driven by current is achieved for junctions smaller than 10 nm where sufficient
thermal stability is provided by shape anisotropy without adopting new material
systems. This work is expected to push forward the development of magnetic
tunnel junctions towards single-digit-nm-scale nano-magnetics/spintronics.",1712.08774v1
2018-01-17,Hartman effect for spin waves in exchange regime,"Hartman effect for spin waves tunnelling through a barrier in a thin magnetic
film is considered theoretically. The barrier is assumed to be created by a
locally increased magnetic anisotropy field. The considerations are focused on
a nanoscale system operating in the exchange-dominated regime. We derive the
formula for group delay $\tau_{gr}$ of spin wave package and show that
$\tau_{gr}$ saturates with increasing barrier width, which is a signature of
the Hartman effect predicted earlier for photonic and electronic systems. In
our calculations we consider the general boundary exchange conditions which
take into account different strength of exchange coupling between the barrier
and its surrounding. As a system suitable for experimental observation of the
Hartman effect we propose a CoFeB layer with perpendicular magnetic anisotropy
induced by a MgO overlayer.",1801.05876v3
2018-03-22,Voltage Control of Magnetic Monopoles in Artificial Spin Ice,"Current research on artificial spin ice (ASI) systems has revealed unique
hysteretic memory effects and mobile quasi-particle monopoles controlled by
externally applied magnetic fields. Here, we numerically demonstrate a
strain-mediated multiferroic approach to locally control the ASI monopoles. The
magnetization of individual lattice elements is controlled by applying voltage
pulses to the piezoelectric layer resulting in strain-induced magnetic
precession timed for 180 degree reorientation. The model demonstrates localized
voltage control to move the magnetic monopoles across lattice sites, in CoFeB,
Ni, and FeGa based ASI$'$s. The switching is achieved at frequencies near
ferromagnetic resonance and requires energies below 620 aJ. The results
demonstrate that ASI monopoles can be efficiently and locally controlled with a
strain-mediated multiferroic approach.",1803.08598v1
2018-05-09,Programmable control of spin-wave transmission in a domain-wall spin valve,"Active manipulation of spin waves is essential for the development of
magnon-based technologies. Here, we demonstrate programmable spin-wave
filtering by resetting the spin structure of a pinned 90$^\circ$ N\'{e}el
domain wall in a continuous CoFeB film with abrupt rotations of uniaxial
magnetic anisotropy. Using phase-resolved micro-focused Brillouin light
scattering and micromagnetic simulations, we show that broad 90$^\circ$
head-to-head or tail-to-tail magnetic domain walls are transparent to spin
waves over a broad frequency range. In contrast, magnetic switching to a
90$^\circ$ head-to-tail configuration produces much narrower domain walls at
the same pinning locations. Spin waves are strongly reflected by a resonance
mode in these magnetic domain walls. Based on these results, we propose a
magnetic spin-wave valve with two parallel domain walls. Switching the
spin-wave valve from an open to a close state changes the transmission of spin
waves from nearly 100% to 0% at the resonance frequency. This active control
over spin-wave transport could be utilized in magnonic logic devices or
non-volatile memory elements.",1805.03470v1
2018-05-10,Anomalous spin Hall magnetoresistance in Pt/Co bilayers,"We have studied the spin Hall magnetoresistance (SMR), the magnetoresistance
within the plane transverse to the current flow, of Pt/Co bilayers. We find
that the SMR increases with increasing Co thickness: the effective spin Hall
angle for bilayers with thick Co exceeds the reported values of Pt when a
conventional drift-diffusion model is used. An extended model including spin
transport within the Co layer cannot account for the large SMR. To identify its
origin, contributions from other sources are studied. For most bilayers, the
SMR increases with decreasing temperature and increasing magnetic field,
indicating that magnon-related effects in the Co layer play little role.
Without the Pt layer, we do not observe the large SMR found for the Pt/Co
bilayers with thick Co. Implementing the effect of the so-called interface
magnetoresistance and the textured induced anisotropic scattering cannot
account for the Co thickness dependent SMR. Since the large SMR is present for
W/Co but its magnitude reduces in W/CoFeB, we infer its origin is associated
with a particular property of Co.",1805.03843v1
2019-04-25,Low damping magnetic properties and perpendicular magnetic anisotropy with strong volume contribution in the Heusler alloy Fe1.5CoGe,"We present a study of the dynamic magnetic properties of TiN-buffered
epitaxial thin films of the Heusler alloy Fe$_{1.5}$CoGe. Thickness series
annealed at different temperatures are prepared and the magnetic damping is
measured, a lowest value of $\alpha=2.18\times 10^{-3}$ is obtained. The
perpendicular magnetic anisotropy properties in Fe$_{1.5}$CoGe/MgO are also
characterized. The evolution of the interfacial perpendicular anisotropy
constant $K^{\perp}_{\rm S}$ with the annealing temperature is shown and
compared with the widely used CoFeB/MgO interface. A large volume contribution
to the perpendicular anisotropy of $(4.3\pm0.5)\times 10^{5}$ $\rm J/m^3$ is
also found, in contrast with vanishing bulk contribution in common Co- and
Fe-based Heusler alloys.",1904.11247v1
2019-10-11,Investigation of spin orbit torque driven dynamics in ferromagnetic heterostructures,"We use time-resolved (TR) measurements based on the polar magneto-optical
Kerr effect (MOKE) to study the magnetization dynamics excited by spin orbit
torques in Py (Permalloy)/Pt and Ta/CoFeB bilayers. The analysis reveals that
the field-like (FL) spin orbit torque (SOT) dominates the amplitude of the
first oscillation cycle of the magnetization precession and the damping-like
(DL) torque determines the final steady-state magnetization. In our bilayer
samples, we have extracted the effective fields, hFL and hDL, of the two SOTs
from the time-resolved magnetization oscillation spectrum. The extracted values
are in good agreement with those extracted from time-integrated DCMOKE
measurements, suggesting that the SOTs do not change at high frequencies. We
also find that the amplitude ratio of the first oscillation to steady state is
linearly proportional to the ratio hFL/hDL. The first oscillation amplitude is
inversely proportional to, whereas the steady state value is independent of,
the applied external field along the current direction.",1910.04945v1
2019-10-28,The spin Hall effect of Bi-Sb alloys driven by thermally excited Dirac-like electrons,"We have studied the charge to spin conversion in Bi$_{1-x}$Sb$_x$/CoFeB
heterostructures. The spin Hall conductivity (SHC) of the sputter deposited
heterostructures exhibits a high plateau at Bi-rich compositions, corresponding
to the topological insulator phase, followed by a decrease of SHC for Sb-richer
alloys, in agreement with the calculated intrinsic spin Hall effect of
Bi$_{1-x}$Sb$_x$ alloy. The SHC increases with increasing thickness of the
Bi$_{1-x}$Sb$_x$ alloy before it saturates, indicating that it is the bulk of
the alloy that predominantly contributes to the generation of spin current; the
topological surface states, if present in the films, play little role.
Surprisingly, the SHC is found to increase with increasing temperature,
following the trend of carrier density. These results suggest that the large
SHC at room temperature, with a spin Hall efficiency exceeding 1 and an
extremely large spin current mobility, is due to increased number of
Dirac-like, thermally-excited electrons in the $L$ valley of the narrow gap
Bi$_{1-x}$Sb$_x$ alloy.",1910.12433v1
2018-02-07,Breaking the current density threshold in spin-orbit-torque magnetic random access memory,"Spin-orbit-torque magnetic random access memory (SOT-MRAM) is a promising
technology for the next generation of data storage devices. The main bottleneck
of this technology is the high reversal current density threshold. This
outstanding problem of SOT-MRAM is now solved by using a current density of
constant magnitude and varying flow direction that reduces the reversal current
density threshold by a factor of more than the Gilbert damping coefficient. The
Euler-Lagrange equation for the fastest magnetization reversal path and the
optimal current pulse are derived for an arbitrary magnetic cell. The
theoretical limit of minimal reversal current density and current density for a
GHz switching rate of the new reversal strategy for CoFeB/Ta SOT-MRAMs are
respectively of the order of $10^5$ A/cm$^2$ and $10^6$ A/cm$^2$ far below
$10^7$ A/cm$^2$ and $10^8$ A/cm$^2$ in the conventional strategy. Furthermore,
no external magnetic field is needed for a deterministic reversal in the new
strategy.",1802.02415v1
2018-12-05,Single spin sensing of domain wall structure and dynamics in a thin film skyrmion host,"Skyrmions are nanoscale magnetic structures with features promising for
future low-power memory or logic devices. In this work, we demonstrate novel
scanning techniques based on nitrogen vacancy center magnetometry that
simultaneously probe both the magnetic dynamics and structure of room
temperature skyrmion bubbles in a thin film system Ta/CoFeB/MgO. We confirm the
handedness of the Dzyaloshinskii-Moriya interaction in this material and
extract the helicity angle of the skyrmion bubbles. Our measurements also show
that the skyrmion bubbles in this material change size in discrete steps,
dependent on the local pinning environment, with their average size determined
dynamically as their domain walls hop between pinning sites. In addition, an
increase in magnetic field noise is observed near all skyrmion bubble domain
walls. These measurements highlight the importance of interactions between
internal degrees of freedom of skyrmion bubble domain walls and pinning sites
in thin film systems. Our observations have relevance for future devices based
on skyrmion bubbles where pinning interactions will determine important aspects
of current-driven motion.",1812.01764v1
2019-06-04,High frequency voltage-induced ferromagnetic resonance in magnetic tunnel junctions,"Voltage-induced ferromagnetic resonance (V-FMR) in magnetic tunnel junctions
(MTJs) with a W buffer is investigated. Perpendicular magnetic anisotropy (PMA)
energy is controlled by both thickness of a CoFeB free layer deposited directly
on the W buffer and a post-annealing process at different temperatures. The PMA
energy as well as the magnetization damping are determined by analysing
field-dependent FMR signals in different field geometries. An optimized MTJ
structure enabled excitation of V-FMR at frequencies exceeding 30 GHz. The
macrospin modelling is used to analyse the field- and angular-dependence of the
V-FMR signal and to support experimental magnetization damping extraction.",1906.01301v1
2022-02-01,Numerical Model Of Harmonic Hall Voltage Detection For Spintronic Devices,"We present a numerical macrospin model for harmonic voltage detection in
multilayer spintronic devices. The core of the computational backend is based
on the Landau-Lifshitz-Gilbert-Slonczewski equation, which combines high
performance with satisfactory, for large-scale applications, agreement with the
experimental results. We compare the simulations with the experimental findings
in Ta/CoFeB bilayer system for angular- and magnetic field-dependent resistance
measurements, electrically detected magnetisation dynamics, and harmonic Hall
voltage detection. Using simulated scans of the selected system parameters such
as the polar angle $\theta$, magnetisation saturation
($\mu_\textrm{0}M_\textrm{s}$) or uniaxial magnetic anisotropy ($K_\textrm{u}$)
we show the resultant changes in the harmonic Hall voltage, demonstrating the
dominating influence of the $\mu_\textrm{0}M_\textrm{s}$ on the first and
second harmonics. In the spin-diode ferromagnetic resonance (SD-FMR) technique
resonance method the ($\mu_\textrm{0}M_\textrm{s}$, $K_\textrm{u}$) parameter
space may be optimised numerically to obtain a set of viable curves that fit
the experimental data.",2202.00364v1
2022-02-03,Element Doping Enhanced Charge-to-Spin Conversion Efficiency in Amorphous PtSn4 Dirac Semimetal,"Topological semimetals (TSs) are promising candidates for low-power
spin-orbit torque (SOT) devices due to their large charge-to-spin conversion
efficiency. Here, we investigated the charge-to-spin conversion efficiency of
amorphous PtSn4 (5 nm)/CoFeB (2.5-12.5 nm) layered structures prepared by a
magnetron sputtering method at room temperature. The charge-to-spin ratio of
PtSn4/CoFeB bilayers was 0.08, characterized by a spin torque ferromagnetic
resonance (ST-FMR) technique. This ratio can further increase to 0.14 by
inducing dopants, like Al and CoSi, into PtSn4. The dopants can also decrease
(Al doping) or increase (CoSi doping) the resistivity of PtSn4. The work
proposed a way to enhance the spin-orbit coupling (SOC) in amorphous TSs with
dopants.",2202.01384v1
2022-02-04,A Magnetoelectric Memory Device Based on Pseudo-Magnetization,"We propose a new type of magnetoelectric memory device that stores magnetic
easy-axis information or pseudo-magnetization, rather than a definite
magnetization direction, in piezoelectric/ferromagnetic (PE/FM)
heterostructures. Theoretically, we show how a PE/FM combination can lead to
non-volatility in pseudo-magnetization exhibiting ferroelectric-like behavior.
The pseudo-magnetization can be manipulated by extremely low voltages
especially when the FM is a low-barrier nanomagnet. Using a circuit model
benchmarked against experiments, we determine the switching energy, delay,
switching probability and retention time of the envisioned 1T/1C memory device
in terms of magnetic and circuit parameters and discuss its thermal stability
in terms of a key parameter called back-voltage vm which is an electrical
measure of the strain-induced magnetic field. Taking advantage of ferromagnetic
resonance (FMR) measurements, we experimentally extract values for vm in CoFeB
films and circular nano-magnets deposited on Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT)
which agree well with the theoretical values. Our experimental findings indeed
indicate the feasibility of the proposed novel device and confirm the assumed
parameters in our modeling effort.",2202.02203v2
2016-03-09,Evolution of perpendicular magnetized tunnel junctions upon annealing,"We study the evolution of perpendicularly magnetized tunnel junctions under
300 to 400 $^{\circ}$C annealing. The hysteresis loops do not evolve much
during annealing and they are not informative of the underlying structural
evolutions. These evolutions are better revealed by the frequencies of the
ferromagnetic resonance eigenmodes of the tunnel junction. Their modeling
provides the exchange couplings and the layers' anisotropies within the stack
which can serve as a diagnosis of the tunnel junction state after each
annealing step. The anisotropies of the two CoFeB-based parts and the two
Co/Pt-based parts of the tunnel junction decay at different rates during
annealing. The ferromagnet exchange coupling through the texture-breaking Ta
layer fails above 375$^{\circ}$C. The Ru spacer meant to promote a synthetic
antiferromagnet behavior is also insufficiently robust to annealing. Based on
these evolutions we propose optimization routes for the next generation tunnel
junctions.",1603.02824v1
2017-06-02,Dzyaloshinskii Moriya interaction across antiferromagnet / ferromagnet interface,"The antiferromagnet (AFM) / ferromagnet (FM) interfaces are of central
importance in recently developed pure electric or ultrafast control of FM
spins, where the underlying mechanisms remain unresolved. Here we report the
direct observation of Dzyaloshinskii Moriya interaction (DMI) across the AFM/FM
interface of IrMn/CoFeB thin films. The interfacial DMI is quantitatively
measured from the asymmetric spin wave dispersion in the FM layer using
Brillouin light scattering. The DMI strength is enhanced by a factor of 7 with
increasing IrMn layer thickness in the range of 1- 7.5 nm. Our findings provide
deeper insight into the coupling at AFM/FM interface and may stimulate new
device concepts utilizing chiral spin textures such as magnetic skyrmions in
AFM/FM heterostructures.",1706.00535v1
2017-06-26,Enhancement of thermovoltage and tunnel magneto-Seebeck effect in CoFeB based magnetic tunnel junctions by variation of the MgAl$_2$O$_4$ and MgO barrier thickness,"We investigate the influence of the barrier thickness of
Co$_{40}$Fe$_{40}$B$_{20}$ based magnetic tunnel junctions on the laser-induced
tunnel magneto-Seebeck effect. Varying the barrier thickness from 1nm to 3nm,
we find a distinct maximum in the tunnel magneto-Seebeck effect for 2.6nm
barrier thickness. This maximum is independently measured for two barrier
materials, namely MgAl$_2$O$_4$ and MgO. Additionally, samples with an
MgAl$_2$O$_4$ barrier exhibit a high thermovoltage of more than 350$\mu$V in
comparison to 90$\mu$V for the MTJs with MgO barrier when heated with the
maximum laser power of 150mW. Our results allow for the fabrication of improved
stacks when dealing with temperature differences across magnetic tunnel
junctions for future applications in spin caloritronics, the emerging research
field that combines spintronics and themoelectrics.",1706.08287v1
2018-04-11,Broadband voltage rectifier induced by linear bias dependence in CoFeB/MgO magnetic tunnel junctions,"In this paper, the perpendicular magnetic anisotropy (PMA) is tailored by
changing the thickness of the free layer with the objective of producing MTJ
nano-pillars with smooth linear resistance dependence with both in-plane
magnetic field and DC bias. We furthermore demonstrate how this linear bias
dependence can be used to create a zero-threshold broadband voltage rectifier,
a feature which is important for rectification in wireless charging and energy
harvesting applications. By carefully balancing the amount of PMA acting in the
free layer the measured RF to DC voltage conversion efficiency can be made as
large as 11%.",1804.04104v1
2018-04-14,Correlation between Dzyaloshinskii Moriya interaction and spin mixing conductance at antiferromagnet / ferromagnet interface,"The rich interaction phenomena at antiferromagnet (AFM)/ ferromagnet (FM)
interfaces are key ingredients in AFM spintronics, where many underlying
mechanisms remain unclear. Here we report a correlation observed between
interfacial Dzyaloshinskii-Moriya interaction (DMI) Ds and effective spin
mixing conductance g at IrMn/CoFeB interface. Both Ds and g are quantitatively
determined with Brillouin light scattering measurements, and increase with IrMn
thickness in the range of 2.5~7.5 nm. Such correlation likely originates from
the AFM-states-mediated spin-flip transitions in FM, which promote both
interfacial DMI and spin pumping effect. Our findings provide deeper insight
into the AFM-FM interfacial coupling for future spintronic design.",1804.05151v2
2018-04-18,Ferromagnetic resonance linewidth in coupled layers with easy-plane and perpendicular magnetic anisotropies,"Magnetic bilayers with different magnetic anisotropy directions are
interesting for spintronic appli- cations as they offer the possibility to
engineer tilted remnant magnetization states. We investigate the ferromagnetic
resonance (FMR) linewidth of modes associated with two interlayer exchange-
coupled ferromagnetic layers, the first a CoNi multilayer with a perpendicular
magnetic anisotropy, and the second a CoFeB layer with an easy-plane
anisotropy. For antiferromagnetic interlayer ex- change coupling, elevated FMR
linewidths are observed below a characteristic field. This is in contrast to
what is found in uncoupled, ferromagnetically coupled and single ferromagnetic
layers in which the FMR linewidth increases monotonically with field. We show
that the characteristic field at which there is a dramatic increase in FMR
linewidth can be understood using a macrospin model with Heisenberg-type
exchange coupling between the layers.",1804.06796v1
2019-03-01,Individual skyrmion manipulation by local magnetic field gradients,"Magnetic skyrmions are topologically protected spin textures, stabilised in
systems with strong Dzyaloshinskii-Moriya interaction (DMI). Several studies
have shown that electrical currents can move skyrmions efficiently through
spin-orbit torques. While promising for technological applications,
current-driven skyrmion motion is intrinsically collective and accompanied by
undesired heating effects. Here we demonstrate a new approach to control
individual skyrmion positions precisely, which relies on the magnetic
interaction between sample and a magnetic force microscopy (MFM) probe. We
investigate perpendicularly magnetised X/CoFeB/MgO multilayers, where for X = W
or Pt the DMI is sufficiently strong to allow for skyrmion nucleation in an
applied field. We show that these skyrmions can be manipulated individually
through the local field gradient generated by the scanning MFM probe with an
unprecedented level of accuracy. Furthermore, we show that the probe stray
field can assist skyrmion nucleation. Our proof-of-concepts results offer
current-free paradigms to efficient individual skyrmion control.",1903.00367v1
2019-03-04,Thermoelectric microscopy of magnetic skyrmions,"The magnetic skyrmion is a nanoscale topological object characterized by the
winding of the magnetic moments, appearing in magnetic materials with broken
inversion symmetry. Because of its low current threshold for driving, the
skyrmions have been intensely studied toward novel storage applications by
using electron-beam, X-ray, and visible light microscopies. Here, we show that
the skyrmions can be imaged via thermoelectric signals of spin-caloritronic
phenomena in combination with focused heating. We applied a local temperature
gradient to a CoFeB/Ta/W multilayer film with scanning a heating position and,
by exploiting a Hall bar structure, mapped the magnitude as well as the
direction of the resultant thermoelectric current distributions, which link to
skyrmions' inner magnetic textures. This method also enables the observation of
skyrmion dynamics under driving current pulses, being a useful imaging
technique for the development of skyrmion devices.",1903.01037v2
2019-03-08,MFM and FORC+ study of switching mechanism in Co$_{25}$Pd$_{75}$ films,"Recent research on CoPd alloys with perpendicular magnetic anisotropy (PMA)
has suggested that they might be useful as the pinning layer in CoFeB/MgO-based
perpendicular magnetic tunnel junctions (pMTJ's) for various spintronic
applications such as spin-torque transfer random access memory (STT-RAM). We
have previously studied the effect of seed layer and composition on the
structure (by XRD, SEM, AFM and TEM) and performance (coercivity) of these CoPd
films. These films do not switch coherently, so the coercivity is determined by
the details of the switching mechanism, which was not studied in our previous
paper. In the present paper, we show that information can be obtained about the
switching mechanism from magnetic force microscopy (MFM) together with first
order reversal curves (FORC), despite the fact that MFM can only be used at
zero field. We find that these films switch by a mechanism of domain nucleation
and dendritic growth into a labyrinthine structure, after which the unreversed
domains gradually shrink to small dots and then disappear.",1903.03568v2
2020-03-26,Bipolar spin Hall nano-oscillators,"We demonstrate a novel type of spin Hall nano-oscillator (SHNO) that allows
for efficient tuning of magnetic auto-oscillations over an extended range of
gigahertz frequencies, using bipolar direct currents at constant magnetic
fields. This is achieved by stacking two distinct ferromagnetic layers with a
platinum interlayer. In this device, the orientation of the spin polarised
electrons accumulated at the top and bottom interfaces of the platinum layer is
switched upon changing the polarity of the direct current. As a result, the
effective anti-damping required to drive large amplitude auto-oscillations can
appear either at the top or bottom magnetic layer. Tuning of the
auto-oscillation frequencies by several gigahertz can be obtained by combining
two materials with sufficiently different saturation magnetization. Here we
show that the combination of NiFe and CoFeB can result in 3 GHz shifts in the
auto-oscillation frequencies. Bipolar SHNOs as such may bring enhanced
synchronisation capabilities to neuromorphic computing applications.",2003.11776v1
2020-06-02,Spin pumping and inverse spin Hall effect in iridium oxide,"Large charge-to-spin conversion (spin Hall angle) and spin Hall conductivity
are prerequisites for development of next generation power efficient spintronic
devices. In this context, heavy metals (e.g. Pt, W etc.), topological
insulators, antiferromagnets are usually considered because they exhibit high
spin-orbit coupling (SOC). In addition to the above materials, 5d transition
metal oxide e.g. Iridium Oxide (IrO 2 ) is a potential candidate which exhibits
high SOC strength. Here we report a study of spin pumping and inverse spin Hall
effect (ISHE), via ferromagnetic resonance (FMR), in IrO 2 /CoFeB system. We
identify the individual contribution of spin pumping and other spin
rectification effects in the magnetic layer, by investigating the in-plane
angular dependence of ISHE signal. Our analysis shows significant contribution
of spin pumping effect to the ISHE signal. We show that polycrystalline IrO 2
thin film exhibits high spin Hall conductivity and spin Hall angle which are
comparable to the values of Pt.",2006.01865v2
2020-06-03,Giant voltage control of spin Hall nano-oscillator damping,"Spin Hall nano-oscillators (SHNOs) are emerging spintronic devices for
microwave signal generation and oscillator based neuromorphic computing
combining nano-scale footprint, fast and ultra-wide microwave frequency
tunability, CMOS compatibility, and strong non-linear properties providing
robust large-scale mutual synchronization in chains and two-dimensional arrays.
While SHNOs can be tuned via magnetic fields and the drive current, neither
approach is conducive for individual SHNO control in large arrays. Here, we
demonstrate electrically gated W/CoFeB/MgO nano-constrictions in which the
voltage-dependent perpendicular magnetic anisotropy, tunes the frequency and,
thanks to nano-constriction geometry, drastically modifies the spin-wave
localization in the constriction region resulting in a giant 42 % variation of
the effective damping over four volts. As a consequence, the SHNO threshold
current can be strongly tuned. Our demonstration adds key functionality to
nano-constriction SHNOs and paves the way for energy-efficient control of
individual oscillators in SHNO chains and arrays for neuromorphic computing.",2006.02151v1
2020-07-04,Entropy-reduced retention times in magnetic memory elements: A case of the Meyer-Neldel Compensation Rule,"We compute mean waiting times between thermally-activated magnetization
reversals in a nanodisk with parameters similar to a free CoFeB layer used in
magnetic random access memories. By combining Langer's theory and forward flux
sampling simulations, we show that the Arrhenius prefactor can take values up
to 10$^{21}$ Hz, orders of magnitude beyond the value of 10$^{9}$ Hz typically
assumed, and varies drastically as a function of material parameters. We show
that the prefactor behaves like an exponential of the activation energy, which
highlights a case of the Meyer-Neldel compensation rule. This suggests that
modeling information retention times with a barrier-independent prefactor in
such magnetic storage elements is not justified.",2007.02152v1
2020-10-14,Spin torque gate magnetic field sensor,"Spin-orbit torque provides an efficient pathway to manipulate the magnetic
state and magnetization dynamics of magnetic materials, which is crucial for
energy-efficient operation of a variety of spintronic devices such as magnetic
memory, logic, oscillator, and neuromorphic computing. Here, we describe and
experimentally demonstrate a strategy for the realization of a spin torque gate
magnetic field sensor with extremely simple structure by exploiting the
longitudinal field dependence of the spin torque driven magnetization
switching. Unlike most magnetoresistance sensors which require a delicate
magnetic bias to achieve a linear response to the external field, the spin
torque gate sensor can achieve the same without any magnetic bias, which
greatly simplifies the sensor structure. Furthermore, by driving the sensor
using an ac current, the dc offset is automatically suppressed, which
eliminates the need for a bridge or compensation circuit. We verify the concept
using the newly developed WTe2/Ti/CoFeB trilayer and demonstrate that the
sensor can work linearly in the range of 3-10 Oe with negligible dc offset.",2010.07158v1
2020-11-06,Picosecond Switching of Optomagnetic Tunnel Junctions,"Perpendicular magnetic tunnel junctions are one of the building blocks for
spintronic memories, which allow fast nonvolatile data access, offering
substantial potentials to revolutionize the mainstream computing architecture.
However, conventional switching mechanisms of such devices are fundamentally
hindered by spin polarized currents4, either spin transfer torque or spin orbit
torque with spin precession time limitation and excessive power dissipation.
These physical constraints significantly stimulate the advancement of modern
spintronics. Here, we report an optomagnetic tunnel junction using a
spintronic-photonic combination. This composite device incorporates an
all-optically switchable Co/Gd bilayer coupled to a CoFeB/MgO-based
perpendicular magnetic tunnel junction by the Ruderman-Kittel-Kasuya-Yosida
interaction. A picosecond all-optical operation of the optomagnetic tunnel
junction is explicitly confirmed by time-resolved measurements. Moreover, the
device shows a considerable tunnel magnetoresistance and thermal stability.
This proof-of-concept device represents an essential step towards ultrafast
spintronic memories with THz data access, as well as ultralow power
consumption.",2011.03612v1
2020-12-10,Observation of Magnetic Droplets in Magnetic Tunnel Junctions,"Magnetic droplets, a class of highly non-linear magnetodynamical solitons,
can be nucleated and stabilized in nanocontact spin-torque nano-oscillators
where they greatly increase the microwave output power. Here, we experimentally
demonstrate magnetic droplets in magnetic tunnel junctions (MTJs). The droplet
nucleation is accompanied by a power increase of over 300 times compared to its
ferromagnetic resonance modes. The nucleation and stabilization of droplets are
ascribed to the double-CoFeB free layer structure in the all-perpendicular MTJ
which provides a low Zhang-Li torque and a high pinning field. Our results
enable better electrical sensitivity in the fundamental studies of droplets and
show that the droplets can be utilized in MTJ-based applications.",2012.05596v1
2020-12-20,Creation of a Chiral Bobber Lattice in Helimagnet-Multilayer Heterostructures,"A chiral bobber is a localized three-dimensional magnetization configuration,
terminated by a singularity. Chiral bobbers coexist with magnetic skyrmions in
chiral magnets, lending themselves to new types of skyrmion-complementary bits
of information. However, the on-demand creation of bobbers, as well as their
direct observation remained elusive. Here, we introduce a new mechanism for
creating a stable chiral bobber lattice state via the proximity of two skyrmion
species with comparable size. This effect is experimentally demonstrated in a
Cu$_2$OSeO$_3$/[Ta/CoFeB/MgO]$_4$ heterostructure in which an exotic bobber
lattice state emerges in the phase diagram of Cu$_2$OSeO$_3$. To unambiguously
reveal the existence of the chiral bobber lattice state, we have developed a
novel characterization technique, magnetic truncation rod analysis, which is
based on resonant elastic x-ray scattering.",2012.10924v1
2021-01-10,A Thermodynamic Core using Voltage-Controlled Spin-Orbit-Torque Magnetic Tunnel Junctions,"We present a magnetic implementation of a thermodynamic computing fabric.
Magnetic devices within computing cores harness thermodynamics through its
voltage-controlled thermal stability; while the evolution of network states is
guided by the spin-orbit-torque effect. We theoretically derive the dynamics of
the cores and show that the computing fabric can successfully compute ground
states of a Boltzmann Machine. Subsequently, we demonstrate the physical
realization of these devices based on a CoFeB-MgO magnetic tunnel junction
structure. The results of this work pave the path towards the realization of
highly efficient, high-performance thermodynamic computing hardware. Finally,
this paper will also give a perspective of computing beyond thermodynamic
computing.",2101.03448v3
2021-04-07,Magnetic Reversal and Critical Current Transparency of CoFeB Superconductor-Ferromagnet-Superconductor Heterostructures,"In this work, we show fundamental low temperature (T) magnetic and Ic
responses of a magnetic Josephson Junction (MJJ) S/F/S heterostructure - Nb/
Co56Fe24B20 /Nb. The ultra-thin Co56Fe24B20 (CFB) films (0.6-1.3 nm) were
deposited onto two separate buffer layers: 150 nm Nb/5 nm Cu and 150 nm Nb/ (1
nm Cu/0.5 nm Nb)6/1 nm Cu. Both film sets were capped with 5 nm Cu/50 nm Nb.
Magnetic results show reduced switching distributions in patterned arrays
measured at near liquid Helium temperature (~ 10 K), with the incorporation of
the (1 nm Cu/0.5 nm Nb)6/1 nm multilayer. In electrical devices, the critical
current (Ic) through the CFB layer decays exponentially with increasing
ferromagnetic layer thickness and shows a dip in Ic at 0.8 nm, characteristic
of a change in the equilibrium Josephson phase in an S/F/S structure.",2104.03188v1
2021-06-22,Room And Cryogenic Temperature Behaviour of Magnetic Sensors Based on Gan/Si Single Saw Resonators,"This work analyzes resonance frequency shift vs. the applied magnetic field
strength for GHz operating GaN/Si SAW single resonators. Magnetostrictive
elements (Ni and CoFeB) were deposited in the proximity of the interdigitated
transducers (IDTs) of the resonators (A-type structures) and also over the
IDTs, after covering them with a BCB layer to avoid short circuits with IDTs
metal (B-type structures). This work targets emerging applications of SAW
resonators in driving spin wave pumping and in coupling of surface acoustic
waves (SAW) with superconducting Q-bits. Magnetic sensitivity of the SAWs was
analyzed at room temperature (RT) and at cryogenic temperatures, obtaining high
magnetic sensitivities at 16 K. According to our knowledge, GaN based SAWs are
first time used in magnetic applications; also, cryogenic behavior of magnetic
SAW sensors is first time analyzed.",2106.11605v1
2021-08-01,Terahertz charge and spin transport in metallic ferromagnets: the role of crystalline and magnetic order,"We study the charge and spin dependent scattering in a set of CoFeB thin
films whose crystalline order is systematically enhanced and controlled by
annealing at increasingly higher temperatures. Terahertz conductivity
measurements reveal that charge transport closely follows the development of
the crystalline phase, with increasing structural order leading to higher
conductivity. The terahertz-induced ultrafast demagnetization, driven by
spin-flip scattering mediated by the spin-orbit interaction, is measurable in
the pristine amorphous sample and much reduced in the sample with highest
crystalline order. Surprisingly, the largest demagnetization is observed at
intermediate annealing temperatures, where the enhancement in spin-flip
probability is not associated with an increased charge scattering. We are able
to correlate the demagnetization amplitude with the magnitude of the in-plane
magnetic anisotropy, which we characterize independently, suggesting a
magnetoresistance-like description of the phenomenon.",2108.00456v1
2021-11-12,Fabrication of voltage gated spin Hall nano-oscillators,"We demonstrate an optimized fabrication process for electric field (voltage
gate) controlled nano-constriction spin Hall nano-oscillators (SHNOs),
achieving feature sizes of <30 nm with easy to handle ma-N 2401 e-beam
lithography negative tone resist. For the nanoscopic voltage gates, we utilize
a two-step tilted ion beam etching approach and through-hole encapsulation
using 30 nm HfO<sub>x</sub>. The optimized tilted etching process reduces
sidewalls by 75% compared to no tilting. Moreover, the HfO<sub>x</sub>
encapsulation avoids any sidewall shunting and improves gate breakdown. Our
experimental results on W/CoFeB/MgO/SiO<sub>2</sub> SHNOs show significant
frequency tunability (6 MHz/V) even for moderate perpendicular magnetic
anisotropy. Circular patterns with diameter of 45 nm are achieved with an
aspect ratio better than 0.85 for 80% of the population. The optimized
fabrication process allows incorporating a large number of individual gates to
interface to SHNO arrays for unconventional computing and densely packed
spintronic neural networks.",2111.06957v1
2021-11-26,Three-dimensional Resonant Magnetization Dynamics Unraveled by Time-Resolved Soft X-ray Laminography,"The imaging of magneto-dynamical processes has been, so far, mostly a
two-dimensional business, due to the constraints of the available experimental
techniques. In this manuscript, building on the recent developments of soft
X-ray magnetic laminography, we present an experimental setup where
magneto-dynamical processes can be resolved in all three spatial dimensions and
in time, with the possibility to freely tune the frequency of the dynamical
process. We then employ this setup to investigate the three-dimensional
dynamics of two resonant magneto-dynamical modes in a CoFeB microstructure
occurring at different frequencies, namely the fundamental vortex gyration mode
and a magnetic field-induced domain wall excitation mode. This new technique
provides much needed capabilities for the experimental investigation of the
magnetization dynamics of three-dimensional magnetic systems.",2111.13533v1
2022-01-11,Perpendicular magnetic tunnel junctions with multi-interface free layer,"Future generations of magnetic random access memory demand magnetic tunnel
junctions that can provide simultaneously high magnetoresistance, strong
retention, low switching energy and small cell size below 10nm. Here we study
perpendicular magnetic tunnel junctions with composite free layers where
multiple ferromagnet/nonmagnet interfaces can contribute to the thermal
stability. Different nonmagnetic materials (MgO, Ta, Mo) have been employed as
the coupling layers in these multi-interface free layers. The evolution of
junction properties under different annealing conditions is investigated. A
strong dependence of tunneling magnetoresistance on the thickness of the first
CoFeB layer has been observed. In junctions where Mo and MgO are used as
coupling layers, large tunneling magnetoresistance above 200% has been achieved
after 400{\deg}C annealing.",2201.03798v1
2022-04-06,Power transfer in magnetoelectric resonators,"We derive an analytical model for the power transfer in a magnetoelectric
film bulk acoustic resonator consisting of a piezoelectric--magnetostrictive
bilayer. The model describes the dynamic magnetostrictive influence on the
elastodynamics via an effective frequency-dependent stiffness constant. This
allows for the calculation of both the magnetic and elastic power absorption in
the resonator as well as of its energy efficiency when such a resonator is
considered as a magnetic transducer. The model is then applied to example
systems consisting of piezoelectric ScAlN and magnetostrictive CoFeB, Ni, or
Terfenol-D layers.",2204.03072v2
2022-10-03,"Voltage control of frequency, effective damping and threshold current in nano-constriction-based spin Hall nano-oscillators","Using micromagnetic simulations, we study the interplay between strongly
voltage-controlled magnetic anisotropy (VCMA), $\Delta K = \pm$200 kJ/m$^3$,
and gate width, $w=$ 10--400 nm, in voltage-gated W/CoFeB/MgO based
nano-constriction spin Hall nano-oscillators. The VCMA modifies the local
magnetic properties such that the magnetodynamics transitions between regimes
of \emph{i}) confinement, \emph{ii}) tuning, and \emph{iii}) separation, with
qualitatively different behavior. We find that the strongest tuning is achieved
for gate widths of the same size as the the constriction width, for which the
effective damping can be increased an order of magnitude compared to its
intrinsic value. As a consequence, voltage control remains efficient over a
very large frequency range, and subsequent manufacturing advances could allow
SHNOs to be easily integrated into next-generation electronics for further
fundamental studies and industrial applications.",2210.01042v1
2022-10-11,Thermally-generated spin current in the topological insulator Bi$_2$Se$_3$,"We complete measurements of interconversions among the full triad of thermal
gradients, charge currents, and spin currents in the topological insulator
Bi$_2$Se$_3$ by quantifying the efficiency with which thermal gradients can
generate transverse spin currents. We accomplish this by comparing the spin
Nernst magneto-thermopower to the spin Hall magnesistance for bilayers of
Bi$_2$Se$_3$/CoFeB. We find that Bi$_2$Se$_3$ does generate substantial
thermally-driven spin currents. A lower bound for the ratio of spin current to
thermal gradient is $J_s/\nabla_x T$ = (4.9 $\pm$ 0.9) $\times$ 10$^{6}$
($\hbar/2e$) A m$^{-2}$ / K $\mu$m$^{-1}$, and a lower bound for the magnitude
of the spin Nernst ratio is $-$0.61 $\pm$ 0.11. The spin Nernst ratio for
Bi$_2$Se$_3$ is the largest among all materials measured to date, 2-3 times
larger compared to previous measurements for the heavy metals Pt and W.",2210.05636v1
2022-10-26,"Perpendicular magnetic anisotropy, tunneling magnetoresistance and spin-transfer torque effect in magnetic tunnel junctions with Nb layers","Nb and its compounds are widely used in quantum computing due to their high
superconducting transition temperatures and high critical fields. Devices that
combine superconducting performance and spintronic non-volatility could deliver
unique functionality. Here we report the study of magnetic tunnel junctions
with Nb as the heavy metal layers. An interfacial perpendicular magnetic
anisotropy energy density of 1.85 mJ/m2 was obtained in Nb/CoFeB/MgO
heterostructures. The tunneling magnetoresistance was evaluated in junctions
with different thickness combinations and different annealing conditions. An
optimized magnetoresistance of 120% was obtained at room temperature, with a
damping parameter of 0.011 determined by ferromagnetic resonance. In addition,
spin-transfer torque switching has also been successfully observed in these
junctions with a quasistatic switching current density of 7.3*10^5 A/cm2.",2210.14969v1
2022-10-30,"Interplay of symmetry-conserved tunneling, interfacial oxidation and perpendicular magnetic anisotropy in CoFeB/MgO-based junctions","The interfacial oxidation level and thermodynamic properties of the MgO-based
perpendicular magnetic tunneling junctions are investigated. The
symmetry-conserved tunneling effect depends sensitively on the MgO adatom
energy during the RF sputtering, as well as the thermal stability of the
structure during the post-growth thermal annealing. Two different failure modes
of the magnetoresistance are highlighted, involving with the decay of
perpendicular magnetic anisotropy and destruction of coherent tunneling
channels, respectively. Through the careful control of interfacial oxidation
level and proper selection of the heavy metal layers, both perpendicular
magnetic anisotropy and tunneling magnetoresistance of the junctions can be
increased.",2210.16734v1
2022-12-08,Acoustic-Driven Magnetic Skyrmion Motion,"Magnetic skyrmions have great potential for developing novel spintronic
devices. The electrical manipulation of skyrmions has mainly relied on
current-induced spin-orbit torques. A recent theoretical model suggested that
the skyrmions could be more efficiently manipulated by surface acoustic waves
(SAW), an elastic wave that can couple with magnetic moment through
magnetoelastic effect. However, the directional motion of skyrmions that is
driven by SAW is still missing. Here, we experimentally demonstrate the motion
of N\'eel-type skyrmions in Ta/CoFeB/MgO/Ta multilayers driven by propagating
SAW pulses from on-chip piezoelectric transducers. Our results reveal that the
elastic wave with longitudinal and shear vertical displacements (Rayleigh wave)
traps skyrmions, while the shear horizontal wave effectively drives the motion
of skyrmions. In particular, a longitudinal motion along the SAW propagation
direction and a transverse motion due to topological charge, are observed and
further confirmed by our micromagnetic simulations. This work demonstrates a
promising approach based on acoustic waves for manipulating skyrmions, which
could offer new opportunities for ultra-low power spintronics.",2212.04049v1
2023-04-30,Specific features of g $\approx$ 4.3 EPR line behavior in magnetic nanogranular composites,"Films of metal-insulator nanogranular composites M$_x$D$_{100-x}$ with
different composition and percentage of metal and dielectric phases (M = Fe,
Co, CoFeB; D = Al$_2$O$_3$, SiO$_2$, LiNbO$_3$; x $\approx$ 15-70 at.%) are
investigated by magnetic resonance in a wide range of frequencies (f = 7-37
GHz) and temperatures (T = 4.2-360 K). In addition to the usual ferromagnetic
resonance signal from an array of nanogranules, the experimental spectra
contain an additional absorption peak, which we associate with the electron
paramagnetic resonance (EPR) of Fe and Co ions dispersed in the insulating
space between the granules. In contrast to the traditional EPR of Fe and Co
ions in weakly doped non-magnetic matrices, the observed peak demonstrates a
number of unusual properties, which we explain by the presence of magnetic
interactions between ions and granules.",2305.00551v1
2023-05-16,Terahertz spin conductance probes of coherent and incoherent spin tunneling through MgO tunnel junctions,"We study femtosecond spin currents through MgO tunneling barriers in CoFeB(2
nm)|MgO($d$)|Pt(2 nm) stacks by terahertz emission spectroscopy. To obtain
transport information independent of extrinsic experimental factors, we
determine the complex-valued spin conductance $\tilde{G}_d (\omega)$ of the MgO
layer (thickness d= 0-6 {\AA} over a wide frequency range $(\omega/2\pi=$ 0.5-8
THz). In the time $(t)$ domain,$ G_d (t)$ has an instantaneous and delayed
component that point to (i) spin transport through Pt pinholes in MgO, (ii)
coherent spin tunneling and (iii) incoherent resonant spin tunneling mediated
by defect states in MgO. A remarkable signature of (iii) is its relaxation time
that grows monotonically with $d$ to as much as 270 fs at $d= 6$ {\AA}, in full
agreement with an analytical model. Our results indicate that terahertz spin
conductance spectroscopy will yield new and relevant insights into ultrafast
spin transport for a wide range of materials.",2305.09074v2
2023-05-18,Observation and enhancement of room temperature bilinear magnetoelectric resistance in sputtered topological semimetal Pt3Sn,"Topological semimetal materials have become a research hotspot due to their
intrinsic strong spin-orbit coupling which leads to large charge-to-spin
conversion efficiency and novel transport behaviors. In this work, we have
observed a bilinear magnetoelectric resistance (BMER) of up to 0.1 nm2A-1Oe-1
in a singlelayer of sputtered semimetal Pt3Sn at room temperature. Different
from previous observations, the value of BMER in sputtered Pt3Sn does not
change out-of-plane due to the polycrystalline nature of Pt3Sn. The observation
of BMER provides strong evidence of the existence of spin-momentum locking in
the sputtered polycrystalline Pt3Sn. By adding an adjacent CoFeB magnetic
layer, the BMER value of this bilayer system is doubled compared to the single
Pt3Sn layer. This work broadens the material system in BMER study, which paves
the way for the characterization of topological states and applications for
spin memory and logic devices.",2305.10720v2
2023-07-07,Orbitronics: Light-induced Orbit Currents in Terahertz Emission Experiments,"Orbitronics is based on the use of orbit currents as information carriers. Up
to now, orbit currents were created from the conversion of charge or spin
currents, and inversely, they could be converted back to charge or spin
currents. Here we demonstrate that orbit currents can also be generated by
femtosecond light pulses on Ni. In multilayers associating Ni with oxides and
nonmagnetic metals such as Cu, we detect the orbit currents by their conversion
into charge currents and the resulting terahertz emission. We show that the
orbit currents extraordinarily predominate the light-induced spin currents in
Ni-based systems, whereas only spin currents can be detected with CoFeB-based
systems. In addition, the analysis of the time delays of the terahertz pulses
leads to relevant information on the velocity and propagation of orbit
carriers. Our finding of light-induced orbit currents and our observation of
their conversion into charge currents opens new avenues in orbitronics,
including the development of orbitronic terahertz devices.",2307.03490v1
2023-09-06,Shaping magnetization dynamics in a planar square dot by adjusting its surface anisotropy,"A planar square dot is one of the simplest structures confined to three
dimensions. Despite its geometrical simplicity, the description of the spin
wave modes in this structure is not trivial due to the competition of dipolar
and exchange interactions. An additional factor that makes this description
challenging are the boundary conditions depend both on non-local dipolar
interactions and local surface parameters such as surface anisotropy. In the
presented work, we showed how the surface anisotropy applied at the lateral
faces of the dot can tune the frequency of fundamental mode in the planar CoFeB
dot, magnetized in an out-of-plane direction. Moreover, we analyzed the spin
wave profile of the fundamental mode and the corresponding dynamic stray field.
We showed that the asymmetric application of surface anisotropy produces an
asymmetric profile of dynamic stray field for square dot and can be used to
tailor inter-dot coupling. The calculations were performed with the use of the
finite-element method.",2309.02984v1
2023-09-22,Strongly Coupled Spin Waves and Surface Acoustic Waves at Room Temperature,"Here, we report the observation of strong coupling between magnons and
surface acoustic wave (SAW) phonons in a thin CoFeB film constructed in an
on-chip SAW resonator by analyzing SAW phonon dispersion anticrossings. Our
device design provides the tunability of the film thickness with a fixed phonon
wavelength, which is a departure from the conventional approach in strong
magnon--phonon coupling research. We detect a monotonic increase in the
coupling strength by expanding the film thickness, which agrees with our
theoretical model. Our work offers a significant way to advance fundamental
research and the development of devices based on magnon--phonon hybrid
quasiparticles.",2309.12690v1
2023-10-20,Visualization of skyrmion-superconducting vortex pairs in a chiral magnet-superconductor heterostructure,"Magnetic skyrmions, the topological states possessing chiral magnetic
structure with non-trivial topology, have been widely investigated as a
promising candidate for spintronic devices. They can also couple with
superconducting vortices to form skyrmion-vortex pairs, hosting Majorana zero
mode which is a potential candidate for topological quantum computering. A lot
of theoretical proposals have been put forward on constructing skyrmion-vortex
pairs in heterostructures of chiral magnet and superconductor. Nevertheless,
how to generate skyrmion-vortex pairs in a controllable way experimentally
remains a significant challenge. We have designed a heterostructure of chiral
magnet and superconductor [CoFeB/Ir/Ta]7/Nb in which zero field N\'eel-type
skyrmions can be stabilized and the superconducting vortices can couple with
the skyrmions when Nb is in the superconducting state. We have directly
observed the formation of skyrmion-superconducting vortex pairs which is
dependent on the direction of the applied magnetic field. Our results provide
an effective method to manipulate the quantum states of skyrmions with the help
of superconducting vortices, which can be used to explore the possible
existence of Majorana zero mode for future quantum computation.",2310.13363v1
2023-10-26,On-chip all-electrical determination of the magnetoelastic coupling constant of magnetic heterostructures,"We have developed an approach to determine the magnetoelastic coupling
constant of magnetic layers in thin film heterostructures. The film is formed
on a piezoelectric substrate between two interdigital transducers (IDT), a
platform often used to construct a surface acoustic wave device. With the
substrate piezoelectricity, strain is induced into the film by applying a dc
voltage to the IDTs. The strain causes changes in the magnetization direction
of the magnetic layer, which is probed by measuring changes, if any, in the
transverse resistance of the heterostructure. We find the extracted
magnetoelastic coupling constant of the magnetic layer (CoFeB) depends on the
film stacking. Such change can be accounted for provided that the elastic
properties of the layers that constitute the heterostructures are taken into
account. The on-chip all-electrical approach described here provides a
versatile means to quantitatively assess the magnetoelastic coupling constant
of thin film heterostructures.",2310.17215v1
2023-12-06,Large Non-Volatile Frequency Tuning of Spin Hall Nano-Oscillators using Circular Memristive Nano-Gates,"Spin Hall nano oscillators (SHNOs) are promising candidates for neuromorphic
computing due to their miniaturized dimensions, non-linearity, fast dynamics,
and ability to synchronize in long chains and arrays. However, tuning the
individual SHNOs in large chains/arrays, which is key to implementing synaptic
control, has remained a challenge. Here, we demonstrate circular memristive
nano-gates, both precisely aligned and shifted with respect to
nano-constriction SHNOs of W/CoFeB/HfOx, with increased quality of the device
tunability. Gating at the exact center of the nano-constriction region is found
to cause irreversible degradation to the oxide layer, resulting in a permanent
frequency shift of the auto-oscillating modes. As a remedy, gates shifted
outside of the immediate nano-constriction region can tune the frequency
dramatically (>200 MHz) without causing any permanent change to the
constriction region. Circular memristive nano-gates can, therefore, be used in
SHNO chains/arrays to manipulate the synchronization states precisely over
large networks of oscillators.",2312.03352v2
2008-05-16,The influence of intergranular interaction on the magnetization of the ensemble of oriented Stoner-Wohlfarth nanoparticles,"We consider the influence of interparticle interaction on the magnetization
reversal in the oriented Stoner-Wohlfarth nanoparticles ensemble. To do so, we
solve a kinetic equation for the relaxation of the overall ensemble
magnetization to its equilibrium value in some effective mean field. Latter
field consists of external magnetic field and interaction mean field
proportional to the instantaneous value of above magnetization. We show that
the interparticle interaction influences the temperature dependence of a
coercive field. This influence manifests itself in the noticeable coercivity at
$T>T_{b}$ ($T_{b}$ is so-called blocking temperature). The above interaction
can also lead to a formation of the ""superferromagnetic"" state with correlated
directions of particle magnetic moments at $T>T_{b}$. This state possesses
coercivity if the overall magnetization has a component directed along the easy
axis of each particle. We have shown that the coercive field in the
""superferromagnetic"" state does not depend on measuring time. This time
influences both $T_{b}$ and the temperature dependence of coercive field at
$T<T_{b}$. We corroborate our theoretical results by measurements on
nanogranular films (CoFeB)$_{x}$-(SiO$_{2}$)$_{1-x}$ with concentration of
ferromagnetic particles close, but below percolation threshold.",0805.2463v3
2009-07-22,Quantized spin wave modes in magnetic tunnel junction nanopillars,"We present an experimental and theoretical study of the magnetic field
dependence of the mode frequency of thermally excited spin waves in rectangular
shaped nanopillars of lateral sizes 60x100, 75x150, and 105x190 nm2, patterned
from MgO-based magnetic tunnel junctions. The spin wave frequencies were
measured using spectrally resolved electrical noise measurements. In all
spectra, several independent quantized spin wave modes have been observed and
could be identified as eigenexcitations of the free layer and of the synthetic
antiferromagnet of the junction. Using a theoretical approach based on the
diagonalization of the dynamical matrix of a system of three coupled, spatially
confined magnetic layers, we have modeled the spectra for the smallest pillar
and have extracted its material parameters. The magnetization and exchange
stiffness constant of the CoFeB free layer are thereby found to be
substantially reduced compared to the corresponding thin film values. Moreover,
we could infer that the pinning of the magnetization at the lateral boundaries
must be weak. Finally, the interlayer dipolar coupling between the free layer
and the synthetic antiferromagnet causes mode anticrossings with gap openings
up to 2 GHz. At low fields and in the larger pillars, there is clear evidence
for strong non-uniformities of the layer magnetizations. In particular, at zero
field the lowest mode is not the fundamental mode, but a mode most likely
localized near the layer edges.",0907.3792v2
2014-09-02,Switching of Perpendicularly Polarized Nanomagnets with Spin Orbit Torque without an External Magnetic Field by Engineering a Tilted Anisotropy,"Spin orbit torque (SOT) provides an efficient way of generating spin current
that promises to significantly reduce the current required for switching
nanomagnets. However, an in-plane current generated SOT cannot
deterministically switch a perpendicularly polarized magnet due to symmetry
reasons. On the other hand, perpendicularly polarized magnets are preferred
over in-plane magnets for high-density data storage applications due to their
significantly larger thermal stability in ultra-scaled dimensions. Here we show
that it is possible switch a perpendicularly polarized magnet by SOT without
needing an external magnetic field. This is accomplished by engineering an
anisotropy in the magnets such that the magnetic easy axis slightly tilts away
from the film-normal. Such a tilted anisotropy breaks the symmetry of the
problem and makes it possible to switch the magnet deterministically. Using a
simple Ta/CoFeB/MgO/Ta heterostructure, we demonstrate reversible switching of
the magnetization by reversing the polarity of the applied current. This
demonstration presents a new approach for controlling nanomagnets with spin
orbit torque.",1409.0620v3
2015-10-13,Efficient metallic spintronic emitters of ultrabroadband terahertz radiation,"Terahertz electromagnetic radiation is extremely useful for numerous
applications such as imaging and spectroscopy. Therefore, it is highly
desirable to have an efficient table-top emitter covering the 1-to-30-THz
window whilst being driven by a low-cost, low-power femtosecond laser
oscillator. So far, all solid-state emitters solely exploit physics related to
the electron charge and deliver emission spectra with substantial gaps. Here,
we take advantage of the electron spin to realize a conceptually new terahertz
source which relies on tailored fundamental spintronic and photonic phenomena
in magnetic metal multilayers: ultrafast photo-induced spin currents, the
inverse spin-Hall effect and a broadband Fabry-P\'erot resonance. Guided by an
analytical model, such spintronic route offers unique possibilities for
systematic optimization. We find that a 5.8-nm-thick W/CoFeB/Pt trilayer
generates ultrashort pulses fully covering the 1-to-30-THz range. Our novel
source outperforms laser-oscillator-driven emitters such as ZnTe(110) crystals
in terms of bandwidth, terahertz-field amplitude, flexibility, scalability and
cost.",1510.03729v2
2016-07-22,The Spin Nernst effect in Tungsten,"The spin Hall effect allows generation of spin current when charge current is
passed along materials with large spin orbit coupling. It has been recently
predicted that heat current in a non-magnetic metal can be converted into spin
current via a process referred to as the spin Nernst effect. Here we report the
observation of the spin Nernst effect in W. In W/CoFeB/MgO heterostructures, we
find changes in the longitudinal and transverse voltages with magnetic field
when temperature gradient is applied across the film. The field-dependence of
the voltage resembles that of the spin Hall magnetoresistance. A comparison of
the temperature gradient induced voltage and the spin Hall magnetoresistance
allows direct estimation of the spin Nernst angle. We find the spin Nernst
angle of W to be similar in magnitude but opposite in sign with its spin Hall
angle. Interestingly, under an open circuit condition, such sign difference
results in spin current generation larger than otherwise. These results
highlight the distinct characteristics of the spin Nernst and spin Hall
effects, providing pathways to explore materials with unique band structures
that may generate large spin current with high efficiency.",1607.06594v2
2017-09-08,Observation of Magnetic Radial Vortex Nucleation in a Multilayer Stack with Tunable Anisotropy,"Recently discovered exotic magnetic configurations, namely magnetic solitons
appearing in the presence of bulk or interfacial Dzyaloshinskii-Moriya
Interaction (i-DMI), have excited scientists to explore their potential
applications in emerging spintronic technologies such as race-track magnetic
memory, spin logic, radio frequency nano-oscillators and sensors. Such studies
are motivated by their foreseeable advantages over conventional micro-magnetic
structures due to their small size, topological stability and easy spin-torque
driven manipulation with much lower threshold current densities giving way to
improved storage capacity, and faster operation with efficient use of energy.
In this work, we show that in the presence of i-DMI in Pt/CoFeB/Ti multilayers
by tuning the magnetic anisotropy (both in-plane and perpendicular-to-plane)
via interface engineering and postproduction treatments, we can stabilize a
variety of magnetic configurations such as N\'eel skyrmions, horseshoes and
most importantly for the first time, the recently predicted isolated radial
vortices at room temperature and under zero bias field. Especially, the radial
vortex state with its absolute convergence to or divergence from a single point
can potentially offer exciting new applications such as particle
trapping/detrapping in addition to magnetoresistive memories with efficient
switching, where the radial vortex state can act as a source of spin-polarized
current with radial polarization.",1709.02876v1
2018-03-12,Electrical initialization of electron and nuclear spins in a single quantum dot at zero magnetic field,"The emission of circularly polarized light from a single quantum dot relies
on the injection of carriers with well-defined spin polarization. Here we
demonstrate single dot electroluminescence (EL) with a circular polarization
degree up to 35% at zero applied magnetic field. The injection of spin
polarized electrons is achieved by combining ultrathin CoFeB electrodes on top
of a spin-LED device with p-type InGaAs quantum dots in the active region. We
measure an Overhauser shift of several $\mu$eV at zero magnetic field for the
positively charged exciton (trion X$^+$) EL emission, which changes sign as we
reverse the injected electron spin orientation. This is a signature of dynamic
polarization of the nuclear spins in the quantum dot induced by the hyperfine
interaction with the electrically injected electron spin. This study paves the
way for electrical control of nuclear spin polarization in a single quantum dot
without any external magnetic field.",1803.04309v1
2018-06-20,Current-induced modulation of interfacial Dzyaloshinskii-Moriya interaction,"The Dzyaloshinskii-Moriya (DM) interaction is an antisymmetric exchange
interaction that is responsible for the emergence of chiral magnetism. The
origin of the DM interaction, however, remains to be identified albeit the
large number of studies reported on related effects. It has been recently
suggested that the DM interaction is equivalent to an equilibrium spin current
density originating from spin-orbit coupling, an effect referred to as the spin
Doppler effect. The model predicts that the DM interaction can be controlled by
spin current injected externally. Here we show that the DM exchange constant
($D$) in W/CoFeB based heterostructures can be modulated with external current
passed along the film plane. At higher current, $D$ decreases with increasing
current, which we infer is partly due to the adiabatic spin transfer torque. At
lower current, $D$ increases linearly with current regardless of the polarity
of current flow. The rate of increase in $D$ with the current density agrees
with that predicted by the model based on the spin Doppler effect. These
results imply that the DM interaction at the HM/FM interface partly originates
from an equilibrium interface spin (polarized) current which can be modulated
externally.",1806.07746v2
2018-06-25,Two-terminal spin-orbit torque magnetoresistive random access memory,"Spin-transfer torque magnetoresistive random access memory (STT-MRAM) is an
attractive alternative to current random access memory technologies due to its
non-volatility, fast operation and high endurance. STT-MRAM does though have
limitations including the stochastic nature of the STT-switching and a high
critical switching current, which makes it unsuitable for ultrafast operation
at nanosecond and sub-nanosecond regimes. Spin-orbit torque (SOT) switching,
which relies on the torque generated by an in-plane current, has the potential
to overcome these limitations. However, SOT-MRAM cells studied so far use a
three-terminal structure in order to apply the in-plane current, which
increases the size of the cells. Here we report a two-terminal SOT-MRAM cell
based on a CoFeB/MgO magnetic tunnel junction pillar on an ultrathin and narrow
Ta underlayer. In this device, an in-plane and out-of-plane current are
simultaneously generated upon application of a voltage, and we demonstrate that
the switching mechanism is dominated by SOT. We also compare our device to a
STT-MRAM cell built with the same architecture and show that critical write
current in the SOT-MRAM cell is reduced by more than 70%.",1806.09713v2
2018-09-14,Femtosecond control of terahertz spin-charge conversion in ferromagnetic heterostructures,"Employing electron spin instead of charge to develop spintronic devices holds
the merits of low-power consumption in information technologies. Meanwhile, the
demand for increasing speed in spintronics beyond current CMOS technology has
further triggered intensive researches for ultrafast control of spins even up
to unprecedent terahertz regime. The femtosecond laser has been emerging as a
potential technique to generate an ultrafast spin-current burst for
magnetization manipulation. However, there is a great challenge to establish
all-optical control and monitor of the femtosecond transient spin current. Deep
insights into the physics and mechanism are extremely essential for the
technique. Here, we demonstrate coherently nonthermal excitation of femtosecond
spin-charge current conversion parallel to the magnetization in W/CoFeB/Pt
heterostructures driven by linearly polarized femtosecond laser pulses. Through
systematical investigation we observe the terahertz emission polarization
depends on both the magnetization direction and structural asymmetry. We
attribute this phenomenon of the terahertz generation parallel to the
magnetization induced by linearly polarized femtosecond laser pulses probably
to inverse spin-orbit torque effect. Our work not only is beneficial to the
deep understanding of spin-charge conversion and spin transportation, but also
helps develop novel on-chip terahertz opto-spintronic devices.",1809.05391v2
2018-11-21,Strong Rashba-Edelstein Effect-Induced Spin-Orbit Torques in Monolayer Transition-Metal Dichalcogenide/Ferromagnet Bilayers,"The electronic and optoelectronic properties of two dimensional materials
have been extensively explored in graphene and layered transition metal
dichalcogenides (TMDs). Spintronics in these two-dimensional materials could
provide novel opportunities for future electronics, for example, efficient
generation of spin current, which should enable the efficient manipulation of
magnetic elements. So far, the quantitative determination of charge current
induced spin current and spin-orbit torques (SOTs) on the magnetic layer
adjacent to two-dimensional materials is still lacking. Here, we report a large
SOT generated by current-induced spin accumulation through the Rashba-Edelstein
effect in the composites of monolayer TMD (MoS$_2$ or WSe$_2$)/CoFeB bilayer.
The effective spin conductivity corresponding to the SOT turns out to be almost
temperature-independent. Our results suggest that the charge-spin conversion in
the chemical vapor deposition-grown large-scale monolayer TMDs could
potentially lead to high energy efficiency for magnetization reversal and
convenient device integration for future spintronics based on two-dimensional
materials.",1811.08583v1
2020-01-15,Nonreciprocal surface acoustic wave propagation via magneto-rotation coupling,"One of the most fundamental forms of magnon-phonon interaction is an
intrinsic property of magnetic materials, the ""magnetoelastic coupling"". This
particular form of interaction has been the basis for describing magnetic
materials and their strain related applications, where strain induces changes
of internal magnetic fields. Different from the magnetoelastic coupling, more
than 40 years ago, it was proposed that surface acoustic waves may induce
surface magnons via rotational motion of the lattice in anisotropic magnets.
However, a signature of this magnon-phonon coupling mechanism, termed
magneto-rotation coupling, has been elusive. Here, we report the first
observation and theoretical framework of the magneto-rotation coupling in a
perpendicularly anisotropic ultra-thin film Ta/CoFeB(1.6 nm)/MgO, which
consequently induces nonreciprocal acoustic wave attenuation with a
unprecedented ratio up to 100$\%$ rectification at the theoretically predicted
optimized condition. Our work not only experimentally demonstrates a
fundamentally new path for investigating magnon-phonon coupling, but also
justify the feasibility of the magneto-rotation coupling based application.",2001.05135v3
2020-01-15,Backward volume vs Damon-Eshbach: a travelling spin wave spectroscopy comparison,"We compare the characteristics of electrically transduced Damon-Eshbach
(DESWs) and backward volume (BVSWs) configurations within the same, 30 nm
thick, ferromagnetic, CoFeB waveguide. Sub-micron U-shaped antennas are used to
deliver the necessary in-plane and out-of-plane RF fields. We measure the
spin-wave transmission with respect to in-plane field orientation, frequency
and propagation distance. Unlike DESW, BVSWs are reciprocally transduced and
collected for either direction of propagation, but their ability to transport
energy is lower than DESWs for two reasons. This arises first because BVSW are
inductively transduced less efficiently than DESWs. Also, in the range of
wavevectors (~5 rad. um-1) typically excited by our antennas, the group
velocity of BVSWs stays lower than that of DESW, which leads to reduced
propagation ability that impact transmission signals in an exponential manner.
In contrast, the group velocity of DESWs is maximum at low fields and decreases
continuously with the applied field. The essential features of the measured SW
characteristics are well reciprocated by a simple, 1-D analytical model which
can be used to assess the potential of each configuration.",2001.05221v2
2021-07-20,Spin-wave dispersion measurement by variable-gap propagating spin-wave spectroscopy,"Magnonics is seen nowadays as a candidate technology for energy-efficient
data processing in classical and quantum systems. Pronounced nonlinearity,
anisotropy of dispersion relations and phase degree of freedom of spin waves
require advanced methodology for probing spin waves at room as well as at mK
temperatures. Yet, the use of the established optical techniques like Brillouin
light scattering (BLS) or magneto optical Kerr effect (MOKE) at ultra-low
temperatures is forbiddingly complicated. By contrast, microwave spectroscopy
can be used at all temperatures but is usually lacking spatial and wavenumber
resolution. Here, we develop a variable-gap propagating spin-wave spectroscopy
(VG-PSWS) method for the deduction of the dispersion relation of spin waves in
wide frequency and wavenumber range. The method is based on the phase-resolved
analysis of the spin-wave transmission between two antennas with variable
spacing, in conjunction with theoretical data treatment. We validate the method
for the in-plane magnetized CoFeB and YIG thin films in $k\perp B$ and
$k\parallel B$ geometries by deducing the full set of material and spin-wave
parameters, including spin-wave dispersion, hybridization of the fundamental
mode with the higher-order perpendicular standing spin-wave modes and surface
spin pinning. The compatibility of microwaves with low temperatures makes this
approach attractive for cryogenic magnonics at the nanoscale.",2107.09363v1
2017-03-18,Spin Based Biosensor with Hard Axis Assist for Enhanced Sensitivity,"We demonstrate the influence of hard axis assist on an in-plane polarized
nanomagnet layer to greatly enhance the sensitivity of a magnetic nano particle
(MNP) based MTJ biosensor. The hard axis assist has been provided to the
sensing layer in the forms of spin Hall Effect (SHE) induced spin injected
torque and stress based effective magnetic field induced torque separately.
Present work mainly focuses on the efficient and qualitative detection of a
single magnetic bead with the aim of detecting a single bimolecular recognition
event at an extremely low analyte concentration. Interfacial spin current
arising from spin orbit metal is expected to impose a torque on the free layer
along sensitive direction improving the signal strength by a factor of ~ 6.5
for a 100 nm bead at a height of 500 nm above the sensor surface. Furthermore,
the potentiality of a multiferroic composite consisting of a piezoelectric
layer coupled with magnetostrictive CoFeB based MTJ has been investigated in
the Biosensing applications. An external stress voltage of 500 mV has been
observed to be sufficient to enhance the sensitivity (~ 6 times). The use of
nanoscaled spin devices and the absence of external magnetic field operated
magnetization rotation facilitates in achieving highly compact and extremely
low power designs. This establishes the possibility of utilizing the present
schemes for advanced, highly sensitive, miniaturized and low power bioassaying
system-on-chip applications. Numerical results were compared with some earlier
reported experimental results to validate our proposed model.",1704.01555v1
2017-04-20,Coupled mode theory for the acoustic wave and spin wave interaction in the magphonic crystals: Propagating magnetoelastic waves,"We have investigated co-directional and contra-directional couplings between
spin wave and acoustic wave in one-dimensional periodic structure (magphonic
crystal). The system consists of two ferromagnetic layers alternating in space.
We have taken into consideration materials commonly used in magnonics: yttrium
iron garnet, CoFeB, permalloy, and cobalt. The coupled mode theory (CMT)
formalism have been successfully implemented to describe magnetoelastic
interaction as a periodic perturbation in the magphonic crystal. The results of
CMT calculations have been verified by more rigorous simulations by
frequency-domain plane wave method and time-domain finite element method. The
presented resonant coupling in the magphonic crystal is an active in-space
mechanism which spatially transfers energy between propagating spin and
acoustic modes, thus creating propagating magnetoelastic wave. We have shown,
that CMT analysis of the magnetoelastic coupling is an useful tool to optimize
and design a spin wave - acoustic wave transducer based on a magphonic
crystals. The effect of spin wave damping has been included to the model to
discuss the efficiency of such a device. Our model shows that it is possible to
obtain forward conversion of the acoustic wave to the spin wave in case of
co-directional coupling and backward conversion in case of contra-directional
coupling.",1704.06118v1
2018-07-17,Large anomalous Nernst effect in thin films of the Weyl semimetal Co2MnGa,"The magneto-thermoelectric properties of Heusler compound thin films are very
diverse. Here, we discuss the anomalous Nernst response of Co$_2$MnGa thin
films. We systematically study the anomalous Nernst coefficient as a function
of temperature, and we show that unlike the anomalous Hall effect, the
anomalous Nernst effect in Co$_2$MnGa strongly varies with temperature. We
exploit the on-chip thermometry technique to quantify the thermal gradient,
which enables us to directly evaluate the anomalous Nernst coefficient. We
compare these results to a reference CoFeB thin film. We show that the
50-nm-thick Co$_2$MnGa films exhibit a large anomalous Nernst effect of
-2$\mu$V/K at 300 K, whereas the 10-nm-thick Co$_2$MnGa film exhibits a
significantly smaller anomalous Nernst coefficient despite having similar
volume magnetizations. These findings suggest that the microscopic origin of
the anomalous Nernst effect in Co$_2$MnGa is complex and may contain
contributions from skew-scattering, side-jump or intrinsic Berry phase. In any
case, the large anomalous Nernst coefficent of Co$_2$MnGa thin films at room
temperature makes this material system a very promising candidate for efficient
spin-caloritronic devices.",1807.06487v1
2019-01-05,Demonstration of a strain-mediated magnetoelectric write and read unit in a Co60Fe20B20/ Pb(Mg1/3Nb2/3)0.7Ti0.3O3 heterostructure,"Taking advantage of the Magnetoelectric (ME) and its inverse effect, this
article demonstrates strain-mediated magnetoelectric write and read operations
simultaneously in Co60Fe20B20/ Pb(Mg1/3Nb2/3)0.7Ti0.3O3 heterostructures
without using any symmetry breaking magnetic field at room temperature. By
applying an external DC-voltage across a (011)-cut PMN-PT substrate, the
ferroelectric polarization is re-oriented, which results in an anisotropic
in-plane strain that transfers to the CoFeB thin film and changes its magnetic
anisotropy Hk. The change in Hk in-turn results in a 90o rotation of the
magnetic easy axis for sufficiently high voltages. Simultaneously, the inverse
effect is employed to read changes of the magnetic properties. Because the
Piezoelectric (PE)/FerroMagnetic (FM) system is fully coupled, the change of
magnetization in FM induces an elastic stress in the PE layer, which generates
a piezoelectric potential in the system that can be used to readout the
magnetic state of the FM layer. Our experimental results are in excellent
qualitative agreement with a recently proposed, experimentally benchmarked
equivalent circuit model that considers how magnetic properties are
electrically controlled in such ME/PE heterostructure and how a back-voltage is
generated due to changing magnetic properties in a self-consistent model.",1901.01368v1
2019-01-24,Topology-Dependent Brownian Gyromotion of a Single Skyrmion,"Non-interacting particles exhibiting Brownian motion have been observed in
many occasions of sciences, such as molecules suspended in liquids, optically
trapped microbeads, and spin textures in magnetic materials. In particular, a
detailed examination of Brownian motion of spin textures is important for
designing thermally stable spintronic devices which motivates the present
study. In this Letter, through using temporally and spatially resolved polar
magneto-optic Kerr effect (MOKE) microscopy, we have experimentally observed
the thermal fluctuation-induced random walk of a single isolated N\'eel-type
magnetic skyrmion in an interfacially asymmetric Ta/CoFeB/TaOx multilayer. An
intriguing topology dependent Brownian gyromotion behavior of skyrmions has
been identified. The onset of Brownian gyromotion of a single skyrmion induced
by the thermal effects, including a nonlinear temperature-dependent diffusion
coefficient and topology-dependent gyromotion are further formulated based on
the stochastic Thiele equation. The experimental and numerical demonstration of
topology-dependent Brownian gyromotion of skyrmions can be useful for
understanding the nonequilibrium magnetization dynamics and implementing
spintronic devices.",1901.08206v2
2018-02-02,Spin wave emission by spin-orbit torque antennas,"We study the generation of propagating spin waves in Ta/CoFeB waveguides by
spin-orbit torque antennas and compare them to conventional inductive antennas.
The spin-orbit torque was generated by a transverse microwave current across
the magnetic waveguide. The detected spin wave signals for an in-plane
magnetization across the waveguide (Damon-Eshbach configuration) exhibited the
expected phase rotation and amplitude decay upon propagation when the current
spreading was taken into account. Wavevectors up to about 6 rad/$\mu$m could be
excited by the spin-orbit torque antennas despite the current spreading,
presumably due to the non-uniformity of the microwave current. The relative
magnitude of generated anti-damping spin-Hall and Oersted fields was calculated
within an analytic model and it was found that they contribute approximately
equally to the total effective field generated by the spin-orbit torque
antenna. Due to the ellipticity of the precession in the ultrathin waveguide
and the different orientation of the anti-damping spin-Hall and Oersted fields,
the torque was however still dominated by the Oersted field. The prospects for
obtaining a pure spin-orbit torque response are discussed, as are the energy
efficiency and the scaling properties of spin-orbit torque antennas.",1802.00861v2
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
2019-02-19,Generation and Hall effect of skyrmions enabled via using nonmagnetic point contacts,"To enable functional skyrmion based spintronic devices, the controllable
generation and manipulation of skyrmions is essential. While the generation of
skyrmions by using a magnetic geometrical constriction has already been
demonstrated, this approach is difficult to combine with a subsequent
controlled manipulation of skyrmions. The high efficiency of skyrmion
generation from magnetic constrictions limits the useful current density,
resulting in stochastic skyrmion motion, which may obscure topological
phenomena such as the skyrmion Hall effect. In order to address this issue, we
designed a nonmagnetic conducting Ti/Au point contact in devices made of
Ta/CoFeB/TaOx trilayer films. By applying high voltage pulses, we
experimentally demonstrated that skyrmions can be dynamically generated.
Moreover, the accompanied spin topology dependent skyrmion dynamics, the
skyrmion Hall effect is also experimentally observed in the same devices. The
creation process has been numerically reproduced through micromagnetic
simulations in which the important role of skyrmion-antiskyrmion pair
generation is identified. The motion and Hall effect of the skyrmions,
immediately after their creation is described using a modified Thiele equation
after taking into account the contribution from spatially inhomogeneous
spin-orbit torques and the Magnus force. The simultaneous generation and
manipulation of skyrmions using a nonmagnetic point contact could provide a
useful pathway for designing novel skyrmion based devices.",1902.06954v1
2020-04-05,Spin wave based tunable switch between superconducting flux qubits,"Quantum computing hardware has received world-wide attention and made
considerable progress recently. YIG thin film have spin wave (magnon) modes
with low dissipation and reliable control for quantum information processing.
However, the coherent coupling between a quantum device and YIG thin film has
yet been demonstrated. Here, we propose a scheme to achieve strong coupling
between superconducting flux qubits and magnon modes in YIG thin film. Unlike
the direct $\sqrt{N}$ enhancement factor in coupling to the Kittel mode or
other spin ensembles, with N the total number of spins, an additional spatial
dependent phase factor needs to be considered when the qubits are magnetically
coupled with the magnon modes of finite wavelength. To avoid undesirable
cancelation of coupling caused by the symmetrical boundary condition, a CoFeB
thin layer is added to one side of the YIG thin film to break the symmetry. Our
numerical simulation demonstrates avoided crossing and coherent transfer of
quantum information between the flux qubits and the standing spin waves in YIG
thin films. We show that the YIG thin film can be used as a tunable switch
between two flux qubits, which have modified shape with small direct inductive
coupling between them. Our results manifest that it is possible to couple flux
qubits while suppressing undesirable cross-talk.",2004.02156v1
2020-08-14,Spin-injection and spin-relaxation in p-doped InGaAs/GaAs quantum-dot spin light emitting diode at zero magnetic field,"We report on efficient spin injection in p-doped InGaAs/GaAs quantum-dot (QD)
spin light emitting diode (spin-LED) under zero applied magnetic field. A high
degree of electroluminescence circular polarization (Pc) ~19% is measured in
remanence up to 100K. This result is obtained thanks to the combination of a
perpendicularly magnetized CoFeB/MgO spin injector allowing efficient spin
injection and an appropriate p-doped InGaAs/GaAs QD layer in the active region.
By analyzing the bias and temperature dependence of the electroluminescence
circular polarization, we have evidenced a two-step spin relaxation process.
The first step occurs when electrons tunnel through the MgO barrier and travel
across the GaAs depletion layer. The spin relaxation is dominated by the
Dyakonov-Perel mechanism related to the kinetic energy of electrons, which is
characterized by a bias dependent Pc. The second step occurs when electrons are
captured into QDs prior to their radiative recombination with holes. The
temperature dependence of Pc reflects the temperature induced modification of
the QDs doping, together with the variation of the ratio between the charge
carrier lifetime and the spin relaxation time inside the QDs. The understanding
of these spin relaxation mechanisms is essential to improve the performance of
spin LED for future spin optoelectronic applications at room temperature under
zero applied magnetic field.",2008.06407v1
2020-08-17,Enhancement of spin Hall conductivity in W-Ta alloy,"Generating pure spin currents via the spin Hall effect in heavy metals has
been an active topic of research in the last decade. In order to reduce the
energy required to efficiently switch neighbouring ferromagnetic layers for
applications, one should not only increase the charge- to-spin conversion
efficiency but also decrease the longitudinal resistivity of the heavy metal.
In this work, we investigate the spin Hall conductivity in W_{1-x}Ta_{x} /
CoFeB / MgO (x = 0 - 0.2) using spin torque ferromagnetic resonance
measurements. Alloying W with Ta leads to a factor of two change in both the
damping-like effective spin Hall angle (from - 0.15 to - 0.3) and longitudinal
resistivity (60 - 120 {\mu}W cm). At 11% Ta concentration, a remarkably high
spin Hall angle value of - 0.3 is achieved with a low longitudinal resistivity
100 {\mu}W cm, which could lead to a very low power consumption for this
W-based alloy. This work demonstrates sputter-deposited W-Ta alloys could be a
promising material for power-efficient spin current generation.",2008.07572v1
2016-02-08,External-Field-Free Spin Hall Switching of Perpendicular Magnetic Nanopillar with a Dipole-Coupled Composite Structure,"Spin Hall effect (SHE) induced reversal of perpendicular magnetization has
attracted significant interest, due to its potential to lead to low power
memory and logic devices. However, the switching requires an assisted in-plane
magnetic field, which hampers its practical applications. Here, we introduce a
novel approach for external-field-free spin Hall switching of a perpendicular
nanomagnet by utilizing a local dipolar field arising from an adjacent in-plane
magnetic layer. Robust switching of perpendicular CoFeB nanopillars in a
dipole-coupled composite stack is experimentally demonstrated in the absence of
any external magnetic field, in consistent with the results of micromagnetic
simulation. Large in-plane compensation field of about 135 Oe and out-of-plane
loop shift of about 45 Oe / 10 7 A cm-2 are obtained in the nanopillar devices
with composite structure. By performing micromagnetic simulations, we confirm
the composite external-field-free switching strategy can also work for a 10 x
10 nm2 circular pillar. Compared with other proposed methods for
external-field-free spin Hall switching of perpendicular magnetization, the
dipole-coupled composite structure is compatible with a wide range of spin Hall
systems and perpendicular magnetic tunnel junctions, paving the way towards
practical SHE-based MRAM and logic applications.",1603.09624v2
2017-02-16,Pumping laser excited spins through MgO barriers,"We present a study of the tunnel magneto-Seebeck (TMS) effect in MgO based
magnetic tunnel junctions (MTJs). The electrodes consist of CoFeB with in-plane
magnetic anisotropy. The temperature gradients which generate a voltage across
the MTJs layer stack are created using laser heating. Using this method, the
temperature can be controlled on the micrometer length scale: here, we
investigate, how both, the TMS voltage and the TMS effect, depend on the size,
position and intensity of the applied laser spot. For this study, a large
variety of different temperature distributions was created across the junction.
We recorded two-dimensional maps of voltages generated by heating in dependence
of the laser spot position and the corresponding calculated TMS values. The
voltages change in value and sign, from large positive values when heating the
MTJ directly in the centre to small values when heating the junction on the
edges and even small negative values when heating the sample away from the
junction. Those zero crossings lead to very high calculated TMS ratios. Our
systematic analysis shows, that the distribution of the temperature gradient is
essential, to achieve high voltage signals and reasonable resulting TMS ratios.
Furthermore, artefacts on the edges produce misleading results, but also open
up further possibilities of more complex heating scenarios for
spincaloritronics in spintronic devices.",1702.05038v1
2017-02-21,All-optical Detection of Spin Hall Angle in W/CoFeB/SiO2 Heterostructures by Varying Tungsten Layer Thickness,"The development of advanced spintronics devices hinges on the efficient
generation and utilization of pure spin current. In materials with large
spin-orbit coupling, the spin Hall effect may convert charge current to pure
spin current and a large conversion efficiency, which is quantified by spin
Hall angle (SHA), is desirable for the realization of miniaturized and energy
efficient spintronic devices. Here, we report a giant SHA in beta-tungsten
(\b{eta}-W) thin films in Sub/W(t)/Co20Fe60B20(3 nm)/SiO2(2 nm)
heterostructures with variable W thickness. We employed an all-optical
time-resolved magneto-optical Kerr effect microscope for an unambiguous
determination of SHA using the principle of modulation of Gilbert damping of
the adjacent ferromagnetic layer by the spin-orbit torque from the W layer. A
non-monotonic variation of SHA with W layer thickness (t) is observed with a
maximum of about 0.4 at about t = 3 nm, followed by a sudden reduction to a
very low value at t = 6 nm. This variation of SHA with W-thickness correlates
well with the thickness dependent structural phase transition and resistivity
variation of W above the spin diffusion length of W, while below this length
the interfacial electronic effect at W/CoFeB influences the estimation of SHA.",1702.06258v1
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
2020-07-23,Electrical Detection of Light Helicity using a Quantum Dots based Hybrid Device at Zero Magnetic Field,"Photon helicity-dependent photocurrent is measured at zero magnetic field on
a device based on an ensemble of InGaAs/GaAs quantum dots that are embedded
into a GaAs-based p-i-n diode. Our main goal is to take advantage of the long
electron spin relaxation time expected in these nano-objects. In these
experiments, no external magnetic field is required thanks to the use of an
ultrathin magnetic CoFeB/MgO electrode, presenting perpendicular magnetic
anisotropy (PMA). We observe a clear asymmetry of the photocurrent measured
under respective right and left polarized light that follows the hysteresis of
the magnetic layer. The amplitude of this asymmetry at zero magnetic field
decreases with increasing temperatures and can be controlled with the bias.
Polarization-resolved photoluminescence is detected in parallel while the
device is operated as a photodetector. This demonstrates the multifunctional
capabilities of the device and gives valuable insights into the spin relaxation
of the electrons in the quantum dots.",2007.12054v1
2020-12-02,Engineered magnetization and exchange stiffness in direct-write Co-Fe nanoelements,"Media with engineered magnetization are essential building blocks in
superconductivity, magnetism and magnon spintronics. However, the established
thin-film and lithographic techniques insufficiently suit the realization of
planar components with on-demand-tailored magnetization in the lateral
dimension. Here, we demonstrate the engineering of the magnetic properties of
CoFe-based nanodisks fabricated by the mask-less technique of focused electron
beam induced deposition (FEBID). The material composition in the nanodisks is
tuned \emph{in-situ} via the e-beam waiting time in the FEBID process and their
post-growth irradiation with Ga ions. The magnetization $M_s$ and exchange
stiffness $A$ of the disks are deduced from perpendicular ferromagnetic
resonance measurements. The achieved $M_s$ variation in the broad range from
$720$ emu/cm$^3$ to $1430$ emu/cm$^3$ continuously bridges the gap between the
$M_s$ values of such widely used magnonic materials as permalloy and CoFeB. The
presented approach paves a way towards nanoscale 2D and 3D systems with
controllable and space-varied magnetic properties.",2012.01481v1
2021-04-05,"Analyser-free, intensity-based wide-field magneto-optical microscopy","In conventional Kerr- and Faraday microscopy the sample is illuminated with
plane-polarised light and a magnetic domain contrast is generated by an
analyser making use of the Kerr- or Faraday rotation. In this paper we
demonstrate possibilities of analyser-free magneto-optical microscopy based on
magnetisation-dependent intensity modulations of the light: (i) The transverse
Kerr effect can be applied for in-plane magnetised material, demonstrated for
an FeSi sheet. (ii) Illuminating the same sample with circularly polarised
light leads to a domain contrast with a different symmetry as the conventional
Kerr contrast. (iii) Circular polarisation can also be used for perpendicularly
magnetised material, demonstrated for a garnet film and an ultrathin CoFeB
film. (iv) Plane-polarised light at a specific angle can be employed for both,
in-plane and perpendicular media. (v) Perpendicular light incidence leads to a
domain contrast on in-plane materials that is quadratic in the magnetisation
and to a domain boundary contrast. (vi) Domain contrast can even be obtained
without polariser. In cases (ii) and (iii), the contrast is generated by MCD
(Magnetic Circular Dichroism), while MLD (Magnetic Linear Dichroism) is
responsible for the contrast in case (v). The domain boundary contrast is due
to the magneto-optical gradient effect in metallic samples. A domain boundary
contrast can also arise due to interference of phase-shifted magneto-optical
amplitudes. An explanation of these contrast phenomena is provided in terms of
Maxwell-Fresnel theory.",2104.01831v2
2021-06-29,Spin Hall effect in a spin-1 chiral semimetal,"Spin-1 chiral semimetal is a new state of quantum matter hosting
unconventional chiral fermions that extend beyond the common Dirac and Weyl
fermions. B20-type CoSi is a prototypal material that accommodates such an
exotic quasiparticle. To date, the spin transport properties in the spin-1
chiral semimetals, have not been explored yet. In this work, we fabricated
B20-CoSi thin films on sapphire c-plane substrates by magnetron sputtering and
studied the spin Hall effect (SHE) by combining experiments and
first-principles calculations. The SHE of CoSi using CoSi/CoFeB/MgO
heterostructures was investigated via spin Hall magnetoresistance and harmonic
Hall measurements. First-principles calculations yield an intrinsic spin Hall
conductivity (SHC) at the Fermi level that is consistent with the experiments
and reveal its unique Fermi-energy dependence. Unlike the Dirac and Weyl
fermion-mediated Hall conductivities that exhibit a peak-like structure
centering around the topological node, SHC of B20-CoSi is odd and crosses zero
at the node with two antisymmetric local extrema of opposite sign situated
below and above in energy. Hybridization between Co d-Si p orbitals and
spin-orbit coupling are essential for the SHC, despite the small (~1%) weight
of Si p-orbital near the Fermi level. This work expands the horizon of
topological spintronics and highlights the importance of Fermi-level tuning in
order to fully exploit the topology of spin-1 chiral fermions for spin current
generation.",2106.15107v1
2021-11-13,360° polarization control of terahertz spintronic emitters using uniaxial FeCo/TbCo2/FeCo trilayers,"Polarization control of THz light is of paramount interest for the numerous
applications offered in this frequency range. Recent developments in THz
spintronic emitters allow for a very efficient broadband emission, and
especially unique is their ability of THz polarization switching through
magnetization control of the ferromagnetic layer. Here we present an improved
scheme to achieve full 360{\deg} nearly coherent polarization rotation that
does not require multipolar or rotating external magnetic bias nor complex
cascaded emitters. By replacing the FM layer of the spintronic emitter with a
carefully designed FeCo/TbCo2/FeCo anisotropic heterostructure, we
experimentally demonstrate Stoner-Wohlfarth-like coherent rotation of the THz
polarization over a full 2pi azimuth only by a bipolar variation of the
strength of the hard axis field, and with only a negligible decrease in the
emission efficiency as compared to standard Pt/CoFeB/W inverse spin Hall
emitters. THz measurements are in agreement with our model of the non-perfect
Stoner-Wohlfarth behaviour. These emitters are well adapted for the
implementation of polarimetric characterization not requiring any mechanically
rotating polarizing elements. An example is given with the characterization of
the birefringence in a quartz plate.",2111.07118v1
2022-06-07,Non-volatile electric control of spin-orbit torques in an oxide two-dimensional electron gas,"Spin-orbit torques (SOTs) have opened a novel way to manipulate the
magnetization using in-plane current, with a great potential for the
development of fast and low power information technologies. It has been
recently shown that two-dimensional electron gases (2DEGs) appearing at oxide
interfaces provide a highly efficient spin-to-charge current interconversion.
The ability to manipulate 2DEGs using gate voltages could offer a degree of
freedom lacking in the classical ferromagnetic/spin Hall effect bilayers for
spin-orbitronics, in which the sign and amplitude of SOTs at a given current
are fixed by the stack structure. Here, we report the non-volatile
electric-field control of SOTs in an oxide-based Rashba-Edelstein 2DEG. We
demonstrate that the 2DEG is controlled using a back-gate electric-field,
providing two remanent and switchable states, with a large resistance contrast
of 1064%. The SOTs can then be controlled electrically in a non-volatile way,
both in amplitude and in sign. This achievement in a 2DEG-CoFeB/MgO
heterostructures with large perpendicular magnetization further validates the
compatibility of oxide 2DEGs for magnetic tunnel junction integration, paving
the way to the advent of electrically reconfigurable SOT MRAMS circuits, SOT
oscillators, skyrmion and domain-wall-based devices, and magnonic circuits.",2206.03068v1
2022-08-09,Growth-dependent Interlayer Chiral Exchange and Field-free Switching,"Interfacial Dzyaloshinskii-Moriya interaction (DMI) has long been observed in
normal metal/ferromagnetic multilayers, enabling the formation of chiral domain
walls, skyrmions and other 2D antisymmetric spin textures confined within a
single ferromagnetic layer, while more recent works on interlayer DMI reveal
new pathways in realizing novel chiral 3D spin textures between two separate
layers.Here, we report on interlayer DMI between two orthogonally magnetized
ferromagnetic layers (CoFeB/Co) mediated by a Pt layer, and confirm the chiral
nature of the observed effective field of up to 37 Oe through asymmetric
hysteresis loops under in-plane field.We highlight the importance of
growth-induced in-plane symmetry breaking, resulting in a sizable interlayer
DMI and a universal characteristic vector through wedge deposition of the
samples.We further perform deterministic current-driven magnetization switching
in the perpendicularly magnetized Co layer utilizing solely the effective field
from the interlayer DMI.These results demonstrate the potential of interlayer
DMI in facilitating deterministic field-free switching in spin memory
applications.",2208.04492v1
2022-10-12,Depth-dependent magnetic crossover in a room-temperature skyrmion-hosting multilayer,"Skyrmion-hosting multilayer stacks are promising avenues for applications,
although little is known about the depth dependence of the magnetism. We
address this by reporting the results of circular dichroic resonant elastic
x-ray scattering (CD-REXS), micromagnetic simulations, and low-energy muon-spin
rotation (LE-$\mu^+$SR) measurements on a stack comprising
[Ta/CoFeB/MgO]$_{16}$/Ta on a Si substrate. Energy-dependent CD-REXS shows a
continuous, monotonic evolution of the domain-wall helicity angle with incident
energy, consistent with a three-dimensional hybrid domain-wall-like structure
that changes from N\'eel-like near the surface to Bloch-like deeper within the
sample. LE-$\mu^+$SR reveals that the magnetic field distribution in the
trilayers near the surface of the stack is distinct from that in trilayers
deeper within the sample. Our micromagnetic simulations support a quantitative
analysis of the $\mu^+$SR results. By increasing the applied magnetic field, we
find a reduction in the volume occupied by domain walls at all depths,
consistent with a crossover into a region dominated by skyrmions above
approximately 180 mT.",2210.06070v2
2022-12-15,Emulation of Neuron and Synaptic Functions in Spin-Orbit Torque Domain Wall Devices,"Neuromorphic computing (NC) architecture has shown its suitability for
energy-efficient computation. Amongst several systems, spin-orbit torque (SOT)
based domain wall (DW) devices are one of the most energy-efficient contenders
for NC. To realize spin-based NC architecture, the computing elements such as
synthetic neurons and synapses need to be developed. However, there are very
few experimental investigations on DW neurons and synapses. The present study
demonstrates the energy-efficient operations of neurons and synapses by using
novel reading and writing strategies. We have used a W/CoFeB-based
energy-efficient SOT mechanism to drive the DWs at low current densities. We
have used the concept of meander devices for achieving synaptic functions. By
doing this, we have achieved 9 different resistive states in experiments. We
have experimentally demonstrated the functional spike and step neurons.
Additionally, we have engineered the anomalous Hall bars by incorporating
several pairs, in comparison to conventional Hall crosses, to increase the
sensitivity as well as signal-to-noise ratio (SNR). We performed micromagnetic
simulations and transport measurements to demonstrate the above-mentioned
functionalities.",2212.07833v1
2023-01-10,Robust mutual synchronization in long spin Hall nano-oscillator chains,"Mutual synchronization of N serially connected spintronic nano-oscillators
increases their coherence by a factor $N$ and their output power by $N^2$.
Increasing the number of mutually synchronized nano-oscillators in chains is
hence of great importance for better signal quality and also for emerging
applications such as oscillator-based neuromorphic computing and Ising machines
where larger N can tackle larger problems. Here we fabricate spin Hall
nano-oscillator chains of up to 50 serially connected nano-constrictions in
W/NiFe, W/CoFeB/MgO, and NiFe/Pt stacks and demonstrate robust and complete
mutual synchronization of up to 21 nano-constrictions, reaching linewidths of
below 200 kHz and quality factors beyond 79,000, while operating at 10 GHz. We
also find a square increase in the peak power with the increasing number of
mutually synchronized oscillators, resulting in a factor of 400 higher peak
power in long chains compared to individual nano-constrictions. Although chains
longer than 21 nano-constrictions also show complete mutual synchronization, it
is not as robust and their signal quality does not improve as much as they
prefer to break up into partially synchronized states. The low current and low
field operation of these oscillators along with their wide frequency tunability
(2-28 GHz) with both current and magnetic fields, make them ideal candidates
for on-chip GHz-range applications and neuromorphic computing.",2301.03859v1
2023-02-02,Controlling the Skyrmion Density and Size for Quantized Convolutional Neural Networks,"Skyrmion devices show energy efficient and high integration data storage and
computing capabilities. Herein, we present the results of experimental and
micromagnetic investigations of the creation and stability of magnetic
skyrmions in the Ta/IrMn/CoFeB/MgO thin film system. We investigate the
magnetic-field dependence of the skyrmion density and size using polar magneto
optical Kerr effect MOKE microscopy supported by a micromagnetic study. The
evolution of the topological charge with time under a magnetic field is
investigated, and the transformation dynamics are explained. Furthermore,
considering the voltage control of these skyrmion devices, we evaluate the
dependence of the skyrmion size and density on the Dzyaloshinskii Moriya
interaction and the magnetic anisotropy. We furthermore propose a skyrmion
based synaptic device based on the results of the MOKE and micromagnetic
investigations. We demonstrate the spin-orbit torque controlled discrete
topological resistance states with high linearity and uniformity in the device.
The discrete nature of the topological resistance makes it a good candidate to
realize hardware implementation of weight quantization in a quantized neural
network (QNN). The neural network is trained and tested on the CIFAR10 dataset,
where the devices act as synapses to achieve a recognition accuracy of 87%,
which is comparable to the result of ideal software-based methods.",2302.01390v1
2023-04-16,Anomalous and Topological Hall Resistivity in Ta/CoFeB/MgO Magnetic Systems for Neuromorphic Computing Applications,"Topologically protected spin textures, such as magnetic skyrmions, have the
potential for dense data storage as well as energy-efficient computing due to
their small size and a low driving current. The evaluation of the writing and
reading of the skyrmion's magnetic and electrical characteristics is a key step
toward the implementation of these devices. In this paper, we present the
magnetic heterostructure Hall bar device and study the anomalous Hall and
topological Hall signals in the device. Using the combination of different
measurements like magnetometry at different temperatures, Hall effect
measurement from 2K to 300K, and magnetic force microscopy imaging, we
investigate the magnetic and electrical characteristics of the magnetic
structure. We measure the skyrmion topological resistivity at different
temperatures as a function of the magnetic field. The topological resistivity
is maximum around the zero magnetic field and it decreases to zero at the
saturating field. This is further supported by MFM imaging. Interestingly the
resistivity decreases linearly with the field, matching the behavior observed
in the corresponding micromagnetic simulations. We combine the experimental
results with micromagnetic simulations, thus propose a skyrmion-based synaptic
device and show spin-orbit torque-controlled potentiation/depression in the
device. The device performance as the synapse for neuromorphic computing is
further evaluated in a convolutional neural network CNN. The neural network is
trained and tested on the MNIST data set we show devices acting as synapses
achieving a recognition accuracy close to 90%, on par with the ideal
software-based weights which offer an accuracy of 92%.",2304.07742v1
2023-06-28,Suppression of the spin waves nonreciprocity due to interfacial Dzyaloshinskii Moriya interaction by lateral confinement in magnetic nanostructures,"Despite the huge recent interest towards chiral magnetism related to the
interfacial Dzyaloshinskii Moriya interaction (iDMI) in layered systems, there
is a lack of experimental data on the effect of iDMI on the spin waves
eigenmodes of laterally confined nanostructures. Here we exploit Brillouin
Light Scattering (BLS) to analyze the spin wave eigenmodes of non-interacting
circular and elliptical dots, as well as of long stripes, patterned starting
from a Pt(3.4 nm)/CoFeB(0.8 nm) bilayer, with lateral dimensions ranging from
100 nm to 400 nm. Our experimental results, corroborated by micromagnetic
simulations based on the GPU-accelerated MuMax3 software package, provide
evidence for a strong suppression of the frequency asymmetry between
counter-propagating spin waves (corresponding to either Stokes or anti-Stokes
peaks in BLS spectra), when the lateral confinement is reduced from 400 nm to
100 nm, i.e. when it becomes lower than the light wavelength. Such an evolution
reflects the modification of the spin wave character from propagating to
stationary and indicates that the BLS based method of quantifying the i-DMI
strength from the frequency difference of counter propagating spin waves is not
applicable in the case of magnetic elements with lateral dimension below about
400 nm.",2306.16310v1
2023-09-27,Impact of surface anisotropy on the spin-wave dynamics in thin ferromagnetic film,"The spin-wave dynamics in the thin CoFeB film in Damon-Eshbach geometry are
studied in three cases of boundary conditions -- free boundary conditions,
symmetrical surface anisotropy, and one-sided surface anisotropy. The
analytical model created by Wolfram and De Wames was extended to include
perpendicular surface anisotropy in boundary conditions. Its comparison with
numerical simulations demonstrate perfect agreement between the approaches. The
analysis of the dispersion relation indicates that the presence of surface
anisotropy increases the avoided crossing size between Damon-Eshbach mode and
perpendicular standing modes. Additionally, asymmetrical one-sided surface
anisotropy induces nonreciprocity in the dispersion relation. In-depth analysis
of the avoided crossing size is conducted for systems with different boundary
conditions, different thicknesses, surface anisotropy constant values, and
external magnetic fields. It shows the significant role of the strength of
surface localization of Damon-Eshbach mode and the symmetry of perpendicular
standing modes in the avoided crossing broadening. Interestingly, for specific
set of parameters the interaction between the particular modes can be
suppressed, resulting in a mode crossing. Such a crossing, which occurs only on
one side of the dispersion relation in a one-sided surface anisotropy system,
can be utilized in nonreciprocal devices.",2309.15583v1
2023-11-15,Low voltage local strain enhanced switching of magnetic tunnel junctions,"Strain-controlled modulation of the magnetic switching behavior in magnetic
tunnel junctions (MTJs) could provide the energy efficiency needed to
accelerate the use of MTJs in memory, logic, and neuromorphic computing, as
well as an additional way to tune MTJ properties for these applications.
State-of-the-art CoFeB-MgO based MTJs still require too high voltages to alter
their magnetic switching behavior with strain. In this study, we demonstrate
strain-enhanced field switching of nanoscale MTJs through electric field
control via voltage applied across local gates. The results show that
record-low voltage down to 200 mV can be used to control the switching field of
the MTJ through enhancing the magnetic anisotropy, and that tunnel
magnetoresistance is linearly enhanced with voltage through straining the
crystal structure of the tunnel barrier. These findings underscore the
potential of electric field manipulation and strain engineering as effective
strategies for tailoring the properties and functionality of nanoscale MTJs.",2311.08984v2
2023-11-24,Even-in-magnetic-field part of transverse resistivity as a probe of magnetic order,"The detection of a voltage transverse to both an applied current and a
magnetic field is one of the most common characterization techniques in
solid-state physics. The corresponding component of the resistivity tensor
$\rho_{ij}$ can be separated into odd and even parts with respect to the
applied magnetic field. The former contains information, for example, about the
ordinary or anomalous Hall effect. The latter is typically ascribed to
experimental artefacts and ignored. We here show that upon suppressing these
artefacts in carefully controlled experiments, useful information remains. We
first investigate the well-explored ferromagnet CoFeB, where the even part of
$\rho_{yx}$ contains a contribution from the anisotropic magnetoresistance,
which we confirm by Stoner-Wohlfarth modelling. We then apply our approach to
magnetotransport measurements in $\rm Mn_5Si_3$ thin films with a complex
compensated magnetic order. In this material, the even part of the transverse
signal is sizable only in the low-spin-symmetry phase below $\approx 80$ K and
thus offers a simple and readily available probe of the magnetic order.",2311.14498v1
2024-03-25,Skyrmionic device for three dimensional magnetic field sensing enabled by spin-orbit torques,"Magnetic skyrmions are topologically protected local magnetic solitons that
are promising for storage, logic or general computing applications. In this
work, we demonstrate that we can use a skyrmion device based on [W/CoFeB/MgO] 1
0 multilayers for three-dimensional magnetic field sensing enabled by
spin-orbit torques (SOT). We stabilize isolated chiral skyrmions and stripe
domains in the multilayers, as shown by magnetic force microscopy images and
micromagnetic simulations. We perform magnetic transport measurements to show
that we can sense both in-plane and out-of-plane magnetic fields by means of a
differential measurement scheme in which the symmetry of the SOT leads to
cancelation of the DC offset. With the magnetic parameters obtained by
vibrating sample magnetometry and ferromagnetic resonance measurements, we
perform finite-temperature micromagnetic simulations, where we investigate the
fundamental origin of the sensing signal. We identify the topological
transformation between skyrmions, stripes and type-II bubbles that leads to a
change in the resistance that is read-out by the anomalous Hall effect. Our
study presents a novel application for skyrmions, where a differential
measurement sensing concept is applied to quantify external magnetic fields
paving the way towards more energy efficient applications in skyrmionics based
spintronics.",2403.16725v1
2020-09-14,Large field-like torque in amorphous Ru2Sn3 originated from the intrinsic spin Hall effect,"We investigated temperature dependent current driven spin-orbit torques in
magnetron sputtered Ru2Sn3 (4 and 10 nm) /Co20Fe60B20 (5 nm) layered structures
with in-plane magnetic anisotropy. The room temperature damping-like and
field-like spin torque efficiencies of the amorphous Ru2Sn3 films were measured
to be 0.14 +- 0.008 (0.07 +- 0.012) and -0.03 +- 0.006 (-0.20 +- 0.009), for
the 4 (10 nm) films respectively, by utilizing the second harmonic Hall
technique. The large field-like torque in the relatively thicker Ru2Sn3 (10 nm)
thin film is unique compared to the traditional spin Hall materials interfaced
with thick magnetic layers with in-plane magnetic anisotropy which typically
have dominant damping-like and negligible field-like torques. Additionally, the
observed room temperature field-like torque efficiency in Ru2Sn3 (10 nm)/CoFeB
(5 nm) is up to three times larger than the damping-like torque (-0.20 +- 0.009
and 0.07 +- 0.012, respectively) and thirty times larger at 50 K (-0.29 +-
0.014 and 0.009 +- 0.017, respectively). The temperature dependence of the
field-like torques show dominant contributions from the intrinsic spin Hall
effect while the damping-like torques show dominate contributions from the
extrinsic spin Hall effects, skew scattering and side jump. Through macro-spin
calculations, we found that including field-like torques on the order or larger
than the damping-like torque can reduce the switching critical current and
decrease magnetization procession for a perpendicular ferromagnetic layer.",2009.06711v2
2021-11-09,Spintronic emitters for super-resolution in THz-spectral imaging,"THz-spectroscopy is an attractive imaging tool for scientific research,
especially in life science, offering non-destructive interaction with matter
due to its low photon energies. However, wavelengths above $100{\mu}m$
principally limit its spatial resolution in the far-field by diffraction to
this regime, making it not sufficient to image biological cells in the
micrometer scale. Therefore, super-resolution imaging techniques are required
to overcome this restriction. Near-field-imaging using spintronic emitters
offers the most feasible approach because of its simplicity and potential for
wide-ranging applications. In our study, we investigate THz-radiation generated
by fs-laser-pulses in CoFeB/Pt heterostructures, based on spin currents,
detected by commercial LT-GaAs Auston switches. The spatial resolution is
evaluated applying a 2D scanning technique with motorized stages allowing
scanning steps in the sub-micrometer range. By applying near-field imaging we
can increase the spatial resolution to the dimensions of the laser spot size in
the micrometer scale. For this purpose, the spintronic emitter is directly
evaporated on a gold test pattern separated by a 300 nm spacer layer. Moving
these structures with respect to the femtosecond laser spot which generates the
THz radiation allows for resolution determination using the knife-edge method.
We observe a full-width half-maximum THz beam diameter of $4.9(4){\mu}$m at 1
THz. The possibility to deposit spintronic emitter heterostructures on simple
glass substrates makes them an interesting candidate for near-field imaging for
a large number of applications.",2111.05023v1