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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 |
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