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2020-04-07 | Nonreciprocal Dzyaloshinskii-Moriya magnetoacoustic waves | We study the interaction of surface acoustic waves with spin waves in
ultra-thin CoFeB/Pt bilayers. Due to the interfacial Dzyaloshinskii-Moriya
interaction (DMI), the spin wave dispersion is non-degenerate for oppositely
propagating spin waves in CoFeB/Pt. In combination with the additional
nonreciprocity of the magnetoacoustic coupling itself, highly nonreciprocal
acoustic wave transmission through the magnetic film is observed. We
systematically characterize the magnetoacoustic wave propagation in a thickness
series of CoFeB($d$)/Pt samples as a function of magnetic field magnitude and
direction, and at frequencies up to 7 GHz. We quantitatively model our results
to extract the strength of the DMI and magnetoacoustic driving fields. | 2004.03535v2 |
2021-04-05 | Optical Damage Threshold and THz Generation Efficiency of (Fe,CoFeB)/(Ta,Pt) Spintronic Emitters | THz pulses are generated from femtosecond pulse-excited
ferromagnetic/nonmagnetic spintronic heterostructures via inverse spin Hall
effect. The contribution from ultrafast demagnetization/remagnetization is
extremely weak, in the comparison. The highest possible THz signal strength
from spintronic THz emitters is limited by the optical damage threshold of the
corresponding heterostructures. The THz generation efficiency does not saturate
with the excitation fluence even up till the damage threshold. Bilayer (Fe,
CoFeB)/(Pt, Ta) based FM/NM spintronic heterostructures have been studied for
an optimized performance for THz generation when pumped by sub-50 fs amplified
laser pulses at 800 nm. Among them, CoFeB/Pt is the best combination for an
efficient THz source. The optimized FM/NM spintronic heterostructure on a
quartz substrate, having alpha-phase Ta as the nonmagnetic layer, show the
highest damage threshold as compared to those with Pt, irrespective of their
generation efficiency. The damage threshold of the Fe/Ta heterostructure on
quartz substrate is ~85 GW/cm2. | 2104.02028v1 |
2023-02-14 | Direct evidence of terahertz emission arising from anomalous Hall effect | A detailed understanding of the different mechanisms being responsible for
terahertz (THz) emission in ferromagnetic (FM) materials will aid in designing
efficient THz emitters. In this report, we present direct evidence of THz
emission from single layer Co$_{0.4}$Fe$_{0.4}$B$_{0.2}$ (CoFeB) FM thin films.
The dominant mechanism being responsible for the THz emission is the anomalous
Hall effect (AHE), which is an effect of a net backflow current in the FM layer
created by the spin-polarized current reflected at the interfaces of the FM
layer. The THz emission from the AHE-based CoFeB emitter is optimized by
varying its thickness, orientation, and pump fluence of the laser beam. Results
from electrical transport measurements show that skew scattering of charge
carriers is responsible for the THz emission in the CoFeB AHE-based THz
emitter. | 2302.07398v2 |
2023-03-21 | Inducing or suppressing the anisotropy in multilayers based on CoFeB | Controlling the uniaxial magnetic anisotropy is of practical interest to a
wide variety of applications. We study Co$_{40}$Fe$_{40}$B$_{20}$ single films
grown on various crystalline orientations of LiNbO$_3$ substrates and on
oxidized silicon. We identify the annealing conditions that are appropriate to
induce or suppress uniaxial anisotropy. Anisotropy fields can be increased by
annealing up to 11 mT when using substrates with anisotropic surfaces. They can
be decreased to below 1 mT when using isotropic surfaces. In the first case,
the observed increase of the anisotropy originates from the biaxial strain in
the film caused by the anisotropic thermal contraction of the substrate when
back at room temperature after strain relaxation during annealing. In the
second case, anisotropy is progressively removed by applying successive
orthogonal fields that are assumed to progressively suppress any chemical
ordering within the magnetic film. The method can be applied to CoFeB/Ru/CoFeB
synthetic antiferromagnets but the tuning of the anisotropy comes with a
decrease of the interlayer exchange coupling and a drastic change of the
exchange stiffness. | 2303.11718v1 |
2023-06-19 | Spin transport and magnetic proximity effect in CoFeB/normal metal/Pt trilayers | We present a study of the damping and spin pumping properties of CoFeB/X/Pt
systems with $\rm X=Al,Cr$ and $\rm Ta$. We show that the total damping of the
CoFeB/Pt systems is strongly reduced when an interlayer is introduced
independently of the material. Using a model that considers spin relaxation, we
identify the origin of this contribution in the magnetically polarized Pt
formed by the magnetic proximity effect (MPE), which is suppressed by the
introduction of the interlayer. The induced ferromagnetic order in the Pt layer
is confirmed by transverse magneto-optical Kerr spectroscopy at the M$_{2,3}$
and N$_7$ absorption edges as an element-sensitive probe. We discuss the impact
of the MPE on parameter extraction in the spin transport model. | 2306.11009v2 |
2023-11-09 | Magnon-phonon coupling of synthetic antiferromagnets in a surface acoustic wave cavity resonator | We use a surface acoustic wave (SAW) cavity resonator to study the coupling
of acoustic magnons in a synthetic antiferromagnet (SAF) and the phonons
carried by SAWs. The SAF is composed of a CoFeB/Ru/CoFeB trilayer and the
scattering matrix of the SAW resonator is studied to assess the coupling. We
find that the spectral linewidth of the SAW resonator is modulated when the
frequency of the excited magnons approaches the SAW resonance frequency.
Moreover, the linewidth modulation varies with the magnitude and orientation of
the external magnetic field. Such change in the spectral linewidth can be well
reproduced using macrospin-like model calculations. From the model analyses, we
estimate the magnon-phonon coupling strength to be $\sim$15.6 MHz at a SAW
resonance frequency of 1.8 GHz: the corresponding magnomechanical cooperativity
is $\sim$0.66. As the spectral shape hardly changes in a CoFeB single layer
reference sample under the same experimental condition, these results show that
SAF provides an ideal platform to study magnon-phonon coupling in a SAW cavity
resonator. | 2311.05275v1 |
2024-01-15 | Multiple Frequency Steps in Synthetic Antiferromagnet Based Double Spin Josephson Junctions Using CoFeB and Fe3Sn | Superconducting quantum interference device (SQUID) which is made of two
parallel Josephson junctions has applications in magnetometry. A similar
spin-based device is proposed here where spin superfluid in ferromagnet (FM)
mimics the superconducting state. Two materials CoFeB and Fe3Sn are used for
spin superfluid-based SQUID like device where easy plane anisotropy in CoFeB
can be engineered and Fe3Sn has inherent easy plane anisotropy. Frequency
varies in spin based proposed devices. Frequency increases and again decreases
with the increase in both applied magnetic field and applied spin current. The
proposed device can be used as nano oscillator and detector. The frequency in
the proposed device shows multiple frequency steps which can be used for
neuromorphic applications. | 2401.08005v1 |
2008-01-09 | Current-induced magnetization switching in MgO barrier magnetic tunnel junctions with CoFeB based synthetic ferrimagnetic free layers | We investigated the effect of using a synthetic ferrimagnetic (SyF) free
layer in MgO-based magnetic tunnel junctions (MTJs) on current-induced
magnetization switching (CIMS), particularly for application to spin-transfer
torque random access memory (SPRAM). The employed SyF free layer had a
Co40Fe40B20/ Ru/ Co40Fe40B20 and Co20Fe60B20/Ru/Co20Fe60B20 structures, and the
MTJs(100x(150-300) nm^2) were annealed at 300oC. The use of SyF free layer
resulted in low intrinsic critical current density (Jc0) without degrading the
thermal-stability factor (E/kBT, where E, kB, and T are the energy potential,
the Boltzmann constant, and temperature,respectively). When the two CoFeB
layers of a strongly antiferromagnetically coupled SyF free layer had the same
thickness, Jc0 was reduced to 2-4x10^6 A/cm^2. This low Jc0 may be due to the
decreased effective volume under the large spin accumulation at the CoFeB/Ru.
The E/kBT was over 60, resulting in a retention time of over ten years and
suppression of the write current dispersion for SPRAM. The use of the SyF free
layer also resulted in a bistable (parallel/antiparallel) magnetization
configuration at zero field, enabling the realization of CIMS without the need
to apply external fields to compensate for the offset field. | 0801.1355v1 |
2012-07-11 | Layer thickness dependence of the current induced effective field vector in Ta|CoFeB|MgO | The role of current induced effective magnetic field in ultrathin magnetic
heterostructures is increasingly gaining interest since it can provide
efficient ways of manipulating magnetization electrically. Two effects, known
as the Rashba spin orbit field and the spin Hall spin torque, have been
reported to be responsible for the generation of the effective field. However,
quantitative understanding of the effective field, including its direction with
respect to the current flow, is lacking. Here we show vector measurements of
the current induced effective field in Ta|CoFeB|MgO heterostructrures. The
effective field shows significant dependence on the Ta and CoFeB layers'
thickness. In particular, 1 nm thickness variation of the Ta layer can result
in nearly two orders of magnitude difference in the effective field. Moreover,
its sign changes when the Ta layer thickness is reduced, indicating that there
are two competing effects that contribute to the effective field. The relative
size of the effective field vector components, directed transverse and parallel
to the current flow, varies as the Ta thickness is changed. Our results
illustrate the profound characteristics of just a few atomic layer thick metals
and their influence on magnetization dynamics. | 1207.2521v1 |
2013-05-12 | Spin Pumping and Inverse Spin Hall Effect in Germanium | We have measured the inverse spin Hall effect (ISHE) in \textit{n}-Ge at room
temperature. The spin current in germanium was generated by spin pumping from a
CoFeB/MgO magnetic tunnel junction in order to prevent the impedance mismatch
issue. A clear electromotive force was measured in Ge at the ferromagnetic
resonance of CoFeB. The same study was then carried out on several test
samples, in particular we have investigated the influence of the MgO tunnel
barrier and sample annealing on the ISHE signal. First, the reference CoFeB/MgO
bilayer grown on SiO$_{2}$ exhibits a clear electromotive force due to
anisotropic magnetoresistance and anomalous Hall effect which is dominated by
an asymmetric contribution with respect to the resonance field. We also found
that the MgO tunnel barrier is essential to observe ISHE in Ge and that sample
annealing systematically lead to an increase of the signal. We propose a
theoretical model based on the presence of localized states at the interface
between the MgO tunnel barrier and Ge to account for these observations.
Finally, all of our results are fully consistent with the observation of ISHE
in heavily doped $n$-Ge and we could estimate the spin Hall angle at room
temperature to be $\approx$0.001. | 1305.2602v1 |
2017-03-10 | Room-temperature perpendicular magnetization switching through giant spin-orbit torque from sputtered BixSe(1-x) topological insulator material | The spin-orbit torque (SOT) arising from materials with large spin-orbit
coupling promises a path for ultra-low power and fast magnetic-based storage
and computational devices. We investigated the SOT from magnetron-sputtered
BixSe(1-x) thin films in BixSe(1-x)/CoFeB heterostructures by using a dc planar
Hall method. Remarkably, the spin Hall angle (SHA) was found to be as large as
18.83, which is the largest ever reported at room temperature (RT). Moreover,
switching of a perpendicular CoFeB multilayer using SOT from the BixSe(1-x) has
been observed with the lowest-ever switching current density reported in a
bilayer system: 2.3 * 105 A/cm2 at RT. The giant SHA, smooth surface, ease of
growth of the films on silicon substrate, successful growth and switching of a
perpendicular CoFeB multilayer on BixSe(1-x) film opens a path for use of
BixSe(1-x) topological insulator (TI) material as a spin-current generator in
SOT-based memory and logic devices. | 1703.03822v1 |
2019-07-08 | Bloch-to-Néel domain wall transition evinced through morphology of magnetic bubble expansion in Ta/CoFeB/MgO layers | Ta/CoFeB/MgO trilayers with perpendicular magnetic anisotropy are often
characterised by vanishing or modest values of interfacial
Dzyaloshinskii-Moriya interaction (DMI), which results in purely Bloch or mixed
Bloch-N\'eel domain walls (DWs). Here we investigate the creep evolution of the
overall magnetic bubble morphology in these systems under the combined presence
of in-plane and out-of-plane magnetic fields and we show that He$^+$ ion
irradiation induces a transition of the internal DW structure towards a fully
N\'eel spin texture. This transition can be correlated to a simultaneous
increase in DMI strength and reduction in saturation magnetisation -- which are
a direct consequence of the effects of ion irradiation on the bottom and top
CoFeB interfaces, respectively. The threshold irradiation dose above which DWs
acquire a pure N\'eel character is experimentally found to be between 12
$\times$ 10$^{18}$ He$^+$/m$^2$ and 16 $\times$ 10$^{18}$ He$^+$/m$^2$,
matching estimations from the one dimensional DW model based on material
parameters. Our results indicate that evaluating the global bubble shape during
its expansion can be an effective tool to sense the internal bubble DW
structure. Furthermore, we show that ion irradiation can be used to achieve
post-growth engineering of a desired DW spin texture. | 1907.03708v1 |
2019-07-14 | Current-Induced magnetization switching by the high spin Hall conductivity $α$-W | The spin Hall effect originating from 5d heavy transition metal thin films
such as Pt, Ta, and W is able to generate efficient spin-orbit torques that can
switch adjacent magnetic layers. This mechanism can serve as an alternative to
conventional spin-transfer torque for controlling next-generation magnetic
memories. Among all 5d transition metals, W in its resistive amorphous phase
typically shows the largest spin-orbit torque efficiency ~ 0.20-0.50. In
contrast, its conductive and crystalline $\alpha$ phase possesses a
significantly smaller efficiency ~ 0.03 and no spin-orbit torque switching has
yet been realized using $\alpha$-W thin films as the spin Hall source. In this
work, through a comprehensive study of high quality W/CoFeB/MgO and the
reversed MgO/CoFeB/W magnetic heterostructures, we show that although
amorphous-W has a greater spin-orbit torque efficiency, the spin Hall
conductivity of $\alpha$-W
($|\sigma_{\operatorname{SH}}^{\alpha\operatorname{-W}}|=3.71\times10^{5}\operatorname{\Omega}^{-1}\operatorname{m}^{-1}$)
is ~3.5 times larger than that of amorphous-W
($|\sigma_{\operatorname{SH}}^{\operatorname{amorphous-W}}|=1.05\times10^{5}\operatorname{\Omega}^{-1}\operatorname{m}^{-1}$).
Moreover, we demonstrate spin-orbit torque driven magnetization switching using
a MgO/CoFeB/$\alpha$-W heterostructure. Our findings suggest that the
conductive and high spin Hall conductivity $\alpha$-W can be a potential
candidate for future low power consumption spin-orbit torque memory
applications. | 1907.06192v1 |
2019-10-02 | A Low Temperature Functioning CoFeB/MgO Based Perpendicular Magnetic Tunnel Junction for Cryogenic Nonvolatile Random Access Memory | We investigated the low temperature performance of CoFeB/MgO based
perpendicular magnetic tunnel junctions (pMTJs) by characterizing their
quasi-static switching voltage, high speed pulse write error rate and endurance
down to 9 K. pMTJ devices exhibited high magnetoresistance (>120%) and reliable
(error rate<10-4) bi-directional switching with 2 to 200 ns voltage pulses. The
endurance of the devices at 9 K surpassed that at 300 K by three orders of
magnitude under the same write conditions, functioning for more than 10^12
cycles with 10 ns write pulses. The critical switching voltage at 9 K was
observed to increase by 33% to 93%, depending on pulse duration, compared to
that at 350 K. Ferromagnetic resonance and magnetization measurements on
blanket pMTJ film stacks suggest that the increased switching voltage is
associated with an increase in effective magnetic anisotropy and magnetization
of free layer with decreasing temperature. Our work demonstrates that CoFeB/MgO
based pMTJs have great potential to enable cryogenic MRAM and that their low
temperature magnetization and effective magnetic anisotropy can be further
optimized to lower operating power and improve endurance. | 1910.01149v1 |
2018-02-22 | Correlation of tunnel magnetoresistance with the magnetic properties in perpendicular CoFeB-based junctions with exchange bias | We investigate the dependence of magnetic properties on the post-annealing
temperature/time, the thickness of soft ferromagnetic electrode and Ta dusting
layer in the pinned electrode as well as their correlation with the tunnel
magnetoresistance ratio, in a series of perpendicular magnetic tunnel junctions
of materials sequence
Ta/Pd/IrMn/CoFe/Ta$(\textit{x})$/CoFeB/MgO$(\textit{y})$/CoFeB$(\textit{z})$/Ta/Pd.
We obtain a large perpendicular exchange bias of 79.6$\,$kA/m for $x=0.3\,$nm.
For stacks with $z=1.05\,$nm, the magnetic properties of the soft electrode
resemble the characteristics of superparamagnetism. For stacks with
$x=0.4\,$nm, $y=2\,$nm, and $z=1.20\,$nm, the exchange bias presents a
significant decrease at post annealing temperature
$T_\textrm{ann}=330\,^{\circ}$C for 60 min, while the interlayer exchange
coupling and the saturation magnetization per unit area sharply decay at
$T_\textrm{ann}=340\,^{\circ}$C for 60 min. Simultaneously, the tunnel
magnetoresistance ratio shows a peak of $65.5\%$ after being annealed at
$T_\textrm{ann}=300\,^{\circ}$C for 60 min, with a significant reduction down
to $10\%$ for higher annealing temperatures
($T_\textrm{ann}\geq330\,^{\circ}$C) and down to $14\%$ for longer annealing
times ($T_\textrm{ann}=300\,^{\circ}$C for 90 min). We attribute the large
decrease of tunnel magnetoresistance ratio to the loss of exchange bias in the
pinned electrode. | 1802.08002v2 |
2020-04-07 | Laser-induced magnetization precession in individual magnetoelastic domains of a multiferroic CoFeB/BaTiO$_3$ composite | Using a magneto-optical pump-probe technique with micrometer spatial
resolution we show that magnetization precession can be launched in individual
magnetic domains imprinted in a Co$_{40}$Fe$_{40}$B$_{20}$ (CoFeB) layer by
elastic coupling to ferroelectric domains in a BaTiO$_{3}$ substrate. The
dependence of the precession parameters on external magnetic field strength and
orientation reveal that by laser-induced ultrafast partial quenching of the
magnetoelastic coupling parameter of CoFeB by $\approx$27% along with 10%
ultrafast demagnetization trigger the magnetization precession. The relation
between the laser-induced reduction of the magnetoelastic coupling and the
demagnetization is approximated by the $n(n+1)/2$-law with n$\approx$2. This
correspondence confirms the thermal origin of the laser-induced anisotropy
change. Based on the analysis and modeling of the excited precession we find
signatures of laser-induced precessional switching, which occurs when the
magnetic field is applied along the hard magnetization axis and its value is
close to the effective magnetoelastic anisotropy field. The precession
excitation process in an individual magnetoelastic domain is found to be
unaffected by neighboring domains. This makes laser-induced changes of
magnetoelastic anisotropy a promising tool for driving magnetization dynamics
and switching in composite multiferroics with spatial selectivity. | 2004.03566v5 |
2018-04-11 | Thermographic measurements of spin-current-induced temperature modulation in metallic bilayers | Spin-to-heat current conversion effects have been investigated in bilayer
films consisting of a paramagnetic metal (PM; Pt, W, or Ta) and a ferromagnetic
metal (FM; CoFeB or permalloy). When a charge current is applied to the PM/FM
bilayer film, a spin current is generated across the PM/FM interface owing to
the spin Hall effect in PM. The spin current was found to exhibit cooling and
heating features depending on the sign of the spin Hall angle of PM, where the
spin-current-induced contribution is estimated by subtracting the contribution
of the anomalous Ettingshausen effect in FM monolayer films. We also found that
the magnitude of the spin-current-induced temperature modulation in the
Pt/CoFeB film is greater than but comparable to that in the Pt/permalloy film,
although the spin dependence of the Peltier coefficient for CoFeB is expected
to be greater than that for permalloy. We discuss the origin of the observed
behaviors with the aid of model calculations; the signals in the PM/FM films
may contain the contributions not only from the electron-driven spin-dependent
Peltier effect but also from the magnon-driven spin Peltier effect. | 1804.04068v1 |
2019-09-20 | Field-free spin-orbit torque switching through domain wall motion | Deterministic current-induced spin-orbit torque (SOT) switching of
magnetization in a heavy transition metal/ferromagnetic metal/oxide magnetic
heterostructure with the ferromagnetic layer being perpendicularly-magnetized
typically requires an externally-applied in-plane field to break the switching
symmetry. We show that by inserting an in-plane magnetized ferromagnetic layer
CoFeB underneath the conventional W/CoFeB/MgO SOT heterostructure,
deterministic SOT switching of the perpendicularly-magnetized top CoFeB layer
can be realized without the need of in-plane bias field. Kerr imaging study
further unveils that the observed switching is mainly dominated by domain
nucleation and domain wall motion, which might limit the potentiality of using
this type of multilayer stack design for nanoscale SOT-MRAM application.
Comparison of the experimental switching behavior with micromagnetic
simulations reveals that the deterministic switching in our devices cannot be
explained by the stray field contribution of the in-plane magnetized layer, and
the roughness-caused N\'eel coupling effect might play a more important role in
achieving the observed field-free deterministic switching. | 1909.09604v1 |
2019-12-16 | Spin-current manipulation of photoinduced magnetization dynamics in heavy metal / ferromagnet double layer based nanostructures | Spin currents offer a way to control static and dynamic magnetic properties,
and therefore they are crucial for next-generation MRAM devices or spin-torque
oscillators. Manipulating the dynamics is especially interesting within the
context of photo-magnonics. In typical $3d$ transition metal ferromagnets like
CoFeB, the lifetime of light-induced magnetization dynamics is restricted to
about 1 ns, which e.g. strongly limits the opportunities to exploit the wave
nature in a magnonic crystal filtering device. Here, we investigate the
potential of spin-currents to increase the spin wave lifetime in a functional
bilayer system, consisting of a heavy metal (8 nm of $\beta$-Tantalum
(Platinum)) and 5 nm CoFeB. Due to the spin Hall effect, the heavy metal layer
generates a transverse spin current when a lateral charge current passes
through the strip. Using time-resolved all-optical pump-probe spectroscopy, we
investigate how this spin current affects the magnetization dynamics in the
adjacent CoFeB layer. We observed a linear spin current manipulation of the
effective Gilbert damping parameter for the Kittel mode from which we were able
to determine the system's spin Hall angles. Furthermore, we measured a strong
influence of the spin current on a high-frequency mode. We interpret this mode
an an exchange dominated higher order spin-wave resonance. Thus we infer a
strong dependence of the exchange constant on the spin current. | 1912.07728v1 |
2019-12-20 | Atomistic investigation of the temperature and size dependence of the energy barrier of CoFeB/MgO nanodots | The balance between low power consumption and high efficiency in memory
devices is a major limiting factor in the development of new technologies.
Magnetic random access memories (MRAM) based on CoFeB/MgO magnetic tunnel
junctions (MTJs) have been proposed as candidates to replace the current
technology due to their non-volatility, high thermal stability and efficient
operational performance. Understanding the size and temperature dependence of
the energy barrier and the nature of the transition mechanism across the
barrier between stable configurations is a key issue in the development of
MRAM. Here we use an atomistic spin model to study the energy barrier to
reversal in CoFeB/MgO nanodots to determine the effects of size, temperature
and external field. We find that for practical device sizes in the 10-50 nm
range the energy barrier has a complex behaviour characteristic of a transition
from a coherent to domain wall driven reversal process. Such a transition
region is not accessible to simple analytical estimates of the energy barrier
preventing a unique theoretical calculation of the thermal stability. The
atomistic simulations of the energy barrier give good agreement with
experimental measurements for similar systems which are at the state of the art
and can provide guidance to experiments identifying suitable materials and MTJ
stacks with the desired thermal stability. | 1912.09761v1 |
2020-07-06 | Transverse and Longitudinal Spin-Torque Ferromagnetic Resonance for Improved Measurements of Spin-Orbit Torques | Spin-torque ferromagnetic resonance (ST-FMR) is a common method used to
measure spin-orbit torques (SOTs) in heavy metal/ferromagnet bilayer
structures. In the course of a measurement, other resonant processes such as
spin pumping (SP) and heating can cause spin current or heat flows between the
layers, inducing additional resonant voltage signals via the inverse spin Hall
effect (ISHE) and Nernst effects (NE). In the standard ST-FMR geometry, these
extra artifacts exhibit a dependence on the angle of an in-plane magnetic field
that is identical to the rectification signal from the SOTs. We show
experimentally that the rectification and artifact voltages can be quantified
separately by measuring the ST-FMR signal transverse to the applied current
(i.e., in a Hall geometry) in addition to the usual longitudinal geometry. We
find that in Pt (6 nm)/CoFeB samples the contribution from the artifacts is
small compared to the SOT rectification signal for CoFeB layers thinner than 6
nm, but can be significant for thicker magnetic layers. We observe a sign
change in the artifact voltage as a function of CoFeB thickness that we suggest
may be due to a competition between a resonant heating effect and the SP/ISHE
contribution. | 2007.02850v1 |
2020-12-10 | Pulse-width and Temperature Dependence of Memristive Spin-Orbit Torque Switching | It is crucial that magnetic memory devices formed from magnetic
heterostructures possess sizable spin-orbit torque (SOT) efficiency and high
thermal stability to realize both efficient SOT control and robust storage of
such memory devices. However, most previous studies on various types of
magnetic heterostructures have focused on only their SOT efficiencies, whereas
the thermal stabilities therein have been largely ignored. In this work, we
study the temperature-dependent SOT and stability properties of two types of
W-based heterostructures, namely W/CoFeB/MgO (standard) and CoFeB/W/CoFeB/MgO
(field-free), from 25 ^{\circ}C (298 K) to 80 ^{\circ}C (353 K). Via
temperature-dependent SOT characterization, the SOT efficacies for both systems
are found to be invariant within the range of studied temperatures.
Temperature-dependent current-induced SOT switching measurements further show
that the critical switching current densities decrease with respect to the
ambient temperature; thermal stability factors ({\Delta}) are also found to
degrade as temperature increases for both standard and field-free systems. The
memristive SOT switching behaviors in both systems with various pulse-widths
and temperatures are also examined. Our results suggest that although the SOT
efficacy is robust against thermal effects, the reduction of {\Delta} at
elevated temperatures could be detrimental to standard memory as well as
neuromorphic (memristive) device applications. | 2012.05531v1 |
2021-02-06 | Spin pumping and inverse spin Hall effect in CoFeB/IrMn heterostructures | High spin to charge conversion efficiency is the requirement for the
spintronics devices which is governed by spin pumping and inverse spin Hall
effect (ISHE). In last one decade, ISHE and spin pumping are heavily
investigated in ferromagnet/ heavy metal (HM) heterostructures. Recently
antiferromagnetic (AFM) materials are found to be good replacement of HMs
because AFMs exhibit terahertz spin dynamics, high spin-orbit coupling, and
absence of stray field. In this context we have performed the ISHE in CoFeB/
IrMn heterostructures. Spin pumping study is carried out for
$Co_{40}Fe_{40}B_{20} (12\ nm)/ Cu (3\ nm)/ Ir_{50}Mn_{50} (t\ nm)/ AlO_{x} (3\
nm)$ samples where \textit{t} value varies from 0 to 10 nm. Damping of all the
samples are higher than the single layer CoFeB which indicates that spin
pumping due to IrMn is the underneath mechanism. Further the spin pumping in
the samples are confirmed by angle dependent ISHE measurements. We have also
disentangled other spin rectifications effects and found that the spin pumping
is dominant in all the samples. From the ISHE analysis the real part of spin
mixing conductance (\textit{$g_{r}^{\uparrow \downarrow}$}) is found to be
0.704 $\pm$ 0.003 $\times$ $10^{18}$ $m^{-2}$. | 2102.03624v2 |
2021-06-10 | Spin-Orbit Torque Engineering in β-W/CoFeB Heterostructures via Ta and V Alloying at Interfaces | Spin-orbit torque manifested as an accumulated spin-polarized moment at
nonmagnetic normal metal, and ferromagnet interfaces is a promising
magnetization switching mechanism for spintronic devices. To fully exploit this
in practice, materials with a high spin Hall angle, i.e., a charge-to-spin
conversion efficiency, are indispensable. To date, very few approaches have
been made to devise new nonmagnetic metal alloys. Moreover, new materials need
to be compatible with semiconductor processing. Here we introduce W-Ta and W-V
alloys and deploy them at the interface between $\beta$-W/CoFeB layers. First,
spin Hall conductivities of W-Ta and W-V structures with various compositions
are carried out by first-principles band calculations, which predict the spin
Hall conductivity of the W-V alloy is improved from $-0.82 \times 10^3$ S/cm
that of W to $-1.98 \times 10^3$ S/cm. Subsequently, heterostructure
fabrication and spin-orbit torque properties are characterized experimentally.
By alloying $\beta$-W with V at a concentration of 20 at%, we observe a large
enhancement of the absolute value of spin Hall conductivity of up to $-(2.77
\pm 0.31) \times 10^3$ S/cm. By employing X-ray diffraction and scanning
transmission electron microscopy, we further explain the enhancement of
spin-orbit torque efficiency is stemmed from W-V alloy between W and CoFeB. | 2106.05460v1 |
2022-11-04 | Exchange energies in CoFeB/Ru/CoFeB Synthetic Antiferromagnets | The interlayer exchange coupling confers specific properties to Synthetic
Antiferromagnets that make them suitable for several applications of
spintronics. The efficient use of this magnetic configuration requires an
in-depth understanding of the magnetic properties and their correlation with
the material structure. Here we establish a reliable procedure to quantify the
interlayer exchange coupling and the intralayer exchange stiffness in synthetic
antiferromagnets; we apply it to the ultrasmooth and amorphous
Co$_{40}$Fe$_{40}$B$_{20}$ (5-40 nm)/Ru/ Co$_{40}$Fe$_{40}$B$_{20}$ material
platform. The complex interplay between the two exchange interactions results
in a gradient of the magnetization orientation across the thickness of the
stack which alters the hysteresis and the spin wave eigenmodes of the stack in
a non trivial way. We measured the field-dependence of the frequencies of the
first four spin waves confined within the thickness of the stack. We modeled
these frequencies and the corresponding thickness profiles of these spin waves
using micromagnetic simulations. The comparison with the experimental results
allows to deduce the magnetic parameters that best account for the sample
behavior. The exchange stiffness is established to be 16 $\pm$ 2 pJ/m,
independently of the Co$_{40}$Fe$_{40}$B$_{20}$ thickness. The interlayer
exchange coupling starts from -1.7 mJ/m$^2$ for the thinnest layers and it can
be maintained above -1.3 mJ/m$^2$ for CoFeB layers as thick as 40 nm. The
comparison of our method with earlier characterizations using the sole
saturation fields argues for a need to revisit the tabulated values of
interlayer exchange coupling in thick synthetic antiferromagnets. | 2211.02497v1 |
2023-06-29 | Ultrafast THz probing of nonlocal orbital current in transverse multilayer metallic heterostructures | THz generation from femtosecond photoexcited spintronic heterostructures has
recently become a versatile tool for investigating ultrafast spin-transport and
transient charge-current in a non-contact and non-invasive manner. The same
from the orbital effects is still in the primitive stage. Here, we
experimentally demonstrate orbital-to-charge current conversion in metallic
heterostructures, consisting of a ferromagnetic layer adjacent to either a
light or a heavy metal layer, through detection of the emitted THz pulses.
Temperature-dependent experiments help to disentangle the orbital and spin
components that are manifested in the respective Hall-conductivities,
contributing to THz emission. NiFe/Nb shows the strongest inverse orbital Hall
effect with an experimentally extracted value of effective Hall-conductivity,
\sigma_SOH^int^eff ~ 195 {\Omega}^(-1){cm}^(-1), while CoFeB/Pt shows maximum
contribution from the inverse spin Hall effect. In addition, we observe nearly
ten-fold enhancement in the THz emission due to pronounced orbital-transport in
W-insertion heavy metal layer in CoFeB/W/Ta heterostructure as compared to the
CoFeB/Ta bilayer counterpart. | 2306.17027v2 |
2008-06-20 | Penetration Depth of Transverse Spin Current in Ferromagnetic Metals | The line width of the ferromagnetic resonance (FMR) spectrum of
Cu/CoFeB/Cu/Co/Cu is studied. Analyzing the FMR spectrum by the theory of spin
pumping, we determined the penetration depth of the transverse spin current in
the Co layer. The obtained penetration depth of Co is 1.7 nm. | 0806.3315v1 |
2019-11-08 | Room Temperature Spin to Charge Conversion in Amorphous Topological Insulating Gd-Alloyed BixSe1-x/CoFeB Bilayers | Disordered topological insulator (TI) films have gained intense interest by
benefiting from both the TIs exotic transport properties and the advantage of
mass production by sputtering. Here, we report on the clear evidence of
spin-charge conversion (SCC) in amorphous Gd-alloyed BixSe1-x (BSG)/CoFeB
bilayers fabricated by sputtering, which could be related to the amorphous TI
surface states. Two methods have been employed to study SCC in BSG/CoFeB(5 nm)
bilayers with different BSG thicknesses. Firstly, spin pumping is used to
generate a spin current in CoFeB and to detect SCC by inverse Edelstein effect.
The maximum SCC efficiency (SCE) is measured as large as 0.035 nm in a 6 nm
thick BSG sample, which shows a strong decay when tBSG increases due to the
increase of BSG surface roughness. The second method is the THz time-domain
spectroscopy, which reveals a small tBSG dependence of SCE, validating the
occurrence of a pure interface state related SCC. Furthermore, our
angle-resolved photoemission spectroscopy data show dispersive two-dimensional
surface states that cross the bulk gap until to the Fermi level, strengthening
the possibility of SCC due to the amorphous TI states. Our studies provide a
new experimental direction towards the search for topological systems in the
amorphous solids. | 1911.03323v12 |
2022-08-10 | Enhancement of Spin-Charge Conversion Efficiency for Co$_{3}$Sn$_{2}$S$_{2}$ across Transition from Paramagnetic to Ferromagnetic Phase | Co$_{3}$Sn$_{2}$S$_{2}$ (CSS) is one of the shandite compounds and becomes a
magnetic Weyl semimetal candidate below the ferromagnetic phase transition
temperature ($\textit{T}_\textrm{C}$). In this paper, we investigate the
temperature ($\textit{T}$) dependence of conversion between charge current and
spin current for the CSS thin film by measuring the spin-torque ferromagnetic
resonance (ST-FMR) for the trilayer consisting of CSS / Cu / CoFeB. Above
$\textit{T}_\textrm{C}$ ~ 170 K, the CSS / Cu / CoFeB trilayer exhibits the
clear ST-FMR signal coming from the spin Hall effect in the paramagnetic CSS
and the anisotropic magnetoresistance (AMR) of CoFeB. Below
$\textit{T}_\textrm{C}$, on the other hand, it is found that the ST-FMR signal
involves the dc voltages ($\textit{V}_\textrm{dc}$) not only through the AMR
but also through the giant magnetoresistance (GMR). Thus, the resistance
changes coming from both AMR and GMR should be taken into account to correctly
understand the characteristic field angular dependence of
$\textit{V}_\textrm{dc}$. The spin Hall torque generated from the ferromagnetic
CSS, which possesses the same symmetry as that for spin Hall effect, dominantly
acts on the magnetization of CoFeB. A definite increase in the spin-charge
conversion efficiency ($\xi$) is observed at $\textit{T}$ <
$\textit{T}_\textrm{C}$, indicating that the phase transition to the
ferromagnetic CSS promotes the highly efficient spin-charge conversion. In
addition, our theoretical calculation shows the increase in spin Hall
conductivity with the emergence of magnetic moment at $\textit{T}$ <
$\textit{T}_\textrm{C}$, which is consistent with the experimental observation. | 2208.05394v1 |
2023-11-10 | Observation by SANS and PNR of pure Néel-type domain wall profiles and skyrmion suppression below room temperature in magnetic [Pt/CoFeB/Ru]$_{10}$ multilayers | We report investigations of the magnetic textures in periodic [Pt(1
nm)/(CoFeB(0.8 nm)/Ru(1.4 nm)]$_{10}$ multilayers using polarised neutron
reflectometry (PNR) and small-angle neutron scattering (SANS). The multilayers
are known to host skyrmions stabilized by Dzyaloshinskii-Moriya interactions
induced by broken inversion symmetry and spin-orbit coupling at the asymmetric
interfaces. From depth-dependent PNR measurements, we observe well-defined
structural features, and obtain the layer-resolved magnetization profiles. The
in-plane magnetization of the CoFeB layers calculated from fitting of the PNR
profiles is found to be in excellent agreement with magnetometry data. Using
SANS as a bulk probe of the entire multilayer, we observe long-period magnetic
stripe domains and skyrmion ensembles with full orientational disorder at room
temperature. No sign of skyrmions is found below 250\,K, which we suggest is
due to an increase of a effective magnetic anisotropy in the CoFeB layer on
cooling that suppresses skyrmion stability. Using polarised SANS at room
temperature, we prove the existence of pure N\'eel-type windings in both stripe
domain and skyrmion regimes. No Bloch-type winding admixture, i.e. an
indication for hybrid windings, is detected within the measurement sensitivity,
in good agreement with expectations according to our micromagnetic modelling of
the multilayers. Our findings using neutron techniques offer valuable
microscopic insights into the rich magnetic behavior of skyrmion-hosting
multilayers, which are essential for the advancement of future skyrmion-based
spintronic devices. | 2311.05959v2 |
2007-12-17 | Spin tunneling in junctions with disordered ferromagnets | We provide compelling evidence to establish that, contrary to one's
elementary guess, the tunneling spin polarization (TSP) of amorphous CoFeB is
larger than that of highly textured fcc CoFeB. First principles atomic and
electronic structure calculations reveal striking agreement between the
measured TSP and the predicted s-electron spin polarization. Given the
disordered structure of the ternary alloy, not only do these results strongly
endorse our communal understanding of tunneling through AlOx, but they also
portray the key concepts that demand primary consideration in such complex
systems. | 0712.2722v1 |
2011-03-22 | Inductive determination of the optimum tunnel barrier thickness in magnetic tunnelling junction stacks for spin torque memory applications | We use pulsed inductive microwave magnetometry to study the precessional
magnetization dynamics of the free layer in CoFeB/MgO/CoFeB based magnetic
tunnelling junction stacks with varying MgO barrier thickness. From the field
dependence of the precession frequency we are able to derive the uniaxial
anisotropy energy and the exchange coupling between the free and the pinned
layer. Furthermore the field dependence of the effective damping parameter is
derived. Below a certain threshold barrier thickness we observe an increased
effective damping for antiparallel orientation of free and pinned layer which
would inhibit reversible low current density spin torque magnetization
reversal. Such inductive measurements, in combination with wafer probe station
based magneto transport experiments, allow a fast determination of the optimum
tunnel barrier thickness range for spin torque memory applications in a
lithography free process. | 1103.4248v1 |
2011-06-03 | Origin of in-plane uniaxial magnetic anisotropy in CoFeB amorphous ferromagnetic thin-films | Describing the origin of uniaxial magnetic anisotropy (UMA) is generally
problematic in systems other than single crystals. We demonstrate an in-plane
UMA in amorphous CoFeB films on GaAs(001) which has the expected symmetry of
the interface anisotropy in ferromagnetic films on GaAs(001), but strength
which is independent of, rather than in inverse proportion to, the film
thickness. We show that this volume UMA is consistent with a bond-orientational
anisotropy, which propagates the interface-induced UMA through the thickness of
the amorphous film. It is explained how, in general, this mechanism may
describe the origin of in-plane UMAs in amorphous ferromagnetic films. | 1106.0606v1 |
2012-05-13 | Voltage-Induced Ferromagnetic Resonance in Magnetic Tunnel Junctions | We demonstrate excitation of ferromagnetic resonance in CoFeB/MgO/CoFeB
magnetic tunnel junctions (MTJs) by the combined action of voltage-controlled
magnetic anisotropy (VCMA) and spin transfer torque (ST). Our measurements
reveal that GHz-frequency VCMA torque and ST in low-resistance MTJs have
similar magnitudes, and thus that both torques are equally important for
understanding high-frequency voltage-driven magnetization dynamics in MTJs. As
an example, we show that VCMA can increase the sensitivity of an MTJ-based
microwave signal detector to the sensitivity level of semiconductor Schottky
diodes. | 1205.2835v2 |
2012-08-08 | Spin transfer torque devices utilizing the giant spin Hall effect of tungsten | We report a giant spin Hall effect (SHE) in {\beta}-W thin films. Using spin
torque induced ferromagnetic resonance with a {\beta}-W/CoFeB bilayer
microstrip we determine the spin Hall angle to be |\theta|=0.30\pm0.02, large
enough for an in-plane current to efficiently reverse the orientation of an
in-plane magnetized CoFeB free layer of a nanoscale magnetic tunnel junction
adjacent to a thin {\beta}-W layer. From switching data obtained with such
3-terminal devices we independently determine |\theta|=0.33\pm0.06. We also
report variation of the spin Hall switching efficiency with W layers of
different resistivities and hence of variable ({\alpha} and {\beta}) phase
composition. | 1208.1711v1 |
2013-03-19 | X-ray absorption spectroscopy and magnetic circular dichroism studies of L1_0-Mn-Ga thin films | Tetragonally distorted \(\rm{Mn}_{3-x}\rm{Ga}_x\) thin films with \(0.1< x <
2\) show a strong perpendicular magnetic anisotropy and low magnetization and
thus have the potential to serve as electrodes in spin transfer torque magnetic
random access memory. Because a direct capping of these films with MgO is
problematic due to oxide formation, we examined the influence of a CoFeB
interlayer, and of two different deposition methods for the MgO barrier on the
formation of interfacial MnO for \(\rm{Mn}_{62}\rm{Ga}_{38}\) by element
specific X-ray absorption spectroscopy (XAS) and magnetic circular dichroism
(XMCD). A highly textured L1\(_0\) crystal structure of the Mn-Ga films was
verified by X-ray diffraction (XRD) measurements. For samples with e-beam
evaporated MgO barrier no evidence for MnO was found, whereas in samples with
magnetron sputtered MgO MnO was detected, even for the thickest interlayer
thickness. Both XAS and XMCD measurements showed an increasing interfacial MnO
amount with decreasing CoFeB interlayer thickness. Additional element specific
full hysteresis loops determined an out-of-plane magnetization axis for the Mn
and Co, respectively. | 1303.4648v2 |
2013-05-27 | Magnetization reversal in sub-100nm magnetic tunnel junctions with ultrathin MgO barrier biased along hard axis | We report on room temperature magnetoresistance and low frequency noise in
sub-100nm elliptic CoFeB/MgO/CoFeB magnetic tunnel junctions with ultrathin
(0.9nm) barriers. For magnetic fields applied along the hard axis, we observe
current induced magnetization switching between the antiparallel and parallel
alignments at DC current densities as low as 4*106A/cm2. We attribute the low
value of the critical current to the influence of localized reductions in the
tunnel barrier, which affects the current distribution. The analysis of random
telegraph noise, which appears in the field interval near a magnetization
switch, provides an estimate to the dimension of the pseudo pinholes that
trigger the magnetization switching via local spin torque. Micromagnetic
simulations qualitatively and quantitatively reproduce the main experimental
observations. | 1305.6209v1 |
2013-05-29 | Enhanced interface perpendicular magnetic anisotropy in Ta|CoFeB|MgO using nitrogen doped Ta underlayers | We show that the magnetic characteristics of Ta|CoFeB|MgO magnetic
heterostructures are strongly influenced by doping the Ta underlayer with
nitrogen. In particular, the saturation magnetization drops upon doping the Ta
underlayer, suggesting that the doped underlayer acts as a boron diffusion
barrier. In addition, the thickness of the magnetic dead layer decreases with
increasing nitrogen doping. Surprisingly, the interface magnetic anisotropy
increases to ~1.8 erg/cm2 when an optimum amount of nitrogen is introduced into
the Ta underlayer. These results show that nitrogen doped Ta serves as a good
underlayer for Spintronics applications including magnetic tunnel junctions and
domain wall devices. | 1305.6660v1 |
2013-08-08 | Interface control of the magnetic chirality in TaN|CoFeB|MgO heterosctructures | Recent advances in the understanding of spin orbital effects in ultrathin
magnetic heterostructures have opened new paradigms to control magnetic moments
electrically. The Dzyaloshinskii-Moriya interaction (DMI) is said to play a key
role in forming a Neel-type domain wall that can be driven by the spin Hall
torque, a torque resulting from the spin current generated in a neighboring
non-magnetic layer via the spin Hall effect. Here we show that the sign of the
DMI, which determines the direction to which a domain wall moves with current,
can be changed by modifying the adjacent non-magnetic layer. We find that the
sense of rotation of a domain wall spiral is reversed when the Ta underlayer is
doped with nitrogen in Ta|CoFeB|MgO heterostructures. The spin Hall angle of
the Ta and nitrogen doped Ta underlayers carry the same sign, suggesting that
the sign of the DMI is defined at the interface. Depending on the sense of
rotation, spin transfer torque and spin Hall torque can either compete or
assist each other, thus influencing the efficiency of moving domain walls with
current. | 1308.1751v1 |
2013-10-18 | Quantitative characterization of the spin orbit torque using harmonic Hall voltage measurements | Solid understanding of current induced torques is key to the development of
current and voltage controlled magnetization dynamics in ultrathin magnetic
heterostructures. To evaluate the size and direction of such torques, or
effective fields, a number of methods have been employed. Here we examine the
adiabatic (low frequency) harmonic Hall voltage measurement that has been used
to study the effective field. We derive an analytical formula for the harmonic
Hall voltages to evaluate the effective field for both out of plane and
in-plane magnetized systems. The formula agrees with numerical calculations
based on a macrospin model. Two different in-plane magnetized films,
Pt|CoFeB|MgO and CuIr|CoFeB|MgO are studied using the formula developed. The
effective field obtained for the latter system shows relatively good agreement
with that estimated using a spin torque switching phase diagram measurements
reported previously. Our results illustrate the versatile applicability of
harmonic Hall voltage measurement for studying current induced torques in
magnetic heterostructures. | 1310.4879v1 |
2013-12-03 | Depth dependent magnetization profiles of hybrid exchange springs | We report on the magnetization depth profile of a hybrid exchange spring
system in which a Co/Pd multilayer with perpendicular anisotropy is coupled to
a CoFeB thin film with in-plane anisotropy. The competition between these two
orthogonal anisotropies promotes a strong depth dependence of the magnetization
orientation. The angle of the magnetization vector is sensitive both to the
strength of the individual anisotropies and to the local exchange constant, and
is thus tunable by changing the thickness of the CoFeB layer and by
substituting Ni for Pd in one layer of the Co/Pd stack. The resulting magnetic
depth profiles are directly probed by element specific x-ray magnetic circular
dichroism (XMCD) of the Co, Fe, and Ni layers located at different average
depths. The experimental results are corroborated by micromagnetic simulations. | 1312.0878v1 |
2014-04-04 | Angular and temperature dependence of current induced spin-orbit effective fields in Ta/CoFeB/MgO nanowires | Current induced spin-orbit effective magnetic fields in
metal/ferromagnet/oxide trilayers provide a new way to manipulate the
magnetization, which is an alternative to the conventional current induced spin
transfer torque arising from noncollinear magnetization. Ta/CoFeB/MgO
structures are expected to be useful for non-volatile memories and logic
devices due to its perpendicular anisotropy and large current induced
spin-orbit effective fields. However many aspects such as the angular and
temperature dependent phenomena of the effective fields are little understood.
Here, we evaluate the angular and temperature dependence of the current-induced
spin-orbit effective fields considering contributions from both the anomalous
and planar Hall effects. The longitudinal and transverse components of
effective fields are found to have strong angular dependence on the
magnetization direction at 300 K. The transverse field decreases significantly
with decreasing temperature, whereas the longitudinal field shows weaker
temperature dependence. Our results reveal important features and provide an
opportunity for a more comprehensive understanding of current induced
spin-orbit effective fields. | 1404.1130v1 |
2014-04-08 | Ferromagnetic resonance spin pumping in CoFeB with highly resistive non-magnetic electrodes | The relative contribution of spin pumping and spin rectification from the
ferromagnetic resonance of CoFeB/non-magnetic bilayers was investigated as a
function of non-magnetic electrode resistance. Samples with highly resistive
electrodes of Ta or Ti exhibit a stronger spin rectification signal, which may
result in over-(or under-)estimation of the spin Hall angle of the materials,
while those with low resistive electrodes of Pt or Pd show the domination of
the inverse spin Hall effect from spin pumping. By comparison with samples of
single FM layer and an inverted structure, we provide a proper analysis method
to extract spin pumping contribution. | 1404.1993v1 |
2014-05-02 | Spin-orbit torque-driven magnetization switching and thermal effects studied in Ta\CoFeB\MgO nanowires | We demonstrate magnetization switching in out-of-plane magnetized
Ta\CoFeB\MgO nanowires by current pulse injection along the nanowires, both
with and without a constant and uniform magnetic field collinear to the current
direction. We deduce that an effective torque arising from spin-orbit effects
in the multilayer drives the switching mechanism. While the generation of a
component of the magnetization along the current direction is crucial for the
switching to occur, we observe that even without a longitudinal field thermally
generated magnetization fluctuations can lead to switching. Analysis using a
generalized N\'eel-Brown model enables key parameters of the thermally induced
spin-orbit torques switching process to be estimated, such as the attempt
frequency and the effective energy barrier. | 1405.0452v1 |
2014-05-08 | Controlling boron redistribution in CoFeB/MgO magnetic tunnel junctions during annealing by variation of cap layer materials and MgO deposition methods | Magnetic tunnel junctions with crystalline MgO tunnel barrier and amorphous
CoFeB electrodes received much attention due to their high tunnel magneto
resistance ratio at room temperature. One important parameter for achieving
high tunnel magneto resistance ratios is to control the boron diffusion from
the electrodes especially during post growth annealing. By high resolution
transmission electron microscopy and electron energy loss spectroscopy
techniques we show that the cap layer material adjacent to the electrodes and
the MgO deposition method are crucial to control boron redistribution. It is
pointed out, that Ta cap layers acts as sinks for boron during annealing in
contrast to Ru layers. Furthermore, radio frequency sputtered MgO tunneling
barriers contain a rather high concentraion of boron in trigonal
[BO$_3$]$^{3-}$ - environment after annealing in contrast to electron beam
evaporated MgO which is virtually free from any boron. Our data further
indicate that neither boron nor oxygen-vacancy-related gap states in the bulk
of MgO barriers affect spin polarized transport for tunnel magneto resistance
ratios at the level of 200%. | 1405.1907v1 |
2014-07-30 | Room Temperature Spin Pumping in Topological Insulator Bi2Se3 | Three-dimensional (3D) topological insulators are known for their strong
spin-orbit coupling and the existence of spin-textured topological surface
states which could be potentially exploited for spintronics. Here, we
investigate spin pumping from a metallic ferromagnet (CoFeB) into a 3D
topological insulator (Bi2Se3) and demonstrate successful spin injection from
CoFeB into Bi2Se3 and the direct detection of the electromotive force generated
by the inverse spin Hal effect (ISHE) at room temperature. The spin pumping,
driven by the magnetization dynamics of the metallic ferromagnet, introduces a
spin current into the topological insulator layer, resulting in a broadening of
the ferromagnetic resonance (FMR) linewidth. We find that the FMR linewidth
more than quintuples, the spin mixing conductance can be as large as
3.4*10^20m^-2 and the spin Hall angle can be as large as 0.23 in the Bi2Se3
layer. | 1407.7940v1 |
2014-11-18 | Towards wafer scale inductive characterization of spin transfer torque critical current density of magnetic tunnel junction stacks | We explore the prospects of wafer scale inductive probing of the critical
current density $j^{c0}$ for spin transfer torque switching of a
CoFeB/MgO/CoFeB magnetic tunnel junction with varying MgO thickness. From
inductive measurements magnetostatic parameters and the effective damping are
derived and $j^{c0}$ is calculated based on spin transfer torque equations. The
inductive values compare well to the values derived from current induced
switching measurements on individual nanopillars. Using a wafer scale inductive
probe head could in the future enable wafer probe station based metrology of
$j^{c0}$. | 1411.4868v1 |
2015-04-25 | Magnetic phase transitions in Ta/CoFeB/MgO multilayers | We study thin films and magnetic tunnel junction nanopillars based on
Ta/Co$_{20}$Fe$_{60}$B$_{20}$/MgO multilayers by electrical transport and
magnetometry measurements. These measurements suggest that an ultrathin
magnetic oxide layer forms at the Co$_{20}$Fe$_{60}$B$_{20}$/MgO interface. At
approximately 160 K, the oxide undergoes a phase transition from an insulating
antiferromagnet at low temperatures to a conductive weak ferromagnet at high
temperatures. This interfacial magnetic oxide is expected to have significant
impact on the magnetic properties of CoFeB-based multilayers used in spin
torque memories. | 1504.06716v1 |
2015-05-21 | Resistance minimum and electrical conduction mechanism in polycrystalline CoFeB thin films | The temperature dependent resistance $R$($T$) of polycrystalline
ferromagnetic CoFeB thin films of varying thickness are analyzed considering
various electrical scattering processes. We observe a resistance minimum in
$R$($T$) curves below $\simeq$ 29 K, which can be explained as an effect of
intergranular Coulomb interaction in a granular system. The structural and
Coulomb interaction related scattering processes contribute more as the film
thickness decreases implying the role of disorder and granularity. Although the
magnetic contribution to the resistance is the weakest compared to these two,
it is the only thickness independent process. On the contrary, the negative
coefficient of resistance can be explained by electron interaction effect in
disordered amorphous films. | 1505.05711v2 |
2015-08-04 | Scanning Kerr microscopy of current induced switching in Ta/CoFeB/MgO films with perpendicular magnetic anisotropy | Ta/CoFeB/MgO trilayers with perpendicular magnetic anisotropy are expected to
play a key role in the next generation of current and electric field switched
memory and logic devices. In this study we use Kerr microscopy alongside
electrical transport measurement to gain insight into the underlying switching
mechanisms of such devices. We find switching to be a stochastic, domain wall
driven process, the speed of which is strongly dependent on the switching
current. Kerr imaging shows domain nucleation at the edge of the device which
modelling reveals is likely assisted by the perpendicular component of the
Oersted field. Further domain growth, leading to magnetisation reversal may be
assisted by spin torques. | 1508.00833v2 |
2015-10-16 | Enhanced orbital magnetic moments in magnetic heterostructures with interface perpendicular magnetic anisotropy | We have studied the magnetic layer thickness dependence of the orbital
magnetic moment in magnetic heterostructures to identify contributions from
interfaces. Three different heterostructures, Ta/CoFeB/MgO, Pt/Co/AlO$_x$ and
Pt/Co/Pt, which possess significant interface contribution to the perpendicular
magnetic anisotropy, are studied as model systems. X-ray magnetic circular
dichroism spectroscopy is used to evaluate the relative orbital moment, i.e.
the ratio of the orbital to spin moments, of the magnetic elements constituting
the heterostructures. We find that the relative orbital moment of Co in
Pt/Co/Pt remains constant against its thickness whereas the moment increases
with decreasing Co layer thickness for Pt/Co/AlO$_x$, suggesting that a
non-zero interface orbital moment exists for the latter system. For
Ta/CoFeB/MgO, a non-zero interface orbital moment is found only for Fe. X-ray
absorption spectra shows that a particular oxidized Co state in Pt/Co/AlO$_x$,
absent in other heterosturctures, may give rise to the interface orbital moment
in this system. These results show element specific contributions to the
interface orbital magnetic moments in ultrathin magnetic heterostructures. | 1510.04756v1 |
2015-12-02 | Dynamic cantilever magnetometry of individual CoFeB nanotubes | We investigate single CoFeB nanotubes with hexagonal cross-section using
dynamic cantilever magnetometry (DCM). We develop both an analytical model
based on the Stoner-Wohlfarth approximation and a broadly applicable numerical
framework for analyzing DCM measurements of magnetic nanostructures.
Magnetometry data show the presence of a uniformly magnetized configuration at
high external fields with $\mu_0 M_s =1.3 \pm 0.1$ T and non-uniform
configurations at low fields. In this low-field regime, comparison between
numerical simulations and DCM measurements supports the existence of
flux-closure configurations. Crucially, evidence of such configurations is only
apparent because of the sensitivity of DCM to single nanotubes, whereas
conventional measurements of ensembles are often obscured by sample-to-sample
inhomogeneities in size, shape, and orientation | 1512.00621v1 |
2016-04-10 | Interfacial Dzyaloshinskii-Moriya interaction in Pt/CoFeB films: effect of the heavy-metal thickness | We report the observation of a Pt layer thickness dependence on the induced
interfacial Dzyaloshinskii-Moriya interaction in ultra-thin
Pt($d_{\text{Pt}}$)/CoFeB films. Taking advantage of the large spin-orbit
coupling of the heavy metal, the interfacial Dzyaloshinskii-Moriya interaction
is quantified by Brillouin light scattering measurements of the frequency
non-reciprocity of spin-waves in the ferromagnet. The magnitude of the induced
Dzyaloshinskii-Moriya coupling is found to saturate to a value $0.45$
mJ$/$m${}^2$ for Pt thicknesses larger than $\sim 2$ nm. The experimental
results are explained by analytical calculations based on the 3-site indirect
exchange mechanism that predicts a Dzyaloshinskii-Moriya interaction at the
interface between a ferromagnetic thin layer and a heavy metal. Our findings
open up a way to control and optimize chiral effects in ferromagnetic thin
films through the thickness of the heavy metal layer. | 1604.02626v2 |
2016-04-14 | THz-driven ultrafast spin-lattice scattering in amorphous metallic ferromagnets | We use single-cycle THz fields and the femtosecond magneto-optical Kerr
effect to respectively excite and probe the magnetization dynamics in two
thin-film ferromagnets with different lattice structure: crystalline Fe and
amorphous CoFeB. We observe Landau-Lifshitz-torque magnetization dynamics of
comparable magnitude in both systems, but only the amorphous sample shows
ultrafast demagnetization caused by the spin-lattice depolarization of the
THz-induced ultrafast spin current. Quantitative modelling shows that such
spin-lattice scattering events occur on similar time scales than the
conventional spin conserving electronic scattering ($\sim30$ fs). This is
significantly faster that optical laser-induced demagnetization. THz
conductivity measurements point towards the influence of lattice disorder in
amorphous CoFeB as the driving force for enhanced spin-lattice scattering. | 1604.04077v1 |
2016-04-26 | Comparison of laser induced and intrinsic tunnel magneto-Seebeck effect in CoFeB/MgAl$_2$O$_4$ and CoFeB/MgO magnetic tunnel junctions | We present a comparison of the tunnel magneto-Seebeck effect for laser
induced and intrinsic heating. Therefore,
Co$_{40}$Fe$_{40}$B$_{20}$/MgAl$_2$O$_4$ and Co$_{25}$Fe$_{55}$B$_{20}$/MgO
magnetic tunnel junctions have been prepared. The TMS ratio of 3\,\% in case of
the MAO MTJ agrees well with ratios found for other barrier materials, while
the TMS ratio of 23\,\% of the MgO MTJ emphasizes the influence of the CoFe
composition. We find results using the intrinsic method that differ in sign and
magnitude in comparison to the results of the laser heating. The intrinsic
contributions can alternatively be explained by the Brinkman model and the
given junction properties. Especially, we are able to demonstrate that the
symmetric contribution is solely influenced by the barrier asymmetry. Thus, we
conclude that the symmetry analysis used for the intrinsic method is not
suitable to unambiguously identify an intrinsic tunnel magneto-Seebeck effect. | 1604.07569v2 |
2016-05-23 | Direct measurement of interfacial Dzyaloshinskii-Moriya interaction in X/CoFeB/MgO heterostructures with a scanning-NV magnetometer | The Dzyaloshinskii-Moriya Interaction (DMI) has recently attracted
considerable interest owing to its fundamental role in the stabilization of
chiral spin textures in ultrathin ferromagnets, which are interesting
candidates for future spintronic technologies. Here we employ a scanning
nano-magnetometer based on a single nitrogen-vacancy (NV) defect in diamond to
locally probe the strength of the interfacial DMI in CoFeB/MgO ultrathin films
grown on different heavy metal underlayers X=Ta,TaN, and W. By measuring the
stray field emanating from DWs in micron-long wires of such materials, we
observe deviations from the Bloch profile for TaN and W underlayers that are
consistent with a positive DMI value favoring right-handed chiral spin
structures. Moreover, our measurements suggest that the DMI constant might vary
locally within a single sample, illustrating the importance of local probes for
the study of magnetic order at the nanoscale. | 1605.07044v1 |
2016-07-21 | Effect of annealing on the interfacial Dzyaloshinskii-Moriya interaction in Ta/CoFeB/MgO trilayers | The interfacial Dzyaloshinskii-Moriya interaction (DMI) has been shown to
stabilize homochiral N\'eel-type domain walls in thin films with perpendicular
magnetic anisotropy and as a result permit them to be propagated by a spin Hall
torque. In this study, we demonstrate that in Ta/Co$_{20}$Fe$_{60}$B$_{20}$/MgO
the DMI may be influenced by annealing. We find that the DMI peaks at
$D=0.057\pm0.003$ mJ/m$^{2}$ at an annealing temperature of 230 $^{\circ}$C.
DMI fields were measured using a purely field-driven creep regime domain
expansion technique. The DMI field and the anisotropy field follow a similar
trend as a function of annealing temperature. We infer that the behavior of the
DMI and the anisotropy are related to interfacial crystal ordering and B
expulsion out of the CoFeB layer as the annealing temperature is increased. | 1607.06405v1 |
2017-07-26 | Reorientable Spin Direction for Spin Current Produced by the Anomalous Hall Effect | We show experimentally that the spin direction of the spin current generated
by spin-orbit interactions within a ferromagnetic layer can be reoriented by
turning the magnetization direction of this layer. We do this by measuring the
field-like component of spin-orbit torque generated by an exchange-biased FeGd
thin film and acting on a nearby CoFeB layer. The relative angle of the CoFeB
and FeGd magnetic moments is varied by applying an external magnetic field. We
find that the resulting torque is in good agreement with predictions that the
spin current generated by the anomalous Hall effect from the FeGd layer depends
on the FeGd magnetization direction $\hat{m}_{FeGd}$ according to
$\vec{\sigma}\propto\left ( \hat{y}\cdot \hat{m}_{FeGd} \right
)\hat{m}_{FeGd}$, where $\hat{y}$ is the in-plane direction perpendicular to
the applied charge current. Because of this angular dependence, the spin-orbit
torque arising from the anomalous Hall effect can be non-zero in a sample
geometry for which the spin Hall torque generated by non-magnetic materials is
identically zero. | 1707.08631v1 |
2017-11-30 | Domain Wall Motion Driven by Laplace Pressure in CoFeB-MgO Nanodots with Perpendicular Anisotropy | We have studied the magnetization reversal of CoFeB-MgO nanodots with
perpendicular anisotropy for size ranging from w=400 nm to 1 {\mu}m. Contrary
to previous experiments, the switching field distribution is shifted toward
lower magnetic fields as the size of the elements is reduced with a mean
switching field varying as 1/w. We show that this mechanism can be explained by
the nucleation of a pinned magnetic domain wall (DW) at the edges of the
nanodots where damages are introduced by the patterning process. As the surface
tension (Laplace pressure) applied on the DW increases when reducing the size
of the nanodots, we demonstrate that the depinning field to reverse the entire
elements varies as 1/w. These results suggest that the presence of DWs has to
be considered in the switching process of nanoscale elements and open a path
toward scalable spintronic devices. | 1711.11334v2 |
2018-03-08 | CMOS compatible W/CoFeB/MgO spin Hall nano-oscillators with wide frequency tunability | We demonstrate low-operational-current W/Co$_{20}$Fe$_{60}$B$_{20}$/MgO spin
Hall nano-oscillators (SHNOs) on highly resistive silicon (HiR-Si) substrates.
Thanks to a record high spin Hall angle of the $\beta$-phase W ($\theta_{SH}$ =
-0.53), a very low threshold current density of 3.3 $\times$ 10$^{7}$ A/cm$^2$
can be achieved. Together with their very wide frequency tunability (7-28 GHz),
promoted by a moderate perpendicular magnetic anisotropy, this makes
HiR-Si/W/CoFeB based SHNOs potential candidates for wide-band microwave signal
generation. Their CMOS compatibility offers a promising route towards the
integration of spintronic microwave devices with other on-chip semiconductor
microwave components. | 1803.03032v1 |
2018-06-13 | Enhanced Spin-Orbit Torques and Magnetization Switching through Interface Engineering | The origin of spin-orbit torques generated from the conversion of
charge-to-spin currents is of considerable debate. Solid understanding of the
physics behind is key to the development of current and voltage controlled
switching dynamics in ultrathin heterostuctures. The field free switching
observed recently (Phys. Rev. Lett. 120, 117703 (2018)) in a Pt/W/CoFeB
structure has intensified such a debate. Here we derive a formula to evaluate a
perpendicular effective field generated when the current flows through the
heterostructure, considering the large resistivity difference between the two
normal metal layers and the chemical potential gradient created at the
interface. Together with recent X-ray photoelectron spectroscopy findings at
the interface of a Pd/CoFeB structure, we conclude that a new torque generated
may play a key role in the field free switching. The model and mechanism
proposed agrees with previous reports on spin current injection and field free
switching using different interface engineering methods. | 1806.05163v1 |
2018-06-20 | Domain wall resistance in CoFeB-based heterostructures with interface Dzyaloshinskii-Moriya interaction | We have studied the domain wall resistance in W/Ta/CoFeB/MgO
heterostructures. The Ta layer thickness is varied to control the type of
domain walls via changes in the interfacial Dzyaloshinskii Moriya interaction.
We find a nearly constant domain wall resistance against the Ta layer
thickness. Adding contributions from the anisotropic magnetoresistance, spin
Hall magnetoresistance and anomalous Hall effect describe well the domain wall
resistance of the thick Ta layer films. However, a discrepancy remains for the
thin Ta layer films wherein chiral N\'eel-like domain walls are found. These
results show the difficulty of studying the domain wall type from resistance
measurements. | 1806.07750v1 |
2018-11-13 | Spin-orbit torques acting upon a perpendicularly-magnetized Py layer | We show that Py, a commonly-used soft ferromagnetic material with weak
anisotropy, can become perpendicularly-magnetized while depositing on Ta buffer
layer with Hf or Zr insertion layers (ILs) and MgO capping layer. By using two
different approaches, namely harmonic voltage measurement and hysteresis loop
shift measurement, the dampinglike spin-orbit torque (DL-SOT) efficiencies from
Ta/IL/Py/IL/MgO magnetic heterostructures with perpendicular magnetic
anisotropy are characterized. We find that though Ta has a significant spin
Hall effect, the DL-SOT efficiencies are small in systems with the Ta/Py
interface compared to that obtained from the control sample with the
traditional Ta/CoFeB interface. Our results indicate that the spin transparency
for the Ta/Py interface is much less than that for the Ta/CoFeB interface,
which might be related to the variation of spin mixing conductance for
different interfaces. | 1811.05164v1 |
2017-04-01 | Thermally nucleated magnetic reversal in CoFeB/MgO nanodots | Power consumption is the main limitation in the development of new high
performance random access memory for portable electronic devices. Magnetic RAM
(MRAM) with CoFeB/MgO based magnetic tunnel junctions (MTJs) is a promising
candidate for reducing the power consumption given its non-volatile nature
while achieveing high performance. The dynamic properties and switching
mechanisms of MTJs are critical to understanding device operation and to enable
scaling of devices below 30 nm in diameter. Here we show that the magnetic
reversal mechanism is incoherent and that the switching is thermally nucleated
at device operating temperatures. Moreover, we find an intrinsic thermal
switching field distribution arising on the sub-nanosecond timescale even in
the absence of size and anisotropy distributions or material defects. These
features represent the characteristic signature of the dynamic properties in
MTJs and give an intrinsic limit to reversal reliability in small magnetic
nanodevices. | 1704.00106v1 |
2017-05-13 | Tuning the perpendicular magnetic anisotropy, spin Hall switching current density and domain wall velocity by submonolayer insertion in Ta / CoFeB / MgO heterostructures | By submonolayer insertion of Au, Pt, or Pd into Ta / CoFeB / MgO / Ta
heterostructures we tune the perpendicular magnetic anisotropy and the coercive
field of the ferromagnetic layer. We demonstrate that this has a major
influence on the spin Hall switching current density and its dependence on the
external magnetic field. Despite a rather small effective spin Hall angle of
$\theta_\mathrm{SH} \approx -0.07$, we obtain switching current densities as
low as $2 \times 10^{10}$ A/m$^2$ with a 2 \AA{} Au interlayer. We find that
the Dzyaloshinskii-Moriya interaction parameter $D$ is reduced with Au or Pd
interlayers, and the perpendicular anisotropy field is reduced by an order of
magnitude with the Pd interlayer. The dependence of the switching current
density on the current pulse width is quantitatively explained with a domain
wall nucleation and propagation model. Interface engineering is thus found to
be a suitable route to tailor the current-induced magnetization switching
properties of magnetic heterostructures. | 1705.04800v1 |
2019-10-17 | Planar Hall driven torque in a FM/NM/FM system | An important goal of spintronics is to covert a charge current into a spin
current with a controlled spin polarization that can exert torques on an
adjacent magnetic layer. Here we demonstrate such torques in a two ferromagnet
system. A CoNi multilayer is used as a spin current source in a sample with
structure CoNi/Au/CoFeB. Spin torque ferromagnetic resonance is used to measure
the torque on the CoFeB layer. The response as a function of the applied field
angle and current is consistent with the symmetry expected for a torques
produced by the planar Hall effect originating in CoNi. We find the strength of
this effect to be comparable to that of the spin Hall effect in platinum,
indicating that the planar Hall effect holds potential as a spin current source
with a controllable polarization direction. | 1910.08039v1 |
2019-06-14 | Mechanisms of FMR line broadening in CoFeB-LiNbO$_3$ granular films in the vicinity of metal-insulator transition | Metal-insulator (CoFeB)$_x$(LiNbO$_3$)$_{100-x}$ nanocomposite films with
different content of the ferromagnetic (FM) phase $x$ are investigated by
ferromagnetic resonance (FMR) technique. A strong change of the FMR line shape
is observed in the vicinity of metal-insulator transition (MIT) of the film,
where the hopping-type conductivity $\sigma$ modifies to the regime of a strong
intergranular tunnelling, characterized by a logarithmic dependence $\sigma(T)$
at high temperatures. It is shown that below MIT, the FMR linewidth is mainly
determined by the inhomogeneous distribution of the local anisotropy axes in
the film plane. Above MIT, the contribution of this inhomogeneity to the line
broadening decreases. At the same time, two-magnon magnetic relaxation
processes begin to play a significant role in the formation of the linewidth.
The observed behaviour indicates the critical role of interparticle exchange in
the tunnelling regime above MIT of the nanocomposite. | 1906.06320v2 |
2020-04-03 | Modulation of field-like spin orbit torque in heavy metal / ferromagnet heterostructure | Recent studies rediscovered the crucial role of field-like spin orbit torque
(SOT) in nanosecond-timescale SOT dynamics. However, there is not yet an
effective way to control its relative amplitude. Here, we experimentally
modulate the field-like SOT in W/CoFeB/MgO trilayers through tuning the
interfacial spin accumulation. By performing spin Hall magnetoresistance
measurement, we find that the CoFeB with enhanced spin dephasing, either
generated from larger layer thickness or from proper annealing, can distinctly
boost the spin absorption and enhance the interfacial spin mixing conductance
G_r. While the damping-like torque efficiency increases with G_r, the
field-like torque efficiency turns out to decrease with it. The results suggest
that the interfacial spin accumulation, which largely contributes to a
field-like torque, is reduced by higher interfacial spin transparency. Our work
shows a new path to further improve the performance of SOT-based magnetic
devices. | 2004.01357v1 |
2020-04-29 | Terahertz Emission From an Exchange-Coupled Synthetic Antiferromagnet | We report on terahertz emission from FeMnPt/Ru/FeMnPt and
Pt/CoFeB/Ru/CoFeB/Pt synthetic antiferromagnet (SAF) structures upon
irradiation by a femtosecond laser; the former is via the anomalous Hall
effect, whereas the latter is through the inverse spin Hall effect. The
antiparallel alignment of the two ferromagnetic layers leads to a terahertz
emission peak amplitude that is almost double that for a corresponding
single-layer or bilayer emitter with the same equivalent thickness. In
addition, we demonstrate by both simulation and experiment that terahertz
emission provides a powerful tool to probe the magnetization reversal processes
of individual ferromagnetic layers in a SAF structure, as the terahertz signal
is proportional to the vector difference of the magnetizations of the two
ferromagnetic layers. | 2004.14128v1 |
2020-05-11 | Improving thermal stability of MnN/CoFeB exchange bias systems by optimizing the Ta buffer layer | We investigated the influence of the Ta buffer layer on the thermal stability
of polycrystalline Ta/ MnN/ CoFeB exchange bias systems, showing high exchange
bias of about 1800 Oe at room temperature. The thermal stability of those
trilayer systems is limited by nitrogen diffusion that occurs during annealing
processes. Most of the nitrogen diffuses into the Ta buffer layer, which is
necessary for good crystal growth of MnN and thus a crucial component of the
exchange bias system. In order to improve the thermal stability, we prepared
exchange bias stacks where we varied the Ta thickness to look for an optimum
value that guarantees stable and high exchange over a broad temperature range.
Our findings show that thin layers of 2-5 nm Ta indeed support stable exchange
bias up to annealing temperatures of more than $550^{\circ}$C. Furthermore, we
found that the introduction of a TaN$_{\text{x}}$ layer between MnN and Ta,
acting as a barrier, can prevent nitrogen diffusion. However, our results show
that those measures, even though being beneficial in terms of thermal
stability, often lead to decreased crystallinity and thus lower the exchange
bias. | 2005.05166v1 |
2017-02-24 | Giant perpendicular exchange bias with antiferromagnetic MnN | We investigated an out-of-plane exchange bias system that is based on the
antiferromagnet MnN. Polycrystalline, highly textured film stacks of Ta / MnN /
CoFeB / MgO / Ta were grown on SiO$_x$ by (reactive) magnetron sputtering and
studied by x-ray diffraction and Kerr magnetometry. Nontrivial modifications of
the exchange bias and the perpendicular magnetic anisotropy were observed both
as functions of film thicknesses as well as field cooling temperatures. In
optimized film stacks, a giant perpendicular exchange bias of 3600 Oe and a
coercive field of 350 Oe were observed at room temperature. The effective
interfacial exchange energy is estimated to be $J_\mathrm{eff} = 0.24$ mJ/m$^2$
and the effective uniaxial anisotropy constant of the antiferromagnet is
$K_\mathrm{eff} = 24$ kJ/m$^3$. The maximum effective perpendicular anisotropy
field of the CoFeB layer is $H_\mathrm{ani} = 3400$ Oe. These values are larger
than any previously reported values. These results possibly open a route to
magnetically stable, exchange biased perpendicularly magnetized spin valves. | 1702.07538v2 |
2019-05-22 | Reduced Exchange Interactions in Magnetic Tunnel Junction Free Layers with Insertion Layers | Perpendicularly magnetized CoFeB layers with ultra-thin non-magnetic
insertion layers are very widely used as the electrodes in magnetic tunnel
junctions for spin transfer magnetic random access memory devices. Exchange
interactions play a critical role in determining the thermal stability of
magnetic states in such devices and their spin torque switching efficiency.
Here the exchange constant of free layers incorporated in full magnetic tunnel
junction layer stacks, specifically CoFeB free layers with W insertion layers
is determined by magnetization measurements in a broad temperature range. A
significant finding is that the exchange constant decreases significantly and
abruptly with W insertion layer thickness. The perpendicular magnetic
anisotropy shows the opposite trend; it initially increases with W insertion
layer thickness and shows a broad maximum for approximately one monolayer (0.3
nm) of W. These results highlight the interdependencies of magnetic
characteristics required to optimize the performance of magnetic tunnel
junction devices. | 1905.09329v1 |
2019-08-06 | Superparamagnetic dwell times and tuning of switching rates in perpendicular CoFeB/MgO/CoFeB tunnel junctions | Thin electrodes of magnetic tunnel junctions can show superparamagnetism at
surprisingly low temperature. We analysed their thermally induced switching for
varying temperature, magnetic and electric field. Although the dwell times
follow an Arrhenius law, they are orders of magnitude too small compared to a
model of single domain activation. Including entropic effects removes this
inconsistency and leads to a magnetic activation volume much smaller than that
of the electrode. Comparing data for varying barrier thickness then allows to
separate the impact of Zeman energy, spin-transfer-torque and voltage induced
anisotropy change on the dwell times. Based on these results, we demonstrate a
tuning of the switching rates by combining magnetic and electric fields, which
opens a path for their application in noisy neural networks. | 1908.02139v3 |
2019-09-06 | Effect of Tantalum spacer thickness and deposition conditions on the properties of MgO/CoFeB/Ta/CoFeB/MgO free layers | To get stable perpendicularly magnetized tunnel junctions at small device
dimensions, composite free layers that comprise two MgO/FeCoB interfaces as
sources of interface anisotropy are generally used. Proper cristallisation and
annealing robustness is typically ensured by the insertion of a spacer layer of
the early transition metal series within the FeCoB layer. We study the
influence of the spacer thickness and growth condition on the switching metrics
of tunnel junctions thermally annealed at 400$^\circ$C for the case of 1-4
\r{A} Ta spacers. Thick Ta spacer results in a large anisotropies indicative of
a better defined top FeCoB/MgO interface, but this is achieved at the
systematic expense of a stronger damping. For the best anisotropy-damping
compromise, junctions of diameter 22 nm can still be stable and spin-torque
switched. Coercivity and inhomogeneous linewidth broadening, likely arising
from roughness at the FeCoB/Ta interface, can be reduced if a sacrificial Mg
layer is inserted before the Ta spacer deposition. | 1909.02741v1 |
2020-02-16 | Electric-field control of spin-orbit torques in perpendicularly magnetized W/CoFeB/MgO films | Controlling magnetism by electric fields offers a highly attractive
perspective for designing future generations of energy-efficient information
technologies. Here, we demonstrate that the magnitude of current-induced
spin-orbit torques in thin perpendicularly magnetized CoFeB films can be tuned
and even increased by electric field generated piezoelectric strain. Using
theoretical calculations, we uncover that the subtle interplay of spin-orbit
coupling, crystal symmetry, and orbital polarization is at the core of the
observed strain dependence of spin-orbit torques. Our results open a path to
integrating two energy efficient spin manipulation approaches, the electric
field-induced strain and the current-induced magnetization switching, thereby
enabling novel device concepts. | 2002.06578v1 |
2020-10-31 | Effect of spin glass frustration on exchange bias in NiMn/CoFeB bilayers | Exchange bias in ferromagnetic/antiferromagnetic systems can be explained in
terms of various interfacial phenomena. Among these spin glass frustration can
affect the magnetic properties in exchange bias systems. Here we have studied a
NiMn/CoFeB exchange bias system in which spin glass frustration seems to play a
crucial role. In order to account the effect of spin glass frustration on
magnetic properties, we have performed the temperature and cooling field
dependence of exchange bias. We have observed the decrease of exchange bias
field (HEB) with cooling field (HFC) whereas there is not significant effect on
coercive field (HC). Exponential decay of HEB and HC is found in these exchange
bias systems. Further, training effect measurements have been performed to
study the spin relaxation mechanism. We have fitted the training effect data
with frozen and rotatable spin relaxation model. We have investigated the ratio
of relaxation rate of interfacial rotatable and frozen spins in this study. The
training effect data are also fitted with various other models. Further, we
observed the shifting of peak temperature towards higher temperature with
frequency from the ac susceptibility data. | 2011.00188v1 |
2021-01-18 | Magnetization switching induced by spin-orbit torque from Co2MnGa magnetic Weyl semimetal thin films | This study reports the magnetization switching induced by spin-orbit torque
(SOT) from the spin current generated in Co2MnGa magnetic Weyl semimetal (WSM)
thin films. We deposited epitaxial Co2MnGa thin films with highly B2-ordered
structure on MgO(001) substrates. The SOT was characterized by harmonic Hall
measurements in a Co2MnGa/Ti/CoFeB heterostructure and a relatively large spin
Hall efficiency of -7.8% was obtained.The SOT-induced magnetization switching
of the perpendicularly magnetized CoFeB layer was further demonstrated using
the structure. The symmetry of second harmonic signals, thickness dependence of
spin Hall efficiency, and shift of anomalous Hall loops under applied currents
were also investigated. This study not only contributes to the understanding of
the mechanisms of spin-current generation from magnetic-WSM-based
heterostructures, but also paves a way for the applications of magnetic WSMs in
spintronic devices. | 2101.06881v1 |
2021-02-07 | Spinterface Induced Modification in Magnetic Properties in Co40Fe40B20/Fullerene Bilayers | Organic semiconductor/ferromagnetic bilayer thin films can exhibit novel
properties due to the formation of the spinterface at the interface.
Buckminsterfullerene (C60) has been shown to exhibit ferromagnetism at the
interface when it is placed next to a ferromagnet (FM) such as Fe or Co.
Formation of spinterface occurs due to the orbital hybridization and spin
polarized charge transfer at the interface. In this work, we have demonstrated
that one can enhance the magnetic anisotropy of the low Gilbert damping alloy
CoFeB by introducing a C60 layer. We have shown that anisotropy increases by
increasing the thickness of C60 which might be a result of the formation of
spinterface. However, the magnetic domain structure remains same in the bilayer
samples as compared to the reference CoFeB film. | 2102.03914v4 |
2021-05-06 | Interfacial and bulk spin Hall contributions to field-like spin-orbit torque generated by Iridium | We present measurements of spin orbit torques generated by Ir as a function
of film thickness in sputtered Ir/CoFeB and Ir/Co samples. We find that Ir
provides a damping-like component of spin orbit torque with a maximum spin
torque conductivity 1.4e5 in SI unit and a maximum spin-torque efficiency of
0.04, which is sufficient to drive switching in an 0.8 nm film of CoFeB with
perpendicular magnetic anisotropy. We also observe a surprisingly large field
like spin orbit torque. Measurements as a function of Ir thickness indicate a
substantial contribution to the FLT from an interface mechanism so that in the
ultrathin limit there is a non-zero FLT with a maximum torque conductivity
-5.0E4 in the SI unit. When the Ir film thickness becomes comparable to or
greater than its spin diffusion length, 1.6 nm, there is also a smaller bulk
contribution to the fieldlike torque. | 2105.02787v1 |
2021-06-04 | Driving skyrmions with low threshold current density in Pt/CoFeB thin film | Magnetic skyrmions are topologically stable spin swirling particle like
entities which are appealing for next generation spintronic devices. The
expected low critical current density for the motion of skyrmions makes them
potential candidates for future energy efficient electronic devices. Several
heavy metal/ferromagnetic (HM/FM) systems have been explored in the past decade
to achieve faster skyrmion velocity at low current densities. In this context,
we have studied Pt/CoFeB/MgO heterostructures in which skyrmions have been
stabilized at room temperature (RT). It has been observed that the shape of the
skyrmions are perturbed even by the small stray field arising from low moment
magnetic tips while performing the magnetic force microscopy (MFM), indicating
presence of low pinning landscape in the samples. This hypothesis is indeed
confirmed by the low threshold current density to drive the skyrmions in our
sample, at velocities of few 10m/s. | 2106.02407v3 |
2021-08-16 | Highly fcc-textured Pt-Al alloy films grown on MgO(001) showing enhanced spin Hall efficiency | We report on a systematic comparative study of the spin Hall efficiency
between highly face-centered cubic (fcc)-textured Pt-Al alloy films grown on
MgO(001) and poorly-crystallized Pt-Al alloy films grown on SiO$_2$. Using
CoFeB as the detector, we show that for Al compositions centering around $x =
25$, mainly L1$_{2}$ ordered Pt$_{100-x}$Al$_x$ alloy films grown on MgO
exhibit outstanding charge-spin conversion efficiency. For
Pt$_{78}$Al$_{22}$/CoFeB bilayer on MgO, we obtain damping-like spin Hall
efficiency as high as $\xi_\textrm{DL} \sim +0.20$ and expect up to seven-fold
reduction of power consumption compared to the polycrystalline bilayer of the
same Al composition on SiO$_2$. This work demonstrates that improving the
crystallinity of fcc Pt-based alloys is a crucial step for achieving large spin
Hall efficiency and low power consumption in this material class. | 2108.06927v1 |
2021-08-23 | Reconfigurable 3D magnonic crystal: tunable and localized spin-wave excitations in CoFeB meander-shaped film | In this work, we study experimentally by broadband ferromagnetic resonance
measurements, the dependence of the spin-wave excitation spectra on the
magnetic applied field in CoFeB meander-shaped films. Two different
orientations of the external magnetic field were explored, namely parallel or
perpendicular to the lattice cores. The interpretation of the field dependence
of the frequency and spatial profiles of major spin-wave modes were obtained by
micromagnetic simulations. We show that the vertical segments lead to the
easy-axis type of magnetic anisotropy and support the in-phase and out-of-phase
spin-wave precession amplitude in the vertical segments. The latter could
potentially be used for the design of tunable metasurfaces or in magnetic
memories based on meandering 3D magnetic films. | 2108.10232v1 |
2021-11-16 | Energy-efficient W$_{\text{100-x}}$Ta$_{\text{x}}$/CoFeB/MgO spin Hall nano-oscillators | We investigate a W-Ta alloying route to reduce the auto-oscillation current
densities and the power consumption of nano-constriction based spin Hall nano
oscillators. Using spin-torque ferromagnetic resonance (ST-FMR) measurements on
microbars of W$_{\text{100-x}}$Ta$_{\text{x}}$(5 nm)/CoFeB(t)/MgO stacks with t
= 1.4, 1.8, and 2.0 nm, we measure a substantial improvement in both the
spin-orbit torque efficiency and the spin Hall conductivity. We demonstrate a
34\% reduction in threshold auto-oscillation current density, which translates
into a 64\% reduction in power consumption as compared to pure W-based SHNOs.
Our work demonstrates the promising aspects of W-Ta alloying for the
energy-efficient operation of emerging spintronic devices. | 2111.08627v1 |
2021-12-23 | Electric-field-induced parametric excitation of exchange magnons in a CoFeB/MgO junction | Electric-field controlled magnetization dynamics is an important integrant in
low power spintronic devices. In this letter, we demonstrate electric-field
induced parametric excitation for CoFeB/MgO junctions by using interfacial
in-plane magnetic anisotropy. When the in-plane magnetic anisotropy and the
external magnetic field are parallel to each other, magnons are efficiently
excited by using electric-field induced parametric excitation. Its wavelength
and wavenumber are tuned by changing input power and frequency of the applied
voltage. A generalized phenomenological model is developed to explain the
underlying role of the electric-field torque. Electrical excitation with no
Joule heating offers a good opportunity for developing magnonic devices and
exploring various nonlinear dynamics in magnetic systems. | 2112.12308v2 |
2022-03-17 | Symmetry effects on the static and dynamic properties of coupled magnetic oscillators | The effect of symmetry on the resonance spectra of antiferromagnetically
coupled oscillators has attracted new interest with the discovery of
symmetry-breaking induced anti-crossings. Here, we experimentally characterise
the resonance spectrum of a synthetic antiferromagnet Pt/CoFeB/Ru/CoFeB/Pt,
where we are able to independently tune the effective magnetisation of the two
coupled magnets. To model our results we apply the mathematical methods of
group theory to the solutions of the Landau Lifshitz Gilbert equation. This
general approach, usually applied to quantum mechanical systems, allows us to
identify the main features of the resonance spectrum in terms of symmetry
breaking and to make a direct comparison with crystal antiferromagnets. | 2204.01490v1 |
2022-04-25 | Ab-initio study of magneto-ionic mechanisms in ferromagnet/oxide multilayers | The application of gate voltages in heavy metal/ferromagnet/Oxide multilayer
stacks has been identified as one possible candidate to manipulate their
anisotropy at will. However, this method has proven to show a wide variety of
behaviours in terms of reversibility, depending on the nature of the
metal/oxide interface and its degree of oxidation. In order to shed light on
the microscopic mechanism governing the complex magneto-ionic behaviour in
$\text{Ta/CoFeB/}\text{HfO}_2$, we perform ab-initio simulations on various
setups comprising $\text{Fe/O, Fe/HfO}_2$ interfaces with different oxygen atom
interfacial geometries. After the determination of the more stable interfacial
configurations, we calculate the magnetic anisotropy energy on the different
unit cell configurations and formulate a possible mechanism that well describes
the recent experimental observations in $\text{Ta/CoFeB/}\text{HfO}_2$. | 2204.11699v1 |
2022-06-21 | Extrinsic tunnel Hall effect in CoFeB/MgO/Pt junctions | The Hall effect that occurs when current flows through a CoFeB/MgO/Pt tunnel
junction is investigated. It is shown that the transverse voltage in Pt
electrode is nonlinear on a DC voltage applied to the tunnel junction. It has
both linear (odd) and quadratic (even) parts. The linear part contains
well-known contributions of the anomalous Hall effect in the ferromagnetic
electrode, inverse spin-hall effect in platinum and others. The quadratic part
is a phenomenon caused by the spin-orbit scattering of electrons in an external
electric field induced by a voltage applied to the barrier. This field reaches
values of $10^9$ V/m which is close to internal atomic fields. The magnitude of
both effects decreases as thickness of Pt electrode is increased due to
shunting effects. | 2206.10264v1 |
2023-06-08 | Unidirectionality of spin waves in Synthetic Antiferromagnets | We study the frequency non-reciprocity of the spin waves in symmetric
CoFeB/Ru/CoFeB synthetic antiferromagnets stacks set in the scissors state by
in-plane applied fields. Using a combination of Brillouin Light Scattering and
propagating spin wave spectroscopy experiments, we show that the acoustical
spin waves in synthetic antiferromagnets possess a unique feature if their
wavevector is parallel to the applied field: the frequency non-reciprocity due
to layer-to-layer dipolar interactions can be so large that the acoustical spin
waves transfer energy in a unidirectional manner for a wide and bipolar
interval of wavevectors. Analytical modeling and full micromagnetic
calculations are conducted to account for the dispersion relations of the
optical and acoustical spin waves for arbitrary field orientations. Our
formalism provides a simple and direct method to understand and design devices
harnessing unidirectional propagation of spin waves in synthetic
antiferromagnets. | 2306.05259v2 |
2024-03-13 | Theoretical limits of magnetic detection of structural surface defects at the nanometer scale | We present a theoretical study on the magnetic signals of structural surface
defects like cracks or indents combined with rough surfaces or subsurface
inclusions of soft ferromagnetic metals like body-centered cubic Fe or
amorphous CoFeB. We discuss limits of early detection of small surface defects
on the basis of calculated magnetic stray fields few tens of nm above the
surface. The considered surface imperfections have extensions of a few nm which
correspond to low multiples of the magnetic exchange lengths of Fe or CoFeB.
The detection of such small inhomogeneities requires that the sensor is about
as close to the surface as the size of the inhomogeneity is. Furthermore, the
step width of a scanning sensor must be of the same size as well. Both these
requirements may be fulfilled for instance by scanning microscopy with diamond
nitrogen-vacancy-center quantum sensors. | 2403.08412v1 |
2005-10-18 | Domain imaging, MOKE and magnetoresistance studies of CoFeB films for MRAM applications | We present a detailed study on domain imaging, Kerr effect magnetometry
(MOKE) and magnetoresistance (MR), for a series of 20 nm
Co$_{73.8}$Fe$_{16.2}$B$_{10}$ thin films, both as-deposited (amorphous) and
annealed (crystalline). By considering the two different (orthogonal) in-plane
magnetization components, obtained by MOKE measurements, we were able to study
the uniaxial anisotropy induced during CoFeB-deposition and to discriminate the
magnetization processes under a magnetic field parallel and perpendicular to
such axis. MOKE magnetic imaging enabled us to observe the dominant
magnetization processes, namely domain wall motion and moment rotation. These
processes were correlated with the behavior of the magnetoresistance, which
depends both on short-range spin disorder electron scattering and on the angle
between the electrical current and the spontaneous magnetization
($\emph{\textbf{M}}_{S}$). A simple numerical treatment based on
Stoner-Wolfarth model enables us to satisfactorily predict the magnetization
behaviour observed in these films. A comparison between the results in
Co$_{73.8}$Fe$_{16.2}$B$_{10}$ films and the previous ones obtained in annealed
Co$_{80}$Fe$_{20}$ films, show that the introduction of boron in CoFe reduces
significatively the coercive and saturation fields along the easy axis (e.g.
$H_{c}$ from $\sim$ 2 down to $\sim$ 0.5 kAm$^{-1}$). Also, the magnetization
along the hard axis saturates at lower fields. We conclude that amorphous and
nanocrystalline CoFeB films show low coercive fields and abrupt switching, as
well as absence of short range spin disorder effects after switching when
compared with Co$_{80}$Fe$_{20}$. | 0510479v2 |
2014-05-05 | On/off switching of bit readout in bias-enhanced tunnel magneto-Seebeck effect | Thermoelectric effects in magnetic tunnel junctions are currently an
attractive research topic. Here, we demonstrate that the tunnel magneto-Seebeck
effect (TMS) in CoFeB/MgO/CoFeB tunnel junctions can be switched on to a logic
1 state and off to 0 by simply changing the magnetic state of the CoFeB
electrodes. We enable this new functionality of magnetic tunnel junctions by
combining a thermal gradient and an electric field. This new technique unveils
the bias-enhanced tunnel magneto-Seebeck effect, which can serve as the basis
for logic devices or memories in a green information technology with a pure
thermal write and read process. Furthermore, the thermally generated voltages
that are referred to as the Seebeck effect are well known to sensitively depend
on the electronic structure and therefore have been valued in solid-state
physics for nearly one hundred years. Here, we lift Seebeck's historic
discovery from 1821 to a new level of current spintronics. Our results show
that the signal crosses zero and can be adjusted by tuning a bias voltage that
is applied between the electrodes of the junction; hence, the name of the
effect is bias-enhanced tunnel magneto-Seebeck effect (bTMS). Via the spin- and
energy-dependent transmission of electrons in the junction, the bTMS effect can
be configured using the bias voltage with much higher control than the tunnel
magnetoresistance (TMR) and even completely suppressed for only one magnetic
configuration, which is either parallel (P) or anti-parallel (AP). This option
allows a readout contrast for the magnetic information of -3000% at room
temperature while maintaining a large signal for one magnetic orientation. This
contrast is much larger than the value that can be obtained using the TMR
effect. Moreover, our measurements are a step towards the experimental
realization of high TMS ratios, which are predicted for specific Co-Fe
compositions. | 1405.1064v1 |
2020-05-08 | Nanoscale Spin Injector Driven by a Microwave Voltage | We propose an electrically driven spin injector into normal metals and
semiconductors, which is based on a magnetic tunnel junction (MTJ) subjected to
a microwave voltage. Efficient functioning of such an injector is provided by
electrically induced magnetization precession in the "free" layer of MTJ, which
generates the spin pumping into a metallic or semiconducting overlayer. We
theoretically describe the spin and charge dynamics in the
CoFeB/MgO/CoFeB/Au(GaAs) heterostructures. First, the magnedynamics in the free
CoFeB layer is quantified with the account of a spin-transfer torque and a
voltage-controlled magnetic anisotropy. By numerically solving the
magnetodynamics equation, we determine dependences of the precession amplitude
on the frequency $f$ and magnitude $V_\mathrm{max}$ of the ac voltage applied
to the MTJ. It is found that the frequency dependence changes drastically above
the threshold amplitude $V_\mathrm{max} \approx 200$mV, exhibiting a break at
the resonance frequency $f_\mathrm{res}$ due to nonlinear effects. The results
obtained for the magnetization dynamics are used to describe the spin injection
and pumping into the Au and GaAs overlayers. Since the generated spin current
creates additional charge current owing to the inverse spin Hall effect, we
also calculate distribution of the electric potential in the thick Au
overlayer. The calculations show that the arising transverse voltage becomes
experimentally measurable at $f = f_\mathrm{res}$. Finally, we evaluate the
spin accumulation in a long n$^+$-GaAs bar coupled to the MTJ and determine its
temporal variation and spatial distribution along the bar. It is found that the
spin accumulation under resonant excitation is large enough for experimental
detection even at micrometer distances from the MTJ. This result demonstrates
high efficiency of the described nanoscale spin injector. | 2005.03896v1 |
2019-12-08 | Thermal Conductivity of Oxide Tunnel Barriers in Magnetic Tunnel Junctions Measured by Ultrafast Thermoreflectance and Magneto-optic Kerr Effect Thermometry | Spin-dependent charge transport in magnetic tunnel junctions (MTJs) can be
manipulated by a temperature gradient, which can be utilized for spintronic and
spin caloritronic applications. Evaluation of the thermally induced phenomena
requires knowledge of the temperature differences across the oxide tunnel
barrier adjacent to the ferromagnetic (FM) leads. However, it is challenging to
accurately measure thermal properties of an oxide tunnel barrier consisting of
only a few atomic layers. In this work, we experimentally interrogate the
temperature evolutions in Ru/oxide/FM/seed/MgO (oxide=MgO, MgAl2O4; FM=Co,
CoFeB; seed=Pt, Ta) structures having perpendicular magnetic anisotropy using
ultrafast thermometry. The Ru layer is optically thick and heated by ultrafast
laser pulses; the subsequent temperature changes are monitored using
thermoreflectance of Ru and magneto-optic Kerr effect (MOKE) of the FM layers.
We independently measure the response times of Co and CoFeB magnetism using
quadratic MOKE and obtain {\tau}em=0.2 ps for Co and 2 ps for CoFeB. These time
scales are much shorter than the time scale of heat transport through the oxide
tunnel barrier, which occurs at 10-3000 ps. We determine effective thermal
conductivities of MgO and MgAl2O4 tunnel barriers in the range of 0.4-0.6 W m-1
K-1, comparable to an estimate of the series conductance of the Ru/oxide and
oxide/FM interfaces and an order of magnitude smaller than the thermal
conductivity of MgO thin films. We find that the electron-phonon thermal
conductance near the tunnel barrier is only a factor of 5-12 larger than the
thermal conductance of the oxide tunnel barrier. Therefore, the drop in the
electronic temperature is approximately 20-30% larger than the drop in the
phonon temperature across the tunnel barrier. | 1912.03588v1 |
2021-06-18 | Large perpendicular magnetic anisotropy in Ta/CoFeB/MgO on full coverage monolayer MoS2 and first principle study of its electronic structure | Perpendicularly magnetized spin injector with high Curie temperature is a
prerequisite for developing spin optoelectronic devices on 2D materials working
at room temperature (RT) with zero applied magnetic field. Here, we report the
growth of Ta/CoFeB/MgO structures with a large perpendicular magnetic
anisotropy (PMA) on full coverage monolayer (ML) MoS2. A large perpendicular
interface anisotropy energy of 0.975mJ/m2 has been obtained at the CoFeB/MgO
interface, comparable to that observed in magnetic tunnel junction systems. It
is found that the insertion of MgO between the ferromagnetic metal (FM) and the
2D material can effectively prevent the diffusion of the FM atoms into the 2D
material. Moreover, the MoS2 ML favors a MgO(001) texture and plays a critical
role to establish the large PMA. First principle calculations on a similar
Fe/MgO/MoS2 structure reveal that the MgO thickness can modify the MoS2 band
structure, from an indirect bandgap with 7ML-MgO to a direct bandgap with
3ML-MgO. Proximity effect induced by Fe results in a splitting of 10meV in the
valence band at the {\Gamma} point for the 3ML-MgO structure while it is
negligible for the 7ML-MgO structure. These results pave the way to develop RT
spin optoelectronic devices on 2D transition-metal dichalcogenide materials. | 2106.10317v1 |
2022-10-20 | Ultrafast behavior of induced and intrinsic magnetic moments in CoFeB/Pt bilayers probed by element-specific measurements in the extreme ultraviolet spectral range | The ultrafast and element-specific response of magnetic systems containing
ferromagnetic 3d transition metals and 4d/5d heavy metals is of interest both
from a fundamental as well as an applied research perspective. However, to date
no consensus about the main microscopic processes describing the interplay
between intrinsic 3d and induced 4d/5d magnetic moments upon femtosecond laser
excitation exist. In this work, we study the ultrafast response of CoFeB/Pt
bilayers by probing element-specific, core-to-valence band transitions in the
extreme ultraviolet spectral range using high harmonic radiation. We show that
the combination of magnetic scattering simulations and analysis of the energy-
and time-dependent magnetic asymmetries allows to accurately disentangle the
element-specific response in spite of overlapping Co and Fe M$_{2,3}$ as well
as Pt O$_{2,3}$ and N$_7$ resonances. We find a considerably smaller
demagnetization time constant as well as much larger demagnetization amplitudes
of the induced moment of Pt compared to the intrinsic moment of CoFeB. Our
results are in agreement with enhanced spin-flip probabilities due to the high
spin-orbit coupling localized at the heavy metal Pt, as well as with the
recently formulated hypothesis that a laser generated, incoherent magnon
population within the ferromagnetic film leads to an overproportional reduction
of the induced magnetic moment of Pt. | 2210.11390v3 |
2009-08-17 | The superferromagnetic state in the ensemble of oriented Stoner-Wohlfarth particles: a coercivity due to phase stability | It is observed experimentally that the coercive field has an anomalous
angular dependence at temperatures above the blocking temperature in physically
nonpercolated granular films CoFeB-SiO$_{2}$ with anisotropic granules oriented
in the same direction. It is shown that the anomaly is determined by the
singularity of an angular dependence of the critical field causing the absolute
loss of phase stability of the superferromagnetic state of an ensemble of
interacting superparamagnetic granules. | 0908.2286v1 |
2014-04-04 | Electrical detection of microwave assisted magnetization reversal by spin pumping | Microwave assisted magnetization reversal has been investigated in a bilayer
system of Pt/ferromagnet by detecting a change in the polarity of the spin
pumping signal. The reversal process is studied in two material systems,
Pt/CoFeB and Pt/NiFe, for different aspect ratios. The onset of the switching
behavior is indicated by a sharp transition in the spin pumping voltage. At a
threshold value of the external field, the switching process changes from
partial to full reversal with increasing microwave power. The proposed method
provides a simple way to detect microwave assisted magnetization reversal. | 1404.1133v1 |
2014-04-04 | Thermally assisted domain wall nucleation in perpendicular anisotropy trilayer nanowires | We study thermally assisted domain wall generation in perpendicular magnetic
anisotropy CoFeB trilayer nanowires by the effect of Joule heating. The
anomalous Hall effect is utilized to detect magnetization reversal in order to
study the domain wall generation. We observe a statistical distribution in the
switching process which is consistent with the thermal activation process. Our
results show that the proposed method provides an efficient way for generating
domain walls in perpendicular magnetic nanowires at predefined locations. | 1404.1135v1 |
2014-05-14 | Generation of magnonic spin wave traps | Spatially resolved measurements of the magnetization dynamics induced by an
intense laser pump-pulse reveal that the frequencies of resulting spin wave
modes depend strongly on the distance to the pump center. This can be
attributed to a laser generated temperature profile. On a CoFeB thin film
magnonic crystal, Damon-Eshbach modes are expected to propagate away from the
point of excitation. The experiments show that this propagation is frustrated
by the strong temperature gradient | 1405.3470v1 |
2017-01-06 | Imaging magnetic vortex configurations in ferromagnetic nanotubes | We image the remnant magnetization configurations of CoFeB and permalloy
nanotubes (NTs) using x-ray magnetic circular dichroism photo-emission electron
microscopy. The images provide direct evidence for flux-closure configurations,
including a global vortex state, in which magnetization points
circumferentially around the NT axis. Furthermore, micromagnetic simulations
predict and measurements confirm that vortex states can be programmed as the
equilibrium remnant magnetization configurations by reducing the NT aspect
ratio. | 1701.01685v1 |
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