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2021-03-31 | Spin charge conversion in Rashba split ferromagnetic interfaces | We show here theoretically and experimentally that a Rashba-split electron
state inside a ferromagnet can efficiently convert a dynamical spin
accumulation into an electrical voltage. The effect is understood to stem from
the Rashba splitting but with a symmetry linked to the magnetization direction.
It is experimentally measured by spin pumping in a CoFeB/MgO structure where it
is found to be as efficient as the inverse spin Hall effect at play when Pt
replaces MgO, with the extra advantage of not affecting the damping in the
ferromagnet. | 2103.16867v1 |
2019-12-24 | Large spin Hall angle and spin mixing conductance in highly resistive antiferromagnetic Mn2Au | Antiferromagnetic (AFM) materials recently have shown interest in the
research in spintronics due to its zero stray magnetic field, high anisotropy,
and spin orbit coupling. In this context, the bi-metallic AFM Mn2Au has drawn
attention because it exhibits unique properties and its Neel temperature is
very high. Here, we report spin pumping and inverse spin Hall effect
investigations in Mn2Au and CoFeB bilayer system using ferromagnetic resonance.
We found large spin Hall angle {\theta}_SH = 0.22 | 1912.11522v2 |
2008-09-20 | Anomalous low-temperature magnetoelastic properties of nanogranular (CoFeB)$_{x}$-(SiO$_{2}$)$_{1-x}$ | We report magnetostatic measurements for granulated films
(CoFeB)$_{x}$-(SiO$_{2}$)$_{1-x}$ with fabrication induced intraplanar
anisotropy. The measurements have been performed in the film plane in the wide
temperature interval 4.5$\div$300 K. They demonstrate that above films have
low-temperature anomaly below the percolation threshold for conductivity. The
essence of the above peculiarity is that below 100 K the temperature dependence
of coercive field for magnetization along easy direction deviates strongly from
Neel-Brown law. At temperature lowering, the sharp increase of coercivity is
observed, accompanied by the appearance of coercive field for magnetization
along hard direction in the film plane. We establish that observed effect is
related to the properties of individual ferromagnetic granules. The effect
weakens as granules merge into conglomerates at $x$ higher then percolation
threshold and disappears completely at $x>1$. We explain the above effect as a
consequence of the difference in thermal expansion coefficients of granule and
cover material. At temperature lowering this difference weakens the envelopment
of an individual granule by the cover matrix material, thus permitting to
realize the spontaneous magnetostriction of a granule. The latter induces an
additional anisotropy with new easy axis of a granule magnetization along the
external magnetic field direction. Our explanation is tested and corroborated
by the ferromagnetic resonance measurements in the films at $T$ = 300 K and $T$
= 77 K. | 0809.3499v1 |
2013-09-27 | Structural and magnetic properties of Cr-diluted CoFeB | The crystallization process and the magnetization of Cr diluted CoFeB was
investigated in both ribbon samples and thin film samples with Cr content up to
30 at. %. A primary crystallization of bcc phase from an amorphous precursor in
ribbon samples was observed when the annealing temperature rose to between 421
oC and 456 oC, followed by boron segregation at temperatures between 518 oC and
573 oC. The two onset crystallization temperatures showed strong dependences on
both Cr and B concentrations. The impact of Cr concentration on the magnetic
properties including a reduced saturation magnetization and an enhanced
coercive field was also observed. The magnetizations of both ribbon samples and
thin film samples were well fitted using the generalized Slater-Pauling curve
with modified moments for B (-0.94 {\mu}B) and Cr (-3.6 {\mu}B). Possible
origins of the enhanced coercive field were also discussed. We also achieved a
damping parameter in CoFeCrB thin films at the same level as Co40Fe40B20, much
lower than the value reported for CoFeCrB films previously. The results suggest
a possible advantage of CoFeCrB in reducing the critical switching current
density in Spin Transfer Torque Random Access Memory (STT-RAM). | 1309.7331v1 |
2014-01-15 | Interface control of the magnetic chirality in CoFeB|MgO heterosctructures with heavy metal underlayers | Recent advances in the understanding of spin orbital effects in ultrathin
magnetic heterostructures have opened new paradigms to control magnetic moments
electrically. The Dzyaloshinskii-Moriya interaction (DMI) is said to play a key
role in forming a Neel-type domain wall that can be driven by the spin Hall
torque, a torque resulting from the spin current generated in a neighboring
non-magnetic layer via the spin Hall effect. Here we show that the strength and
sign of the DMI can be changed by modifying the adjacent heavy metal underlayer
(X) in perpendicularly magnetized X|CoFeB|MgO heterstructures. Albeit the same
spin Hall angle, a domain wall moves along or against the electron flow
depending on the underlayer. We find that the sense of rotation of a domain
wall spiral11 is reversed when the underlayer is changed from Hf to W and the
strength of DMI varies as the number of 5d electrons of the heavy metal layer
changes. The DMI can even be tuned by adding nitrogen to the underlayer, thus
allowing interface engineering of the magnetic texture in ultrathin magnetic
heterostructures. | 1401.3568v1 |
2014-02-05 | Fieldlike and antidamping spin-orbit torques in as-grown and annealed Ta/CoFeB/MgO layers | We present a comprehensive study of the current-induced spin-orbit torques in
perpendicularly magnetized Ta/CoFeB/MgO layers. The samples were annealed in
steps up to 300 degrees C and characterized using x-ray absorption
spectroscopy, transmission electron microscopy, resistivity, and Hall effect
measurements. By performing adiabatic harmonic Hall voltage measurements, we
show that the transverse (field-like) and longitudinal (antidamping-like)
spin-orbit torques are composed of constant and magnetization-dependent
contributions, both of which vary strongly with annealing. Such variations
correlate with changes of the saturation magnetization and magnetic anisotropy
and are assigned to chemical and structural modifications of the layers. The
relative variation of the constant and anisotropic torque terms as a function
of annealing temperature is opposite for the field-like and antidamping
torques. Measurements of the switching probability using sub-{\mu}s current
pulses show that the critical current increases with the magnetic anisotropy of
the layers, whereas the switching efficiency, measured as the ratio of magnetic
anisotropy energy and pulse energy, decreases. The optimal annealing
temperature to achieve maximum magnetic anisotropy, saturation magnetization,
and switching efficiency is determined to be between 240 degrees and 270
degrees C. | 1402.0986v2 |
2014-02-26 | Anomalous temperature dependence of current induced torques in CoFeB|MgO heterostructures with Ta based underlayers | We have studied the underlayer thickness and temperature dependences of the
current induced effective field in CoFeB|MgO heterostructures with Ta based
underlayers. The underlayer thickness at which the effective field saturates is
found to be different between the two orthogonal components of the effective
field, i.e. the damping-like term tends to saturate at smaller underlayer
thickness than the field-like term. For large underlayer thickness films in
which the effective field saturates, we find that the temperature significantly
influences the size of the effective field. A striking difference is found in
the temperature dependence of the two components: the damping-like term
decreases whereas the field-like term increases with increasing temperature.
Using a simple spin diffusion-spin transfer model, we find that all of these
results can be accounted for provided the real and imaginary parts of an
effective spin mixing conductance are negative. These results imply that either
spin transport in this system is different from conventional metallic
interfaces or effects other spin diffusion into the magnetic layer need to be
taken account in order to model the system accurately. | 1402.6388v1 |
2014-10-28 | Current driven asymmetric magnetization switching in perpendicularly magnetized CoFeB/MgO heterostructures | The flow of in-plane current through ultrathin magnetic heterostructures can
cause magnetization switching or domain wall nucleation owing to bulk and
interfacial effects. Within the magnetic layer, the current can create magnetic
instabilities via spin transfer torques (STT). At interface(s), spin current
generated from the spin Hall effect in a neighboring layer can exert torques,
referred to as the spin Hall torques, on the magnetic moments. Here, we study
current induced magnetization switching in perpendicularly magnetized CoFeB/MgO
heterostructures with a heavy metal (HM) underlayer. Depending on the thickness
of the HM underlayer, we find distinct differences in the inplane field
dependence of the threshold switching current. The STT is likely responsible
for the magnetization reversal for the thinner underlayer films whereas the
spin Hall torques cause the switching for thicker underlayer films. For the
latter, we find differences in the switching current for positive and negative
currents and initial magnetization directions. We find that the growth process
during the film deposition introduces an anisotropy that breaks the symmetry of
the system and causes the asymmetric switching. The presence of such symmetry
breaking anisotropy enables deterministic magnetization switching at zero
external fields. | 1410.7473v2 |
2015-06-12 | Origin of interfacial perpendicular magnetic anisotropy in MgO/CoFe/metallic capping layer structures | Spin-transfer-torque magnetic random access memory (STT-MRAM) attracts
extensive attentions due to its non-volatility, high density and low power
consumption. The core device in STT-MRAM is CoFeB/MgO-based magnetic tunnel
junction (MTJ), which possesses a high tunnel magnetoresistance ratio as well
as a large value of perpendicular magnetic anisotropy (PMA). It has been
experimentally proven that a capping layer coating on CoFeB layer is essential
to obtain a strong PMA. However, the physical mechanism of such effect remains
unclear. In this paper, we investigate the origin of the PMA in
MgO/CoFe/metallic capping layer structures by using a first-principles
computation scheme. The trend of PMA variation with different capping materials
agrees well with experimental results. We find that interfacial PMA in the
three-layer structures comes from both the MgO/CoFe and CoFe/capping layer
interfaces, which can be analyzed separately. Furthermore, the PMAs in the
CoFe/capping layer interfaces are analyzed through resolving the magnetic
anisotropy energy by layer and orbital. The variation of PMA with different
capping materials is attributed to the different hybridizations of both d and p
orbitals via spin-orbital coupling. This work can significantly benefit the
research and development of nanoscale STT-MRAM. | 1506.04078v4 |
2016-05-07 | Giant interfacial perpendicular magnetic anisotropy in MgO/CoFe/capping layer structures | Magnetic tunnel junction (MTJ) based on CoFeB/MgO/CoFeB structures is of
great interest due to its application in the spin-transfer-torque magnetic
random access memory (STT-MRAM). Large interfacial perpendicular magnetic
anisotropy (PMA) is required to achieve high thermal stability. Here we use
first-principles calculations to investigate the magnetic anisotropy energy
(MAE) of MgO/CoFe/capping layer structures, where the capping materials include
5d metals Hf, Ta, Re, Os, Ir, Pt, Au and 6p metals Tl, Pb, Bi. We demonstrate
that it is feasible to enhance PMA by using proper capping materials.
Relatively large PMA is found in the structures with capping materials of Hf,
Ta, Os, Ir and Pb. More importantly, the MgO/CoFe/Bi structure gives rise to
giant PMA (6.09 mJ/m2), which is about three times larger than that of the
MgO/CoFe/Ta structure. The origin of the MAE is elucidated by examining the
contributions to MAE from each atomic layer and orbital. These findings provide
a comprehensive understanding of the PMA and point towards the possibility to
achieve advanced-node STT-MRAM with high thermal stability. | 1605.02247v5 |
2017-08-14 | Current-induced magnetization switching in atom-thick tungsten engineered perpendicular magnetic tunnel junctions with large tunnel magnetoresistance | Perpendicular magnetic tunnel junctions based on MgO/CoFeB structures are of
particular interest for magnetic random-access memories because of their
excellent thermal stability, scaling potential, and power dissipation. However,
the major challenge of current-induced switching in the nanopillars with both a
large tunnel magnetoresistance ratio and a low junction resistance is still to
be met. Here, we report spin transfer torque switching in nano-scale
perpendicular magnetic tunnel junctions with a magnetoresistance ratio up to
249% and a resistance area product as low as 7.0 {\Omega}.{\mu}m2, which
consists of atom-thick W layers and double MgO/CoFeB interfaces. The efficient
resonant tunnelling transmission induced by the atom-thick W layers could
contribute to the larger magnetoresistance ratio than conventional structures
with Ta layers, in addition to the robustness of W layers against high
temperature diffusion during annealing. The switching critical current density
could be lower than 3.0 MA.cm-2 for devices with a 45 nm radius. | 1708.04111v3 |
2018-01-11 | Efficient Charge-Spin Conversion and Magnetization Switching though Rashba Effect at Topological Insulator/Ag Interface | We report the observation of efficient charge-to-spin conversion in the
three-dimensional topological insulator (TI) Bi2Se3 and Ag bilayer by the
spin-torque ferromagnetic resonance technique. The spin orbit torque ratio in
the Bi2Se3/Ag/CoFeB heterostructure shows a significant enhancement as the Ag
thickness increases to ~2 nm and reaches a value of 0.5 for 5 nm Ag, which is
~3 times higher than that of Bi2Se3/CoFeB at room temperature. The observation
reveals the interfacial effect of Bi2Se3/Ag exceeds that of the topological
surface states (TSS) in the Bi2Se3 layer and plays a dominant role in the
charge-to-spin conversion in the Bi2Se3/Ag/CoFeB system. Based on the
first-principles calculations, we attribute our observation to the large
Rashba-splitting bands which wrap the TSS band and has the same net spin
polarization direction as TSS of Bi2Se3. Subsequently, we demonstrate for the
first time the Rashba induced magnetization switching in Bi2Se3/Ag/Py with a
low current density of 5.8 X 10^5 A/cm2. | 1801.03689v1 |
2020-01-13 | Large and Robust Charge-to-Spin Conversion in Sputtered Conductive WTex with Disorder | Topological materials with large spin-orbit coupling and immunity to
disorder-induced symmetry breaking show great promise for efficiently
converting charge to spin. Here, we report that long-range disordered sputtered
WTex thin films exhibit local chemical and structural order as those of Weyl
semimetal WTe2 and conduction behavior that is consistent with semi-metallic
Weyl fermion. We find large charge-to-spin conversion properties and electrical
conductivity in thermally annealed sputtered WTex films that are comparable
with those in crystalline WTe2 flakes. Besides, the strength of unidirectional
spin Hall magnetoresistance in annealed WTex/Mo/CoFeB heterostructure is 5 to
20 times larger than typical SOT layer/ferromagnet heterostructures reported at
room temperature. We further demonstrate room temperature damping-like
SOT-driven magnetization switching of in-plane magnetized CoFeB. These large
charge-to-spin conversion properties that are robust in the presence of
long-range disorder and thermal annealing pave the way for industrial
application of a new class of sputtered semimetals. | 2001.04054v2 |
2021-07-16 | Large Unidirectional Magnetoresistance in Metallic Heterostructures in the Spin Transfer Torque Regime | A large unidirectional magnetoresistance (UMR) ratio of UMR/$R_{xx}\sim$
$0.36\%$ is found in W/CoFeB metallic bilayer heterostructures at room
temperature. Three different regimes in terms of the current dependence of UMR
ratio are identified: A spin-dependent-scattering mechanism regime at small
current densities $J \sim$ $10$$^{9}$A/m$^{2}$ (UMR ratio $\propto$ $J$), a
spin-magnon-interaction mechanism regime at intermediate $J \sim$
$10$$^{10}$A/m$^{2}$ (UMR ratio $\propto$ $J$$^{3}$), and a spin-transfer
torque (STT) regime at $J \sim$ $10$$^{11}$A/m$^{2}$ (UMR ratio independent of
$J$). We verify the direct correlation between this large UMR and the transfer
of spin angular momentum from the W layer to the CoFeB layer by both
field-dependent and current-dependent UMR characterizations. Numerical
simulations further confirm that the large STT-UMR stems from the tilting of
the magnetization affected by the spin Hall effect-induced spin-transfer
torques. An alternative approach to estimate damping-like spin-torque
efficiencies from magnetic heterostructures is also proposed. | 2107.07780v1 |
2017-05-18 | Magnetization dynamics and its scattering mechanism in thin CoFeB films with interfacial anisotropy | Studies of magnetization dynamics have incessantly facilitated the discovery
of fundamentally novel physical phenomena, making steady headway in the
development of magnetic and spintronics devices. The dynamics can be induced
and detected electrically, offering new functionalities in advanced electronics
at the nanoscale. However, its scattering mechanism is still disputed.
Understanding the mechanism in thin films is especially important, because most
spintronics devices are made from stacks of multilayers with nanometer
thickness. The stacks are known to possess interfacial magnetic anisotropy, a
central property for applications, whose influence on the dynamics remains
unknown. Here, we investigate the impact of interfacial anisotropy by adopting
CoFeB/MgO as a model system. Through systematic and complementary measurements
of ferromagnetic resonance (FMR), on a series of thin films, we identify
narrower FMR linewidths at higher temperatures. We explicitly rule out the
temperature dependence of intrinsic damping as a possible cause, and it is also
not expected from existing extrinsic scattering mechanisms for ferromagnets. We
ascribe this observation to motional narrowing, an old concept so far neglected
in the analyses of FMR spectra. The effect is confirmed to originate from
interfacial anisotropy, impacting the practical technology of spin-based
nanodevices up to room temperature. | 1705.06624v1 |
2019-01-26 | Low spin-polarization in the heavy metal\ferromagnet structures detected through the domain wall motion by synchronized magnetic field and current | CoFeB is a very soft material, in which Domain Wall (DW) can be moved easily
under a weak magnetic field. However, it is very difficult to move DWs in
Ta\CoFeB\MgO nanowires with interfacial perpendicular magnetic anisotropy
through a spin-polarized current, and this limits the perspectives of racetrack
memory driven by the current-in-plane mechanism. To investigate this
phenomenon, we performed experiments of DW velocity measurement by applying a
magnetic field and a current simultaneously. Working in the precessional
regime, we have been able to see a very important effect of the spin-polarized
current, which allows evaluating the polarization rate of the charge carriers.
An unexpected quite low spin polarization rate down to 0.26 have been obtained,
which can explain the low efficiency of DW motion induced by the spin-polarized
current. Possible reasons for this low rate are analyzed, such as the spin
relaxation in the Ta layer. | 1901.09256v1 |
2018-12-06 | Optimization of high performance spintronic terahertz sources | To achieve high efficiency and good performance of spintronic terahertz
sources, we propose and corroborate a remnant magnetization method to radiate
continuously and stably terahertz pulses from W/CoFeB/Pt magnetic nanofilms
without carrying magnets on the transmitters driven by femtosecond laser
pulses. We systematically investigate the influences of the pumping central
wavelength and find out the optimal wavelength for a fixed sample thickness. We
also optimize the incidence angle of the pumping laser and find the emission
efficiency is enhanced under oblique incidence. Combing the aforementioned
optimizations, we finally obtain comparable radiation efficiency and broadband
spectra in W/CoFeB/Pt heterostructures compared with that from 1 mm thick ZnTe
nonlinear crystals via optical rectification under the same pumping conditions
of 100 fs pulse duration from a Ti:sapphire laser oscillator, which was not
previously demonstrated under such pulse duration. We believe our observations
not only benefit for a deep insight into the physics of femtosecond spin
dynamics, but also help develop novel and cost-effective ultrabroadband
spintronic terahertz emitters. | 1812.02286v2 |
2020-08-04 | Ultrathin perpendicular free layers for lowering the switching current in STT-MRAM | The critical current density $J_{c0}$ required for switching the
magnetization of the free layer (FL) in a spin-transfer torque magnetic random
access memory (STT-MRAM) cell is proportional to the product of the damping
parameter, saturation magnetization and thickness of the free layer, $\alpha
M_S t_F$. Conventional FLs have the structure CoFeB/nonmagnetic spacer/CoFeB.
By reducing the spacer thickness, W in our case, and also splitting the single
W layer into two layers of sub-monolayer thickness, we have reduced $t_F$ while
minimizing $\alpha$ and maximizing $M_S$, ultimately leading to lower $J_{c0}$
while maintaining high thermal stability. Bottom-pinned MRAM cells with device
diameter in the range of 55-130 nm were fabricated, and $J_{c0}$ is lowest for
the thinnest (1.2 nm) FLs, down to 4 MA/cm$^2$ for 65 nm devices, $\sim$30%
lower than 1.7 nm FLs. The thermal stability factor $\Delta_{\mathrm{dw}}$, as
high as 150 for the smallest device size, was determined using a domain wall
reversal model from field switching probability measurements. With high
$\Delta_{\mathrm{dw}}$ and lowest $J_{c0}$, the thinnest FLs have the highest
spin-transfer torque efficiency. | 2008.01343v1 |
2021-03-08 | Anisotropic magnon-magnon coupling in synthetic antiferromagnets | The magnon-magnon coupling in synthetic antiferromagnets advances it as
hybrid magnonic systems to explore the quantum information technologies. To
induce the magnon-magnon coupling, the parity symmetry between two
magnetization needs to be broken. Here we experimentally demonstrate a
convenient method to break the parity symmetry by the asymmetric thickness of
two magnetic layers and thus introduce a magnon-magnon coupling in Ir-based
synthetic antiferromagnets CoFeB(10 nm)/Ir(tIr=0.6 nm, 1.2 nm)/CoFeB(13 nm).
Remarkably, we find that the weakly uniaxial anisotropy field (~ 20 Oe) makes
the magnon-magnon coupling anisotropic. The coupling strength presented by a
characteristic anticrossing gap varies in the range between 0.54 GHz and 0.90
GHz for tIr =0.6 nm, and between nearly zero to 1.4 GHz for tIr = 1.2 nm,
respectively. Our results demonstrate a feasible way to induce the
magnon-magnon coupling by an asymmetric structure and tune the coupling
strength by varying the direction of in-plane magnetic field. The magnon-magnon
coupling in this highly tunable material system could open exciting
perspectives for exploring quantum-mechanical coupling phenomena. | 2103.04512v2 |
2017-02-01 | Continuous Tuning the Magnitude and Direction of Spin-Orbit Torque Using Bilayer Heavy Metals | Spin-orbit torques (SOTs) have opened a new path to switch the magnetization
in perpendicularly magnetized films and are of great interest due to their
potential applications in novel data storage technology, such as the magnetic
random access memory (MRAM). The effective manipulation of SOT has thus become
an important step towards these applications. Here, current induced spin-orbit
effective fields and magnetization switching are investigated in
Pt/Ta/CoFeB/MgO structures with bilayer heavy metals. With a fixed thickness (1
nm) of the Ta layer, the magnitude and sign of current induced spin-orbit
effective fields can be continuously tuned by changing the Pt layer thickness,
consistent with the current induced magnetization switching data. The ratio of
longitudinal to transverse spin-orbit effective fields is found to be
determined by the Ta/CoFeB interface and can be continuously tuned by changing
the Pt layer thickness. The Dzyaloshinskii-Moriya interaction (DMI) is found to
be weak and shows an insignificant variation with the Pt thickness. The results
demonstrate an effective method to tune SOTs utilizing bilayer heavy metals
without affecting the DMI, a desirable feature which will be useful for the
design of SOT-based devices. | 1702.00147v1 |
2017-06-19 | Investigation of the Dzyaloshinskii-Moriya interaction and room temperature skyrmions in W/CoFeB/MgO thin films and microwires | Recent studies have shown that material structures, which lack structural
inversion symmetry and have high spin-orbit coupling can exhibit chiral
magnetic textures and skyrmions which could be a key component for next
generation storage devices. The Dzyaloshinskii-Moriya Interaction (DMI) that
stabilizes skyrmions is an anti-symmetric exchange interaction favoring
non-collinear orientation of neighboring spins. It has been shown that material
systems with high DMI can lead to very efficient domain wall and skyrmion
motion by spin-orbit torques. To engineer such devices, it is important to
quantify the DMI for a given material system. Here we extract the DMI at the
Heavy Metal (HM) /Ferromagnet (FM) interface using two complementary
measurement schemes namely asymmetric domain wall motion and the magnetic
stripe annihilation. By using the two different measurement schemes, we find
for W(5 nm)/Co20Fe60B20(0.6 nm)/MgO(2 nm) the DMI to be 0.68 +/- 0.05 mJ/m2 and
0.73 +/- 0.5 mJ/m2, respectively. Furthermore, we show that this DMI stabilizes
skyrmions at room temperature and that there is a strong dependence of the DMI
on the relative composition of the CoFeB alloy. Finally we optimize the layers
and the interfaces using different growth conditions and demonstrate that a
higher deposition rate leads to a more uniform film with reduced pinning and
skyrmions that can be manipulated by Spin-Orbit Torques. | 1706.05987v1 |
2018-04-30 | Field-Free Switching of Perpendicular Magnetic Tunnel Junction via Voltage-Gated Spin Hall Effect for Low-Power Spintronic Memory | Spin Hall effect (SHE) and voltage-controlled magnetic anisotropy (VCMA) are
two promising methods for low-power electrical manipulation of magnetization.
Recently, magnetic field-free switching of perpendicular magnetization through
SHE has been reported with the aid of an exchange bias from an
antiferromagnetic IrMn layer. In this letter, we experimentally demonstrate
that the IrMn/CoFeB/MgO structure exhibits a VCMA effect of 39 fJ/Vm, which is
comparable to that of the Ta/CoFeB/MgO structure. Magnetization dynamics under
a combination of the SHE and VCMA are modeled and simulated. It is found that,
by applying a voltage of 1.5 V, the critical SHE switching current can be
decreased by 10 times owing to the VCMA effect, leading to low-power
operations. Furthermore, a high-density spintronic memory structure can be
built with multiple magnetic tunnel junctions (MTJs) located on a single IrMn
strip. Through hybrid CMOS/MTJ simulations, we demonstrate that fast-speed
write operations can be achieved with power consumption of only 8.5 fJ/bit.
These findings reveal the possibility to realize high-density and low-power
spintronic memory manipulated by voltage-gated SHE. | 1804.11025v1 |
2020-06-04 | Defect-Correlated Skyrmions and Controllable Generation in Perpendicularly Magnetized CoFeB Ultrathin Films | Skyrmions have attracted significant interest due to their topological spin
structures and fascinating physical features. The skyrmion phase arises in
materials with Dzyaloshinskii-Moriya (DM) interaction at interfaces or in
volume of non-centrosymmetric materials. However, although skyrmions were
generated experimentally, one critical intrinsic relationship between
fabrication, microstructures, magnetization and the existence of skyrmions
remains to be established. Here, two series of CoFeB ultrathin films with
controlled atomic scale structures are employed to reveal this relationship. By
inverting the growth order, the amount of defects can be artificially tuned,
and skyrmions are shown to be preferentially formed at defect sites. The stable
region and the density of the skyrmions can be efficiently controlled in the
return magnetization loops by utilizing first-order reversal curves to reach
various metastable states. These findings establish the general and intrinsic
relationship from sample preparation to skyrmion generation, offering an
universal method to control skyrmion density. | 2006.02864v1 |
2020-07-27 | Magnonic band structure in vertical meander-shaped CoFeB thin films | The dispersion of spin waves in vertical meander-shaped CoFeB thin films
consisting of segments located at 90{\deg} angles with respect to each other is
investigated by Brillouin light scattering spectroscopy. We reveal the periodic
character of several dispersive branches as well as alternating frequency
ranges where spin waves are allowed or forbidden to propagate. Noteworthy is
the presence of the frequency band gaps between each couple of successive modes
only for wave numbers k=n$\pi$/a, where n is an even integer number and a is
the size of the meander unit cell, whereas the spectra show propagating modes
in the orthogonal film segments for the other wavenumbers. The micromagnetic
simulations and analytical calculations allow us to understand and explain the
results in terms of the mode spatial localization and symmetry. The obtained
results demonstrate the wave propagation in three dimensions opening the path
for multi-level magnonic architectures for signal processing. | 2007.13707v2 |
2020-09-14 | Memristive control of mutual SHNO synchronization for neuromorphic computing | Synchronization of large spin Hall nano-oscillators (SHNO) arrays is an
appealing approach toward ultra-fast non-conventional computing based on
nanoscale coupled oscillator networks. However, for large arrays, interfacing
to the network, tuning its individual oscillators, their coupling, and
providing built-in memory units for training purposes, remain substantial
challenges. Here, we address all these challenges using memristive gating of
W/CoFeB/MgO/AlOx based SHNOs. In its high resistance state (HRS), the memristor
modulates the perpendicular magnetic anisotropy (PMA) at the CoFeB/MgO
interface purely by the applied electric field. In its low resistance state
(LRS), and depending on the voltage polarity, the memristor adds/subtracts
current to/from the SHNO drive. The operation in both the HRS and LRS affects
the SHNO auto-oscillation mode and frequency, which can be tuned up to 28
MHz/V. This tuning allows us to reversibly turn on/off mutual synchronization
in chains of four SHNOs. We also demonstrate two individually controlled
memristors to tailor both the coupling strength and the frequency of the
synchronized state. Memristor gating is therefore an efficient approach to
input, tune, and store the state of the SHNO array for any non-conventional
computing paradigm, all in one platform. | 2009.06594v1 |
2021-02-15 | Controlling Domain-Wall Nucleation in Ta/CoFeB/MgO Nanomagnets via Local Ga+ Ion Irradiation | Comprehensive control of the domain wall nucleation process is crucial for
spin-based emerging technologies ranging from random-access and storage-class
memories over domain-wall logic concepts to nanomagnetic logic. In this work,
focused Ga+ ion-irradiation is investigated as an effective means to control
domain-wall nucleation in Ta/CoFeB/MgO nanostructures. We show that analogously
to He+ irradiation, it is not only possible to reduce the perpendicular
magnetic anisotropy but also to increase it significantly, enabling new,
bidirectional manipulation schemes. First, the irradiation effects are assessed
on film level, sketching an overview of the dose-dependent changes in the
magnetic energy landscape. Subsequent time-domain nucleation characteristics of
irradiated nanostructures reveal substantial increases in the anisotropy fields
but surprisingly small effects on the measured energy barriers, indicating
shrinking nucleation volumes. Spatial control of the domain wall nucleation
point is achieved by employing focused irradiation of pre-irradiated magnets,
with the diameter of the introduced circular defect controlling the coercivity.
Special attention is given to the nucleation mechanisms, changing from a
Stoner-Wohlfarth particle's coherent rotation to depinning from an anisotropy
gradient. Dynamic micromagnetic simulations and related measurements are used
in addition to model and analyze this depinning-dominated magnetization
reversal. | 2102.07540v1 |
2021-06-12 | Spin pumping and inverse spin Hall effect in CoFeB/C$_{60}$ bilayers | Pure spin current based research is mostly focused on ferromagnet (FM)/heavy
metal (HM) system. Because of the high spin orbit coupling (SOC) these HMs
exhibit short spin diffusion length and therefore possess challenges for device
application. Low SOC (elements of light weight) and large spin diffusion length
make the organic semiconductors (OSCs) suitable for future spintronic
applications. From theoretical model it is explained that, due to $\pi$ -
$\sigma$ hybridization the curvature of the C$_{60}$ molecules may increase the
SOC strength. Here, we have investigated spin pumping and inverse spin hall
effect (ISHE) in CoFeB/C$_{60}$ bilayer system using coplanar wave guide based
ferromagnetic resonance (CPW-FMR) set-up. We have performed angle dependent
ISHE measurement to disentangle the spin rectification effects for example
anisotropic magnetoresistance, anomalous Hall effect etc. Further, effective
spin mixing conductance (g$_{eff}^{\uparrow\downarrow}$) and spin Hall angle
($\theta_{SH}$) for C$_{60}$ have been reported here. The evaluated value for
$\theta_{SH}$ is 0.055. | 2106.06829v2 |
2022-06-29 | Tailoring the switching efficiency of magnetic tunnel junctions by the fieldlike spin-orbit torque | Current-induced spin-orbit torques provide a versatile tool for switching
magnetic devices. In perpendicular magnets, the dampinglike component of the
torque is the main driver of magnetization reversal. The degree to which the
fieldlike torque assists the switching is a matter of debate. Here we study the
switching of magnetic tunnel junctions with a CoFeB free layer and either W or
Ta underlayers, which have a ratio of fieldlike to dampinglike torque of 0.3
and 1, respectively. We show that the fieldlike torque can either assist or
hinder the switching of CoFeB when the static in-plane magnetic field required
to define the polarity of spin-orbit torque switching has a component
transverse to the current. In particular, the non-collinear alignment of the
field and current can be exploited to increase the switching efficiency and
reliability compared to the standard collinear alignment. By probing individual
switching events in real-time, we also show that the combination of transverse
magnetic field and fieldlike torque can accelerate or decelerate the reversal
onset. We validate our observations using micromagnetic simulations and
extrapolate the results to materials with different torque ratios. Finally, we
propose device geometries that leverage the fieldlike torque for density
increase in memory applications and synaptic weight generation. | 2206.14587v2 |
2022-08-17 | Magnetic domain scanning imaging using phase-sensitive THz-pulse detection | In our study, we determine the alignment of magnetic domains in a CoFeB layer
using THz radiation. We generate THz-pulses by fs-laser-pulses in magnetized
CoFeB/Pt heterostructures, based on spin currents. An LT-GaAs Auston switch
detects the radiation phase-sensitively and allows to determine the
magnetization alignment. Our scanning technique with motorized stages with step
sizes in the sub-micrometer range, allows to image two dimensional magnetic
structures. Theoretically the resolution is restricted to half of the
wavelength if focusing optics in the far-field limit are used. By applying
near-field imaging, the spatial resolution is enhanced to the single digit
micrometer range. For this purpose, spintronic emitters in diverse geometric
shapes, e.g. circles, triangles, squares, and sizes are prepared to observe the
formation of magnetization patterns. The alignment of the emitted THz radiation
can be influenced by applying unidirectional external magnetic fields. We
demonstrate how magnetic domains with opposite alignment and different shapes
divided by domain walls are created by demagnetizing the patterns using minor
loops and imaged using phase sensitive THz radiation detection. For analysis,
the data is compared to Kerr microscope images. The possibility to combine this
method with THz range spectroscopic information of magnetic texture or
antiferromagnets in direct vicinity to the spintronic emitter, makes this
detection method interesting for much wider applications probing THz excitation
in spin systems with high resolution beyond the Abbe diffraction limit, limited
solely by the laser excitation area. | 2208.08219v1 |
2022-08-19 | Revealing nanoscale disorder in W/CoFeB/MgO ultra-thin films using domain wall motion | Disorder in ultra-thin magnetic films can significantly hinder domain wall
motion. One of the main issues on the path towards efficient domain wall based
devices remains the characterization of the pinning landscape at the nanoscale.
In this paper, we study domain wall motion in W/CoFeB/MgO thin films with
perpendicular magnetic anisotropy crystallized by annealing at 400$^{\circ}$C
and a process based on He$^{+}$ irradiation combined with elevated
temperatures. Magnetic properties are similar for the whole series of samples,
while the magnetic domain wall mobility is critically improved in the
irradiated samples. By using an analytical model to extract nanoscale pinning
parameters, we reveal important variations in the disorder of the crystallized
samples. This work offers a unique opportunity to selectively analyze the
effects of disorder on the domain wall dynamics, without the contribution of
changes in the magnetic properties. Our results highlight the importance of
evaluating the nanoscale pinning parameters of the material when designing
devices based on domain wall motion, which in return can be a powerful tool to
probe the disorder in ultra-thin magnetic films. | 2208.09280v1 |
2022-09-05 | Nonlocal detection of interlayer three-magnon coupling | A leading nonlinear effect in magnonics is the interaction that splits a
high-frequency magnon into two low-frequency ones with conserved linear
momentum. Here, we report experimental observation of nonlocal three-magnon
scattering between spatially separated magnetic systems, viz. a CoFeB nanowire
and an yttrium iron garnet (YIG) thin film. Above a certain threshold power of
an applied microwave field, a CoFeB Kittel magnon splits into a pair of
counter-propagating YIG magnons that induce voltage signals in Pt electrodes on
each side, in excellent agreement with model calculations based on the
interlayer dipolar interaction. The excited YIG magnon pairs reside mainly in
the first excited (n=1) perpdendicular standing spin-wave mode. With increasing
power, the n=1 magnons successively scatter into nodeless (n=0) magnons through
a four-magnon process. Our results help to assess non-local scattering
processes in magnonic circuits that may enable quantum entanglement between
distant magnons for quantum information applications. | 2209.01875v1 |
2022-10-04 | Strain Coupled Domains in BaTiO3(111)-CoFeB Heterostructures | Domain pattern transfer from ferroelectric to ferromagnetic materials is a
critical step for the electric field control of magnetism and has the potential
to provide new schemes for low-power data storage and computing devices. Here
we investigate domain coupling in BaTiO$_3$(111)/CoFeB heterostructures by
direct imaging in a wide-field Kerr microscope. The magnetic easy axis is found
to locally change direction as a result of the underlying ferroelectric domains
and their polarisation. By plotting the remanent magnetisation as a function of
angle in the plane of the CoFeB layer, we find that the magnetic easy axes in
adjacent domains are angled at 60$^\circ$ or 120$^\circ$, corresponding to the
angle of rotation of the polarisation from one ferroelectric domain to the
next, and that the magnetic domain walls may be charged or uncharged depending
on the magnetic field history. Micromagnetic simulations show that the
properties of the domain walls vary depending on the magnetoelastic easy axis
configuration and the charged or uncharged nature of the wall. The
configuration where the easy axis alternates by 60$^\circ$ and a charged wall
is initialised exhibits the largest change in domain wall width from 192 nm to
119 nm as a function of in-plane magnetic field. Domain wall width tuning
provides an additional degree of freedom for devices that seek to manipulate
magnetic domain walls using strain coupling to ferroelectrics. | 2210.01511v1 |
2023-03-10 | Field-driven collapsing dynamics of skyrmions in magnetic multilayers | Magnetic skyrmions are fascinating topological particle-like textures
promoted by a trade-off among interfacial properties (perpendicular anisotropy
and Dzyaloshinskii-Moriya interaction (DMI)) and dipolar interactions.
Depending on the dominant interaction, complex spin textures, including pure
N\'eel and hybrid skyrmions have been observed in multilayer heterostructures.
A quantification of these different spin textures typically requires a
depth-reoslved magnetic imaging or scattering techniques. In the present work,
we will show qualitatively different collapsing dynamics for pure N\'eel and
hybrid skyrmions induced by a perpendicular magnetic field in two
representative systems, [Pt/Co/Ir]15 and [Ta/CoFeB/MgO]15 multilayers.
Skyrmions in the former stack undergo two morphological transitions, upon
reversing the perpendicular field direction. Skyrmions in [Ta/CoFeB/MgO]15
multilayers exhibit a continuous transition, which is mainly linked to a
reversible change of the skyrmion size. A full micromagnetic phase diagram is
presented to identify these two collapsing mechanisms as a function of material
parameters. Since the two distinct collapsing dynamics rely on the detailed
layer-dependent spin structures of skyrmions, they could be used as potential
fingerprints for identifying the skyrmion type in magnetic multilayers. Our
work suggests the employment of pure and hybrid skyrmions for specific
applications, due to the strong correlation between the skyrmion dynamics and
3-dimentional spin profiles. | 2303.05988v1 |
2023-07-05 | Comparative Analysis of THz Signal Emission from SiO$_2$/CoFeB/Metal Heterostructures: Wideband and High-Frequency THz Signal Advantage of PtBi-based Emitter | Spintronic THz emitters have attracted much attention due to their desirable
properties, such as affordability, ultra-wideband capability, high efficiency,
and tunable polarization. In this study, we investigate the characteristics of
THz signals, including their frequency, bandwidth, and amplitude, emitted from
a series of heterostructures with ferromagnetic (FM) and nonmagnetic (NM)
materials. The FM layer consists of a wedge-shaped CoFeB layer with a thickness
of 0 to 5 nm, while the NM materials include various metals such as Pt, Au, W,
Ru, Pt$_{\%92}$Bi$_{\%8}$, and Ag$_{\%90}$Bi$_{\%10}$ alloys. Our experiments
show that the emitter with Pt-NM layer has the highest amplitude of the emitted
THz signal. However, the PtBi-based emitter exhibits a higher central THz peak
and wider bandwidth, making it a promising candidate for broadband THz
emitters. These results pave the way for further exploration of the specific
compositions of Pt$_{1-x}$Bi$_{x}$ for THz emitter design, especially with the
goal of generating higher frequency and wider bandwidth THz signals. These
advances hold significant potential for applications in various fields such as
high-resolution imaging, spectroscopy, communications, medical diagnostics, and
more. | 2307.02232v1 |
2023-09-06 | Mid-infrared optical properties of non-magnetic-metal/CoFeB/MgO heterostructures | We report on the optical characterization of non-magnetic metal/ferromagnetic
(Co$_{20}$Fe$_{60}$B$_{20}$)/MgO heterostructures and interfaces by using mid
infrared spectroscopic ellipsometry at room temperature. We extracted for the
mid-infrared range the dielectric function of Co$_{20}$Fe$_{60}$B$_{20}$, that
is lacking in literature, from a multisample analysis. From the optical
modelling of the heterostructures we detected and determined the dielectric
tensor properties of a two-dimensional gas (2DEG) forming at the non-magnetic
metal and the CoFeB interface. These properties comprise independent Drude
parameters for the in-plane and out-of plane tensor components, with the latter
having an epsilon-near-zero frequency within our working spectral range. A
feature assigned to spin-orbit coupling (SOC) is identified. Furthermore, it is
found that both, the interfacial properties, 2DEG Drude parameters and SOC
strength, and the apparent dielectric function of the MgO layer depend on the
type of the underlying nonmagnetic metal, namely, Pt, W, or Cu. The results
reported here should be useful in tailoring novel phenomena in such types of
heterostructures by assessing their optical response noninvasively,
complementing existing characterization tools such as angle-resolved
photoemission spectroscopy, and those related to electron/spin transport. | 2309.02981v1 |
2023-12-10 | Magnetoelectric Coupling in Pb(Zr,Ti)O3/CoFeB Nanoscale Waveguides Studied by Propagating Spin-Wave Spectroscopy | This study introduces a method for the characterization of the
magnetoelectric coupling in nanoscale Pb(Zr,Ti)O3/CoFeB thin film composites
based on propagating spin-wave spectroscopy. Finite element simulations of the
strain distribution in the devices indicated that the magnetoelastic effective
field in the CoFeB waveguides was maximized in the Damon - Eshbach
configuration. All-electrical broadband propagating spin-wave transmission
measurements were conducted on Pb(Zr,Ti)O3/CoFeB magnetoelectric waveguides
with lateral dimensions down to 700 nm. The results demonstrated that the
spin-wave resonance frequency can be modulated by applying a bias voltage to
Pb(Zr,Ti)O3. The modulation is hysteretic due to the ferroelastic behavior of
Pb(Zr,Ti)O3. An analytical model was then used to correlate the change in
resonance frequency to the induced magnetoelastic field in the magnetostrictive
CoFeB waveguide. We observe a hysteresis magnetoelastic field strength with
values as large as 5.61 mT, and a non-linear magnetoelectric coupling
coefficient with a maximum value of 1.69 mT/V. | 2312.05819v1 |
2018-01-12 | Anomalous Nernst effect on the nanometer scale: Exploring three-dimensional temperature gradients in magnetic tunnel junctions | Localized laser heating creates temperature gradients in all directions and
thus leads to three-dimensional electron flux in metallic materials.
Temperature gradients in combination with material magnetization generate
thermomagnetic voltages. The interplay between these direction-dependent
temperature gradients and the magnetization along with their control enable to
manipulate the generated voltages, e.g. in magnetic nanodevices. We identify
the anomalous Nernst effect (ANE) generated on a nanometer length scale by
micrometer sized temperature gradients in magnetic tunnel junctions (MTJs). In
a systematic study, we extract the ANE by analyzing the influence of in-plane
temperature gradients on the tunnel magneto-Seebeck effect (TMS) in three
dimensional devices. To investigate these effects, we utilize in-plane
magnetized MTJs based on CoFeB electrodes with an MgO tunnel barrier. Due to
our measurement configuration, there is no necessity to disentangle the ANE
from the spin Seebeck effect in inverse spin-Hall measurements. The temperature
gradients are created by a tightly focused laser spot. The spatial extent of
the measured effects is defined by the MTJ size, while the spatial resolution
is given by the laser spot size and the step size of its lateral translation.
This method is highly sensitive to low voltages and yields an ANE coefficient
of $K_N\approx 1.6\cdot 10^{-8}\,\mathrm{V/TK}$ for CoFeB. In general, TMS
investigations in MTJs are motivated by the usage of otherwise wasted heat in
magnetic memory devices for read/write operations. Here, the additionally
generated ANE effect allows to expand the MTJs' functionality from simple
memory storage to nonvolatile logic devices and opens new application fields
such as direction dependent temperature sensing with the potential for further
downscaling. | 1801.04186v1 |
2021-03-14 | Perpendicular magnetic anisotropy in ultra-thin Cu$_2$Sb-type (Mn-Cr)AlGe films onto thermally oxidized silicon substrates | Perpendicularly magnetized films showing small saturation magnetization,
$M_\mathrm{s}$, are essential for spin-transfer-torque writing type
magnetoresistive random access memories, STT-MRAMs. An intermetallic compound,
{(Mn-Cr)AlGe} of the Cu$_2$Sb-type crystal structure was investigated, in this
study, as a material showing the low $M_\mathrm{s}$ ($\sim 300$ kA/m) and
high-perpendicular magnetic anisotropy, $K_\mathrm{u}$. The layer thickness
dependence of $K_\mathrm{u}$ and effects of Mg-insertion layers at top and
bottom (Mn-Cr)AlGe$|$MgO interfaces were studied in film samples fabricated
onto thermally oxidized silicon substrates to realize high-$K_\mathrm{u}$ in
the thickness range of a few nanometer. Optimum Mg-insertion thicknesses were
1.4 and 3.0 nm for the bottom and the top interfaces, respectively, which were
relatively thick compared to results in similar insertion effect investigations
on magnetic tunnel junctions reported in previous studies. The cross-sectional
transmission electron microscope images revealed that the Mg-insertion layers
acted as barriers to interdiffusion of Al-atoms as well as oxidization from the
MgO layers. The values of $K_\mathrm{u}$ were about $7 \times 10^5$ and $2
\times 10^5$ J/m$^3$ at room temperature for 5 and 3 nm-thick (Mn-Cr)AlGe
films, respectively, with the optimum Mg-insertion thicknesses. The
$K_\mathrm{u}$ at a few nanometer thicknesses is comparable or higher than
those reported in perpendicularly magnetized CoFeB films which are
conventionally used in MRAMs, while the $M_\mathrm{s}$ value is one third or
less smaller than those of the CoFeB films. The developed (Mn-Cr)AlGe films are
promising from the viewpoint of not only the magnetic properties, but also the
compatibility to the silicon process in the film fabrication. | 2103.07847v2 |
2019-12-08 | Multifilamentary character of anticorrelated capacitive and resistive switching in memristive structures based on (CoFeB)x(LiNbO3)100-x nanocomposite | Resistive and capacitive switching in capacitor metal/nanocomposite/metal
(M/NC/M) structures based on (CoFeB)x(LiNbO3)100-x NC fabricated by ion-beam
sputtering with metal content x $\approx$ 8-20 at. % is studied. The
peculiarity of the structure synthesis was the use of increased oxygen content
($\approx$ 2*10^-5 Torr) at the initial stage of the NC growth. The NC films,
along with metal nanogranules of 3-6 nm in size, contained a large number of
dispersed Co (Fe) atoms (up to ~10^22 cm^-3). Measurements were performed both
in DC and AC (frequency range 5-13 MHz) regimes. When switching structures from
high-resistance (Roff) to low-resistance (Ron) state, the effect of a strong
increase in their capacity was found, which reaches 8 times at x $\approx$ 15
at. % and the resistance ratio Roff/Ron $\approx$ 40. The effect is explained
by the synergetic combination of the multifilamentary character of resistive
switching (RS) and structural features of the samples associated, in
particular, with the formation of high-resistance and strongly polarizable
LiNbO3 layer near the bottom electrode of the structures. The proposed model is
confirmed by investigations of RS of two-layer nanoscale M/NC/LiNbO3/M
structures as well as by studies of the magnetization of M/NC/M structures in
the pristine state and after RS. | 1912.03726v3 |
2023-05-13 | Temperature dependence of magnetic anisotropy and domain wall tuning in BaTiO3(111)/CoFeB multiferroics | Artificial multiferroics consist of two types of ferroic materials, typically
a ferroelectric and ferromagnet, often coupled interfacially by
magnetostriction induced by the lattice elongations in the ferroelectric. In
BaTiO3 the magnitude of strain induced by these elongations is heavily
temperature dependent, varying greatly between each of the polar crystal phases
and exerting a huge influence over the properties of a coupled magnetic film.
Here we demonstrate that temperature, and thus strain, is an effective means of
controlling the magnetic anisotropy in BaTiO3(111)/CoFeB heterostructures. We
investigate the three polar phases of BaTiO3: tetragonal (T) at room
temperature, orthorhombic (O) below 280 K and rhombohedral (R) below 190 K,
across a total range of 77 K to 420 K. We find two distinct responses; a
step-like change in the anisotropy across the low-temperature phase
transitions, and a sharp high-temperature reduction around the ferroelectric
Curie temperature, measured from hard axis hysteresis loops. Using our
measurements of this anisotropy strength we are then able to show by
micromagnetic simulation the behaviour of all possible magnetic domain wall
states and determine their scaling as a function of temperature. The most
significant changes occur in the head-to-head domain wall states, with a
maximum change of 210 nm predicted across the entire range effectively doubling
the size of the domain wall as compared to room temperature. Notably, similar
changes are seen for both high and low temperatures which suggest different
routes for potential control of magnetic anisotropy and elastically pinned
magnetic domain walls. | 2305.07879v1 |
2006-07-21 | Tunnel magnetoresistance and robust room temperature exchange bias with multiferroic BiFeO3 epitaxial thin films | We report on the functionalization of multiferroic BiFeO3 epitaxial films for
spintronics. A first example is provided by the use of ultrathin layers of
BiFeO3 as tunnel barriers in magnetic tunnel junctions with La2/3Sr1/3MnO3 and
Co electrodes. In such structures, a positive tunnel magnetoresistance up to
30% is obtained at low temperature. A second example is the exploitation of the
antiferromagnetic spin structure of a BiFeO3 film to induce a sizeable (~60 Oe)
exchange bias on a ferromagnetic film of CoFeB, at room temperature.
Remarkably, the exchange bias effect is robust upon magnetic field cycling,
with no indications of training. | 0607563v2 |
2006-08-25 | Dependence of tunnel magnetoresistance on ferromagnetic electrode materials in MgO-barrier magnetic tunnel junctions | We investigated the relationship between the tunnel magnetoresistance (TMR)
ratio and the electrode structure in MgO-barrier magnetic tunnel junctions
(MTJs). The TMR ratio in a MTJ with Co40Fe40B20 reference and free layers
reached 355% at the post-deposition annealing temperature of Ta=400 degree C.
When Co50Fe50 or Co90Fe10 is used for the reference layer material, no high TMR
ratio was observed. The key to have high TMR ratio is to have highly oriented
(001) MgO barrier/CoFeB crystalline electrodes. The highest TMR ratio obtained
so far is 450% at Ta = 450 degree C in a pseudo spin-valve MTJ. | 0608551v1 |
2007-03-22 | Spin-Torque Ferromagnetic Resonance Measurements of Damping in Nanomagnets | We measure the magnetic damping parameter a in thin film CoFeB and permalloy
(Py) nanomagnets at room temperature using ferromagnetic resonance driven by
microwave frequency spin-transfer torque. We obtain $\alpha_{CoFeB} = 0.014 \pm
0.003$ and $\alpha_{Py}=0.010 \pm 0.002$, values comparable to measurements for
extended thin films, but significantly less than the effective damping
determined previously for similar nanomagnets by fits to time-domain studies of
large-angle magnetic excitations and magnetic reversal. The greater damping
found for the large amplitude nanomagnet dynamics is attributed to the
nonlinear excitation of non-uniform magnetic modes. | 0703577v1 |
2007-08-27 | Determination of Penetration Depth of Transverse Spin Current in Ferromagnetic Metals by Spin Pumping | Spin pumping in nonmagnetic/ferromagnetic metal multilayers is studied both
theoretically and experimentally. We show that the line widths of the
ferromagnetic resonance (FMR) spectrum depend on the thickness of the
ferromagnetic metal layers, which must not be in resonance with the oscillating
magnetic field. We also show that the penetration depths of the transverse spin
current in ferromagnetic metals can be determined by analyzing the line widths
of their FMR spectra. The obtained penetration depths in NiFe, CoFe and CoFeB
were 3.7 [nm], 2.5 [nm] and 12.0 [nm], respectively. | 0708.3528v3 |
2007-10-10 | Mechanisms of exchange bias with multiferroic BiFeO3 epitaxial thin films | We have combined neutron scattering and piezoresponse force microscopy to
study the relation between the exchange bias observed in CoFeB/BiFeO3
heterostructures and the multiferroic domain structure of the BiFeO3 films. We
show that the exchange field scales with the inverse of the ferroelectric and
antiferromagnetic domain size, as expected from Malozemoff's model of exchange
bias extended to multiferroics. Accordingly, polarized neutron reflectometry
reveals the presence of uncompensated spins in the BiFeO3 film at the interface
with the CoFeB. In view of these results we discuss possible strategies to
switch the magnetization of a ferromagnet by an electric field using BiFeO3. | 0710.2025v1 |
2009-10-22 | MgO barrier-perpendicular magnetic tunnel junctions with CoFe/Pd multilayers and ferromagnetic insertion layers | The authors studied an effect of ferromagnetic (Co20Fe60B20 or Fe) layer
insertion on tunnel magnetoresistance (TMR) properties of MgO-barrier magnetic
tunnel junctions (MTJs) with CoFe/Pd multilayer electrodes. TMR ratio in MTJs
with CoFeB/MgO/Fe stack reached 67% at an-nealing temperature (Ta) of 200
degree C and then decreased rapidly at Ta over 250 degree C. The degradation of
the TMR ratio may be related to crystallization of CoFe(B) into fcc(111) or
bcc(011) texture result-ing from diffusion of B into Pd layers. MTJs which were
in-situ annealed at 350oC just after depo-siting bottom CoFe/Pd multilayer
showed TMR ratio of 78% by post annealing at Ta =200 degree C. | 0910.4204v1 |
2010-08-09 | Detection of bottom ferromagnetic electrode oxidation in magnetic tunnel junctions by magnetometry measurements | Surface oxidation of the bottom ferromagnetic (FM) electrode, one of the
major detrimental factors to the performance of a Magnetic Tunnel Junction
(MTJ), is difficult to avoid during the fabrication process of the MTJ's tunnel
barrier. Since Co rich alloys are commonly used for the FM electrodes in MTJs,
over-oxidation of the tunnel barrier results in the formation of a CoO
antiferromagnetic (AF) interface layer which couples with the bottom FM
electrode to form a typical AF/FM exchange bias (EB) system. In this work,
surface oxidation of the CoFe and CoFeB bottom electrodes was detected via
magnetometry measurements of exchange-bias characterizations including the EB
field, training effect, uncompensated spin density, and coercivity. Variations
of these parameters were found to be related to the surface oxidation of the
bottom electrode, among them the change of coercivity is most sensitive.
Annealed samples show evidence for an oxygen migration back to the MgO tunnel
barrier by annealing. | 1008.1493v1 |
2011-03-16 | Spin-Torque Diode Measurements of MgO-Based Magnetic Tunnel Junctions with Asymmetric Electrodes | We present a detailed study of the spin-torque diode effect in
CoFeB/MgO/CoFe/NiFe magnetic tunnel junctions. From the evolution of the
resonance frequency with magnetic field at different angles, we clearly
identify the free-layer mode and find an excellent agreement with simulations
by taking into account several terms for magnetic anisotropy. Moreover, we
demonstrate the large contribution of the out-of-plane torque in our junctions
with asymmetric electrodes compared to the in-plane torque. Consequently, we
provide a way to enhance the sensitivity of these devices for the detection of
microwave frequency. | 1103.3207v1 |
2013-04-09 | Paramagnetic Fe_xTa_{1-x} alloys for engineering of perpendicularly magnetized tunnel junctions | Exchange coupling between two magnetic layers through an interlayer is of
broad interest for numerous recent applications of nano-magnetic systems. In
this Letter we study ferromagnetic exchange coupling through amorphous
paramagnetic Fe-Ta alloys. We show that the exchange coupling depends
exponentially on spacer thickness and scales with the Fe-Ta susceptibility,
which can be tuned via the alloy composition and/or temperature. Such materials
are of high interest for the engineering of perpendicularly magnetized
CoFeB-MgO based tunnel junctions as it enables ferromagnetic coupling of
magnetic layers with differing crystalline lattices, suppresses dead layers and
can act as an inter-diffusion barrier during annealing. | 1304.2763v1 |
2013-08-29 | Control of Propagating Spin Waves via Spin Transfer Torque in a Metallic Bilayer Waveguide | We investigate the effect of a direct current on propagating spin waves in a
CoFeB/Ta bilayer structure. Using the micro-Brillouin light scattering
technique, we observe that the spin wave amplitude may be attenuated or
amplified depending on the direction of the current and the applied magnetic
field. Our work suggests an effective approach for electrically controlling the
propagation of spin waves in a magnetic waveguide and may be useful in a number
of applications such as phase locked nano-oscillators and hybrid information
processing devices. | 1308.6357v2 |
2014-01-29 | Nonreciprocal Dispersion of Spin Waves in Ferromagnetic Thin Films Covered with a Finite-Conductivity Metal | We study the effect of one-side metallization of a uniform ferromagnetic thin
film on its spin-wave dispersion relation in the Damon-Eshbach geometry. Due to
the finite conductivity of the metallic cover layer on the ferromagnetic film
the spin-wave dispersion relation may be nonreciprocal only in a limited
wave-vector range. We provide an approximate analytical solution for the
spin-wave frequency, discuss its validity and compare it with numerical
results. The dispersion is analyzed systematically by varying the parameters of
the ferromagnetic film, the metal cover layer and the value of the external
magnetic field. The conclusions drawn from this analysis allow us to define a
structure based on a 30 nm thick CoFeB film with an experimentally accessible
nonreciprocal dispersion relation in a relatively wide wave-vector range. | 1401.7454v1 |
2014-02-18 | Switching Properties in Magnetic Tunnel Junctions with Interfacial Perpendicular Anisotropy: Micromagnetic Study | The role of universal memory can be successfully satisfied by magnetic tunnel
junctions (MTJs) where the writing mechanism is based on spin-transfer torque
(STT). An improvement in the switching properties (lower switching current
density maintaining the thermal stability) has been achieved in MTJs with
interfacial perpendicular anisotropy (IPA) at the interface between CoFeB and
MgO. In this paper, micromagnetic simulations point out the influence of IPA
and saturation magnetization (MS) on the properties of fast magnetization
reversal achieved in 5, 10 and 20 ns. Both cases of in-plane and out-of-plane
free layer are considered. In addition, the thermal effect is included for the
in-plane switching at 20 ns and a complete analysis of energy dissipation
during the switching is illustrated. This study can provide useful information
for the design of STT-based memories. | 1402.4352v1 |
2014-07-23 | Deterministic Domain Wall Motion Orthogonal To Current Flow Due To Spin Orbit Torque | Deterministic control of domain walls orthogonal to the direction of current
flow is demonstrated by exploiting spin orbit torque in a perpendicularly
polarized Ta/CoFeB/MgO multilayer in presence of an in-plane magnetic field.
Notably, such orthogonal motion with respect to current flow is not possible
from traditional spin transfer torque driven domain wall propagation even in
presence of an external magnetic field. Reversing the polarity of either the
current flow or the in-plane field is found to reverse the direction of the
domain wall motion. From these measurements, which are unaffected by any
conventional spin transfer torque by symmetry, we estimate the spin orbit
torque efficiency of Ta to be 0.08. | 1407.6137v1 |
2014-11-19 | Universal domain wall dynamics under electric field in Ta/CoFeB/MgO devices with perpendicular anisotropy | Electric field effects in ferromagnetic/oxide dielectric structures provide a
new route to control domain wall (DW) dynamics with low power dissipation.
However, electric field effects on DW velocities have only been observed so far
in the creep regime where DW velocities are low due to strong interactions with
pinning sites. Here, we show gate voltage modulation of DW velocities ranging
from the creep to the flow regime in Ta/Co40Fe40B20/MgO/TiO2 structures with
perpendicular magnetic anisotropy. We demonstrate a universal description of
the role of applied electric fields in the various pinning dependent regimes by
taking into account an effective magnetic field being linear with the electric
field. In addition, the electric field effect is found to change sign in the
Walker regime. Our work opens new opportunities for the study and optimization
of electric field effect at ferromagnetic metal/insulator interfaces. | 1411.5267v2 |
2015-03-02 | Measuring the magnetic moment density in patterned ultrathin ferromagnets with submicron resolution | We present a new approach to infer the surface density of magnetic moments
$I_s$ in ultrathin ferromagnetic films with perpendicular anisotropy. It relies
on quantitative stray field measurements with an atomic-size magnetometer based
on the nitrogen-vacancy center in diamond. The method is applied to
microstructures patterned in a 1-nm-thick film of CoFeB. We report measurements
of $I_s$ with a few percent uncertainty and a spatial resolution in the range
of $(100$ nm)$^2$, an improvement by several orders of magnitude over existing
methods. As an example of application, we measure the modifications of $I_s$
induced by local irradiation with He$^+$ ions in an ultrathin ferromagnetic
wire. This method offers a new route to study variations of magnetic properties
at the nanoscale. | 1503.00705v1 |
2015-03-25 | Spin Hall torque driven chiral domain walls in magnetic heterostructures | The motion of magnetic domain walls in ultrathin magnetic heterostructures
driven by current via the spin Hall torque is described. We show results from
perpendicularly magnetized CoFeB|MgO heterostructures with various heavy metal
underlayers. The domain wall moves along or against the current flow depending
on the underlayer material. The direction to which the domain wall moves is
associated with the chirality of the domain wall spiral formed in these
heterostructures. The one-dimensional model is used to describe the
experimental results and extract parameters such as the Dzyaloshinskii-Moriya
exchange constant which is responsible for the formation of the domain wall
spiral. Fascinating effects arising from the control of interfaces in magnetic
heterostructures are described. | 1503.07250v1 |
2015-03-31 | Spin Hall magnetoresistance in metallic bilayers | Spin Hall magnetoresistance (SMR) is studied in metallic bilayers that
consist of heavy metal (HM) layer and a ferromagnetic metal (FM) layer. We find
nearly a ten-fold increase of SMR in W/CoFeB compared to previously studied
HM/ferromagnetic insulator (FI) systems. The SMR increases with decreasing
temperature despite the negligible change in the W layer resistivity with
temperature. A model is developed to account for the absorption of the
longitudinal spin current to the FM layer, one of the key characteristics of a
metallic ferromagnet. We find that the model not only quantitatively describes
the HM layer thickness dependence of SMR, allowing accurate estimation of the
spin Hall angle and the spin diffusion length of the HM layer, but also can
account for the temperature dependence of SMR by assuming a temperature
dependent spin polarization of the FM layer. These results illustrate the
unique role a metallic ferromagnetic layer plays in defining spin transmission
across the HM/FM interface. | 1503.08903v2 |
2015-08-18 | Quasistatic and Pulsed Current-Induced Switching with Spin-Orbit Torques in Ultrathin Films with Perpendicular Magnetic Anisotropy | Spin-orbit interaction derived spin torques provide a means of reversing the
magnetization of perpendicularly magnetized ultrathin films with currents that
flow in the plane of the layers. A basic and critical question for applications
is the speed and efficiency of switching with nanosecond current pulses. Here
we investigate and contrast the quasistatic (slowly swept current) and pulsed
current-induced switching characteristics of micron scale Hall crosses
consisting of very thin ($<1$ nm) perpendicularly magnetized CoFeB layers on
$\beta$-Ta. While complete magnetization reversal is found at a threshold
current density in the quasistatic case, short duration ($\leq 10$ ns) larger
amplitude pulses ($\simeq 10 \times$ the quasistatic threshold current) lead to
only partial magnetization reversal and domain formation. We associate the
partial reversal with the limited time for reversed domain expansion during the
pulse. | 1508.04336v1 |
2015-08-24 | Origin of robust interaction of spin waves with a single skyrmion in perpendicularly magnetized nanostripes | We studied interactions between propagating spin waves (SWs) and a single
skyrmion in a perpendicularly magnetized CoFeB nanostripe where the magnetic
layer is interfaced with W and MgO. Micromagnetic numerical calculations
revealed that robust interactions between the incident SWs and the skyrmion
give rise to considerable forward skyrmion motions for specific SW frequencies
(e.g., here: fsw = 12 - 19 GHz). Additionally, it was found that there exists a
sufficiently low threshold field amplitude, e.g., 0.1 kOe for the fsw = 15 GHz
SWs. This frequency-dependent interaction originated from the robust coupling
of the SWs with the internal modes of the skyrmion, through the SWs' linear
momentum transfer torque acting on the skyrmion. This work provides for
all-magnetic control of skyrmion motions without electronic currents, and
facilitates further understanding of the interactions between magnons and
topological solitons in constricted geometries. | 1508.05682v1 |
2015-12-11 | Correlation between the spin Hall angle and the structural phases of early 5d transition metals | We have studied the relationship between the structure and the spin Hall
angle of the early 5d transition metals in X/CoFeB/MgO (X=Hf, Ta, W, Re)
heterostructures. Spin Hall magnetoresistance (SMR) is used to characterize the
spin Hall angle of the heavy metals. Transmission electron microscopy images
show that all underlayers are amorphous-like when their thicknesses are small,
however, crystalline phases emerge as the thickness is increased for certain
elements. We find that the heavy metal layer thickness dependence of the SMR
reflects these changes in structure. The spin Hall angle largest
|\theta$_{SH}$| of Hf, Ta, W and Re (~0.11, 0.10, 0.23 and 0.07, respectively)
is found when the dominant phase is amorphous-like. We find that the
amorphous-like phase not only possesses large resistivity but also exhibits
sizeable spin Hall conductivity, which both contribute to the emergence of the
large spin Hall angle. | 1512.03529v1 |
2016-02-24 | Exchange stiffness in ultrathin perpendicularly-magnetized CoFeB layers determined using spin wave spectroscopy | We measure the frequencies of spin waves in nm-thick perpendicularly
magnetized FeCoB systems, and model the frequencies to deduce the exchange
stiffness of this material in the ultrathin limit. For this, we embody the
layers in magnetic tunnel junctions patterned into circular nanopillars of
diameters ranging from 100 to 300 nm and we use magneto-resistance to determine
which rf-current frequencies are efficient in populating the spin wave modes.
Micromagnetic calculations indicate that the ultrathin nature of the layer and
the large wave vectors used ensure that the spin wave frequencies are
predominantly determined by the exchange stiffness, such that the number of
modes in a given frequency window can be used to estimate the exchange. For 1
nm layers the experimental data are consistent with an exchange stiffness A= 20
pJ/m, which is slightly lower that its bulk counterpart. The thickness
dependence of the exchange stiffness has strong implications for the numerous
situations that involve ultrathin films hosting strong magnetization gradients,
and the micromagnetic description thereof. | 1602.07421v1 |
2016-04-15 | Twist in the bias-dependence of spin-torques in magnetic tunnel junctions | The spin-torque in magnetic tunnel junctions possesses two components that
both depend on the applied voltage. Here, we develop a new method for the
accurate extraction of this bias-dependence from experiments over large voltage
ranges. We study several junctions with different magnetic layer structures of
the top electrode. Our results obtained on junctions with symmetric CoFeB
electrodes agree well with theoretical calculations. The bias-dependences of
asymmetric samples, with top electrodes containing NiFe, however, are twisted
compared to the quadratic form generally assumed. Our measurements reveal the
complexity of spin-torque mechanisms at large bias. | 1604.04517v1 |
2016-04-16 | A broadband Ferromagnetic Resonance dipper probe for magnetic damping measurements from 4.2 K to 300 K | A dipper probe for broadband Ferromagnetic Resonance (FMR) operating from 4.2
K to room temperature is described. The apparatus is based on a 2-port
transmitted microwave signal measurement with a grounded coplanar waveguide.
The waveguide generates a microwave field and records the sample response. A
3-stage dipper design is adopted for fast and stable temperature control. The
temperature variation due to FMR is in the milli-Kelvin range at liquid helium
temperature. We also designed a novel FMR probe head with a spring-loaded
sample holder. Improved signal-to-noise ratio and stability compared to a
common FMR head are achieved. Using a superconducting vector magnet we
demonstrate Gilbert damping measurements on two thin film samples using a
vector network analyzer with frequency up to 26 GHz: 1) A Permalloy film of 5
nm thickness and 2) a CoFeB film of 1.5 nm thickness. Experiments were
performed with the applied magnetic field parallel and perpendicular to the
film plane. | 1604.04688v1 |
2016-05-18 | Electrical control over perpendicular magnetization switching driven by spin-orbit torques | Flexible control of magnetization switching by electrical manners is crucial
for applications of spin-orbitronics. Besides of a switching current that is
parallel to an applied field, a bias current that is normal to the switching
current is introduced to tune the magnitude of effective damping-like and
field-like torques and further to electrically control magnetization switching.
Symmetrical and asymmetrical control over the critical switching current by the
bias current with opposite polarities is both realized in Pt/Co/MgO and
$\alpha$-Ta/CoFeB/MgO systems, respectively. This research not only identifies
the influences of field-like and damping-like torques on switching process but
also demonstrates an electrical method to control it. | 1605.05569v1 |
2016-09-14 | Rashba-Edelstein Magnetoresistance in Metallic Heterostructure | We report the observation of magnetoresistance originating from Rashba
spin-orbit coupling (SOC) in a metallic heterostructure: the Rashba-Edelstein
(RE) magnetoresistance. We show that the simultaneous action of the direct and
inverse RE effects in a Bi/Ag/CoFeB trilayer couples current-induced spin
accumulation to the electric resistance. The electric resistance changes with
the magnetic-field angle, reminiscent of the spin Hall magnetoresistance,
despite the fact that bulk SOC is not responsible for the magnetoresistance. We
further found that, even when the magnetization is saturated, the resistance
increases with increasing the magnetic-field strength, which is attributed to
the Hanle magnetoresistance in this system. | 1609.04122v1 |
2016-11-03 | Scalable synchronization of spin-Hall oscillators in out-of-plane field | A strategy for a scalable synchronization of an array of spin-Hall
oscillators (SHOs) is illustrated. In detail, we present micromagnetic
simulations of two and five SHOs realized by means of couples of triangular
golden contacts on the top of a Pt/CoFeB/Ta trilayer. Results highlight that
the synchronization occurs for the whole current region that gives rise to the
excitation of self-oscillations. This is linked to the role of the
magnetodipolar coupling, which is the phenomenon driving the synchronization
when the distance between oscillators is not too large. Synchronization turns
out to be also robust against geometrical differences of the contacts,
simulated by considering variable distances between the tips ranging from 100nm
to 200nm. Besides, it entails an enlargement of the radiation pattern that can
be useful for the generation of spin-waves in magnonics applications.
Simulations performed to study the effect of the interfacial
Dzyaloshinskii-Moriya interaction show nonreciprocity in spatial propagation of
the synchronized spin-wave. The simplicity of the geometry and the robustness
of the achieved synchronization make this design of array of SHOs scalable for
a larger number of synchronized oscillators. | 1611.01227v1 |
2017-01-23 | Understanding stability diagram of perpendicular magnetic tunnel junctions | Perpendicular magnetic tunnel junctions (MTJ) with a bottom pinned reference
layer and a composite free layer (FL) are investigated. Different thicknesses
of the FL were tested to obtain an optimal balance between tunneling
magnetoresistance (TMR) ratio and perpendicular magnetic anisotropy. After
annealing at 400 $^\circ$C, the TMR ratio for 1.5 nm thick CoFeB sublayer
reached 180 % at room temperature and 280 % at 20 K with an MgO tunnel barrier
thickness corresponding to the resistance area product RA = 10
Ohm$\mathrm{\mu}$m$^2$. The voltage vs. magnetic field stability diagrams
measured in pillar-shaped MTJs with 130 nm diameter indicate the competition
between spin transfer torque (STT), voltage controlled magnetic anisotropy
(VCMA) and temperature effects in the switching process. An extended stability
phase diagram model that takes into account all three parameters and the
effective damping measured independently using broadband ferromagnetic
resonance technique enabled the determination of both STT and VCMA coefficients
that are responsible for the FL magnetization switching. | 1701.06411v1 |
2017-03-29 | Ultrabroadband single-cycle terahertz pulses with peak fields of 300 kV cm$^{-1}$ from a metallic spintronic emitter | To explore the capabilities of metallic spintronic thin-film stacks as a
source of intense and broadband terahertz electromagnetic fields, we excite a
W/CoFeB/Pt trilayer on a large-area glass substrate (diameter of 7.5 cm) by a
femtosecond laser pulse (energy 5.5 mJ, duration 40 fs, wavelength 800 nm).
After focusing, the emitted terahertz pulse is measured to have a duration of
230 fs, a peak field of 300 kV cm$^{-1}$ and an energy of 5 nJ. In particular,
the waveform exhibits a gapless spectrum extending from 1 to 10 THz at 10% of
amplitude maximum, thereby facilitating nonlinear control over matter in this
difficult-to-reach frequency range and on the sub-picosecond time scale. | 1703.09970v1 |
2017-07-15 | Stochastic Spin-Orbit Torque Devices as Elements for Bayesian Inference | Probabilistic inference from real-time input data is becoming increasingly
popular and may be one of the potential pathways at enabling cognitive
intelligence. As a matter of fact, preliminary research has revealed that
stochastic functionalities also underlie the spiking behavior of neurons in
cortical microcircuits of the human brain. In tune with such observations,
neuromorphic and other unconventional computing platforms have recently started
adopting the usage of computational units that generate outputs
probabilistically, depending on the magnitude of the input stimulus. In this
work, we experimentally demonstrate a spintronic device that offers a direct
mapping to the functionality of such a controllable stochastic switching
element. We show that the probabilistic switching of Ta/CoFeB/MgO
heterostructures in presence of spin-orbit torque and thermal noise can be
harnessed to enable probabilistic inference in a plethora of unconventional
computing scenarios. This work can potentially pave the way for hardware that
directly mimics the computational units of Bayesian inference. | 1707.04687v2 |
2017-09-08 | Tunneling magnetoresistance of perpendicular CoFeB-based junctions with exchange bias | Recently, magnetic tunnel junctions with perpendicular magnetized electrodes
combined with exchange bias films have attracted large interest. In this paper
we examine the tunnel magnetoresistance of
Ta/Pd/IrMn/Co-Fe/Ta/Co-Fe-B/MgO/Co-Fe-B/capping/Pd magnetic tunnel junctions in
dependence on the capping layer, i.e., Hf or Ta. In these stacks perpendicular
exchange bias fields of -500\,Oe along with perpendicular magnetic anisotropy
are combined. A tunnel magnetoresistance of $(47.2\pm 1.4)\%$ for the Hf-capped
sample was determined compared to the Ta one $(42.6\pm 0.7)\%$ at room
temperature. Interestingly, this observation is correlated to the higher boron
absorption of Hf compared to Ta which prevents the suppression of
$\Delta_{\textrm{1}}$ channel and leads to higher tunnel magnetoresistance
values. Furthermore, the temperature dependent coercivities of the soft
electrodes of both samples are mainly described by the Stoner-Wohlfarth model
including thermal fluctuations. Slight deviations at low temperatures can be
attributed to a torque on the soft electrode that is generated by the pinned
magnetic layer system. | 1709.02607v1 |
2017-09-12 | Scaling of Dzyaloshinskii Moriya interaction at heavy metal and ferromagnetic metal interfaces | The Dzyaloshinskii Moriya Interaction (DMI) at the heavy metal (HM) and
ferromagnetic metal (FM) interface has been recognized as a key ingredient in
spintronic applications. Here we investigate the chemical trend of DMI on the
5d band filling (5d^3~5d^10) of the HM element in HM/CoFeB/MgO multilayer thin
films. DMI is quantitatively evaluated by measuring asymmetric spin wave
dispersion using Brillouin light scattering. Sign reversal and 20 times
modification of the DMI coefficient D have been measured as the 5d HM element
is varied. The chemical trend can be qualitatively understood by considering
the 5d and 3d bands alignment at the HM/FM interface and the subsequent orbital
hybridization around the Fermi level. Furthermore, a positive correlation is
observed between DMI and spin mixing conductance at the HM/FM interfaces. Our
results provide new insights into the interfacial DMI for designing future
spintronic devices. | 1709.03961v1 |
2017-11-09 | Room Temperature Giant Charge-to-Spin Conversion at SrTiO3/LaAlO3 Oxide Interface | Two-dimensional electron gas (2DEG) formed at the interface between SrTiO3
(STO) and LaAlO3 (LAO) insulating layer is supposed to possess strong Rashba
spin-orbit coupling. To date, the inverse Edelstein effect (i.e. spin-to-charge
conversion) in the 2DEG layer is reported. However, the direct effect of
charge-to-spin conversion, an essential ingredient for spintronic devices in a
current induced spin-orbit torque scheme, has not been demonstrated yet. Here
we show, for the first time, a highly efficient spin generation with the
efficiency of ~6.3 in the STO/LAO/CoFeB structure at room temperature by using
spin torque ferromagnetic resonance. In addition, we suggest that the spin
transmission through the LAO layer at high temperature range is attributed to
the inelastic tunneling via localized states in the LAO band gap. Our findings
may lead to potential applications in the oxide insulator based spintronic
devices. | 1711.03268v1 |
2017-12-05 | Studies of CoFeB crystalline structure grown on PbSnTe topological insulator substrate | Co40Fe40B20 layers were grown on the Pb0.71Sn0.29Te topological insulator
substrates by laser molecular beam epitaxy (LMBE) method, and the growth
conditions were studied. The possibility of growing epitaxial layers of a
ferromagnet on the surface of a topological insulator was demonstrated for the
first time. The Co40Fe40B20 layers obtained have a bcc crystal structure with a
crystalline (111) plane parallel to the (111) PbSnTe plane. The use of
three-dimensional mapping in the reciprocal space of reflection high electron
diffraction (RHEED) patterns made it possible to determine the epitaxial
relationship of main crystallographic axes between the film and the substrate
of topological insulator. Quenching of some reflections in diffraction pattern
allows confirmation of the substrate stoichiometry. | 1712.01585v3 |
2017-12-23 | Shape anisotropy revisited in single-digit nanometer magnetic tunnel junctions | Nanoscale magnetic tunnel junction plays a pivotal role in magnetoresistive
random access memories. Successful implementation depends on a simultaneous
achievement of low switching current for the magnetization switching by
spin-transfer torque and high thermal stability, along with a continuous
reduction of junction size. Perpendicular-easy-axis CoFeB/MgO stacks possessing
interfacial anisotropy have paved the way down to 20-nm scale, below which a
new approach needs to be explored. Here we show magnetic tunnel junctions that
satisfy the requirements at ultrafine scale by revisiting shape anisotropy,
which is a classical part of magnetic anisotropy but has not been fully
utilized in the current perpendicular systems. Magnetization switching solely
driven by current is achieved for junctions smaller than 10 nm where sufficient
thermal stability is provided by shape anisotropy without adopting new material
systems. This work is expected to push forward the development of magnetic
tunnel junctions towards single-digit-nm-scale nano-magnetics/spintronics. | 1712.08774v1 |
2018-01-17 | Hartman effect for spin waves in exchange regime | Hartman effect for spin waves tunnelling through a barrier in a thin magnetic
film is considered theoretically. The barrier is assumed to be created by a
locally increased magnetic anisotropy field. The considerations are focused on
a nanoscale system operating in the exchange-dominated regime. We derive the
formula for group delay $\tau_{gr}$ of spin wave package and show that
$\tau_{gr}$ saturates with increasing barrier width, which is a signature of
the Hartman effect predicted earlier for photonic and electronic systems. In
our calculations we consider the general boundary exchange conditions which
take into account different strength of exchange coupling between the barrier
and its surrounding. As a system suitable for experimental observation of the
Hartman effect we propose a CoFeB layer with perpendicular magnetic anisotropy
induced by a MgO overlayer. | 1801.05876v3 |
2018-03-22 | Voltage Control of Magnetic Monopoles in Artificial Spin Ice | Current research on artificial spin ice (ASI) systems has revealed unique
hysteretic memory effects and mobile quasi-particle monopoles controlled by
externally applied magnetic fields. Here, we numerically demonstrate a
strain-mediated multiferroic approach to locally control the ASI monopoles. The
magnetization of individual lattice elements is controlled by applying voltage
pulses to the piezoelectric layer resulting in strain-induced magnetic
precession timed for 180 degree reorientation. The model demonstrates localized
voltage control to move the magnetic monopoles across lattice sites, in CoFeB,
Ni, and FeGa based ASI$'$s. The switching is achieved at frequencies near
ferromagnetic resonance and requires energies below 620 aJ. The results
demonstrate that ASI monopoles can be efficiently and locally controlled with a
strain-mediated multiferroic approach. | 1803.08598v1 |
2018-05-09 | Programmable control of spin-wave transmission in a domain-wall spin valve | Active manipulation of spin waves is essential for the development of
magnon-based technologies. Here, we demonstrate programmable spin-wave
filtering by resetting the spin structure of a pinned 90$^\circ$ N\'{e}el
domain wall in a continuous CoFeB film with abrupt rotations of uniaxial
magnetic anisotropy. Using phase-resolved micro-focused Brillouin light
scattering and micromagnetic simulations, we show that broad 90$^\circ$
head-to-head or tail-to-tail magnetic domain walls are transparent to spin
waves over a broad frequency range. In contrast, magnetic switching to a
90$^\circ$ head-to-tail configuration produces much narrower domain walls at
the same pinning locations. Spin waves are strongly reflected by a resonance
mode in these magnetic domain walls. Based on these results, we propose a
magnetic spin-wave valve with two parallel domain walls. Switching the
spin-wave valve from an open to a close state changes the transmission of spin
waves from nearly 100% to 0% at the resonance frequency. This active control
over spin-wave transport could be utilized in magnonic logic devices or
non-volatile memory elements. | 1805.03470v1 |
2018-05-10 | Anomalous spin Hall magnetoresistance in Pt/Co bilayers | We have studied the spin Hall magnetoresistance (SMR), the magnetoresistance
within the plane transverse to the current flow, of Pt/Co bilayers. We find
that the SMR increases with increasing Co thickness: the effective spin Hall
angle for bilayers with thick Co exceeds the reported values of Pt when a
conventional drift-diffusion model is used. An extended model including spin
transport within the Co layer cannot account for the large SMR. To identify its
origin, contributions from other sources are studied. For most bilayers, the
SMR increases with decreasing temperature and increasing magnetic field,
indicating that magnon-related effects in the Co layer play little role.
Without the Pt layer, we do not observe the large SMR found for the Pt/Co
bilayers with thick Co. Implementing the effect of the so-called interface
magnetoresistance and the textured induced anisotropic scattering cannot
account for the Co thickness dependent SMR. Since the large SMR is present for
W/Co but its magnitude reduces in W/CoFeB, we infer its origin is associated
with a particular property of Co. | 1805.03843v1 |
2019-04-25 | Low damping magnetic properties and perpendicular magnetic anisotropy with strong volume contribution in the Heusler alloy Fe1.5CoGe | We present a study of the dynamic magnetic properties of TiN-buffered
epitaxial thin films of the Heusler alloy Fe$_{1.5}$CoGe. Thickness series
annealed at different temperatures are prepared and the magnetic damping is
measured, a lowest value of $\alpha=2.18\times 10^{-3}$ is obtained. The
perpendicular magnetic anisotropy properties in Fe$_{1.5}$CoGe/MgO are also
characterized. The evolution of the interfacial perpendicular anisotropy
constant $K^{\perp}_{\rm S}$ with the annealing temperature is shown and
compared with the widely used CoFeB/MgO interface. A large volume contribution
to the perpendicular anisotropy of $(4.3\pm0.5)\times 10^{5}$ $\rm J/m^3$ is
also found, in contrast with vanishing bulk contribution in common Co- and
Fe-based Heusler alloys. | 1904.11247v1 |
2019-10-11 | Investigation of spin orbit torque driven dynamics in ferromagnetic heterostructures | We use time-resolved (TR) measurements based on the polar magneto-optical
Kerr effect (MOKE) to study the magnetization dynamics excited by spin orbit
torques in Py (Permalloy)/Pt and Ta/CoFeB bilayers. The analysis reveals that
the field-like (FL) spin orbit torque (SOT) dominates the amplitude of the
first oscillation cycle of the magnetization precession and the damping-like
(DL) torque determines the final steady-state magnetization. In our bilayer
samples, we have extracted the effective fields, hFL and hDL, of the two SOTs
from the time-resolved magnetization oscillation spectrum. The extracted values
are in good agreement with those extracted from time-integrated DCMOKE
measurements, suggesting that the SOTs do not change at high frequencies. We
also find that the amplitude ratio of the first oscillation to steady state is
linearly proportional to the ratio hFL/hDL. The first oscillation amplitude is
inversely proportional to, whereas the steady state value is independent of,
the applied external field along the current direction. | 1910.04945v1 |
2019-10-28 | The spin Hall effect of Bi-Sb alloys driven by thermally excited Dirac-like electrons | We have studied the charge to spin conversion in Bi$_{1-x}$Sb$_x$/CoFeB
heterostructures. The spin Hall conductivity (SHC) of the sputter deposited
heterostructures exhibits a high plateau at Bi-rich compositions, corresponding
to the topological insulator phase, followed by a decrease of SHC for Sb-richer
alloys, in agreement with the calculated intrinsic spin Hall effect of
Bi$_{1-x}$Sb$_x$ alloy. The SHC increases with increasing thickness of the
Bi$_{1-x}$Sb$_x$ alloy before it saturates, indicating that it is the bulk of
the alloy that predominantly contributes to the generation of spin current; the
topological surface states, if present in the films, play little role.
Surprisingly, the SHC is found to increase with increasing temperature,
following the trend of carrier density. These results suggest that the large
SHC at room temperature, with a spin Hall efficiency exceeding 1 and an
extremely large spin current mobility, is due to increased number of
Dirac-like, thermally-excited electrons in the $L$ valley of the narrow gap
Bi$_{1-x}$Sb$_x$ alloy. | 1910.12433v1 |
2018-02-07 | Breaking the current density threshold in spin-orbit-torque magnetic random access memory | Spin-orbit-torque magnetic random access memory (SOT-MRAM) is a promising
technology for the next generation of data storage devices. The main bottleneck
of this technology is the high reversal current density threshold. This
outstanding problem of SOT-MRAM is now solved by using a current density of
constant magnitude and varying flow direction that reduces the reversal current
density threshold by a factor of more than the Gilbert damping coefficient. The
Euler-Lagrange equation for the fastest magnetization reversal path and the
optimal current pulse are derived for an arbitrary magnetic cell. The
theoretical limit of minimal reversal current density and current density for a
GHz switching rate of the new reversal strategy for CoFeB/Ta SOT-MRAMs are
respectively of the order of $10^5$ A/cm$^2$ and $10^6$ A/cm$^2$ far below
$10^7$ A/cm$^2$ and $10^8$ A/cm$^2$ in the conventional strategy. Furthermore,
no external magnetic field is needed for a deterministic reversal in the new
strategy. | 1802.02415v1 |
2018-12-05 | Single spin sensing of domain wall structure and dynamics in a thin film skyrmion host | Skyrmions are nanoscale magnetic structures with features promising for
future low-power memory or logic devices. In this work, we demonstrate novel
scanning techniques based on nitrogen vacancy center magnetometry that
simultaneously probe both the magnetic dynamics and structure of room
temperature skyrmion bubbles in a thin film system Ta/CoFeB/MgO. We confirm the
handedness of the Dzyaloshinskii-Moriya interaction in this material and
extract the helicity angle of the skyrmion bubbles. Our measurements also show
that the skyrmion bubbles in this material change size in discrete steps,
dependent on the local pinning environment, with their average size determined
dynamically as their domain walls hop between pinning sites. In addition, an
increase in magnetic field noise is observed near all skyrmion bubble domain
walls. These measurements highlight the importance of interactions between
internal degrees of freedom of skyrmion bubble domain walls and pinning sites
in thin film systems. Our observations have relevance for future devices based
on skyrmion bubbles where pinning interactions will determine important aspects
of current-driven motion. | 1812.01764v1 |
2019-06-04 | High frequency voltage-induced ferromagnetic resonance in magnetic tunnel junctions | Voltage-induced ferromagnetic resonance (V-FMR) in magnetic tunnel junctions
(MTJs) with a W buffer is investigated. Perpendicular magnetic anisotropy (PMA)
energy is controlled by both thickness of a CoFeB free layer deposited directly
on the W buffer and a post-annealing process at different temperatures. The PMA
energy as well as the magnetization damping are determined by analysing
field-dependent FMR signals in different field geometries. An optimized MTJ
structure enabled excitation of V-FMR at frequencies exceeding 30 GHz. The
macrospin modelling is used to analyse the field- and angular-dependence of the
V-FMR signal and to support experimental magnetization damping extraction. | 1906.01301v1 |
2022-02-01 | Numerical Model Of Harmonic Hall Voltage Detection For Spintronic Devices | We present a numerical macrospin model for harmonic voltage detection in
multilayer spintronic devices. The core of the computational backend is based
on the Landau-Lifshitz-Gilbert-Slonczewski equation, which combines high
performance with satisfactory, for large-scale applications, agreement with the
experimental results. We compare the simulations with the experimental findings
in Ta/CoFeB bilayer system for angular- and magnetic field-dependent resistance
measurements, electrically detected magnetisation dynamics, and harmonic Hall
voltage detection. Using simulated scans of the selected system parameters such
as the polar angle $\theta$, magnetisation saturation
($\mu_\textrm{0}M_\textrm{s}$) or uniaxial magnetic anisotropy ($K_\textrm{u}$)
we show the resultant changes in the harmonic Hall voltage, demonstrating the
dominating influence of the $\mu_\textrm{0}M_\textrm{s}$ on the first and
second harmonics. In the spin-diode ferromagnetic resonance (SD-FMR) technique
resonance method the ($\mu_\textrm{0}M_\textrm{s}$, $K_\textrm{u}$) parameter
space may be optimised numerically to obtain a set of viable curves that fit
the experimental data. | 2202.00364v1 |
2022-02-03 | Element Doping Enhanced Charge-to-Spin Conversion Efficiency in Amorphous PtSn4 Dirac Semimetal | Topological semimetals (TSs) are promising candidates for low-power
spin-orbit torque (SOT) devices due to their large charge-to-spin conversion
efficiency. Here, we investigated the charge-to-spin conversion efficiency of
amorphous PtSn4 (5 nm)/CoFeB (2.5-12.5 nm) layered structures prepared by a
magnetron sputtering method at room temperature. The charge-to-spin ratio of
PtSn4/CoFeB bilayers was 0.08, characterized by a spin torque ferromagnetic
resonance (ST-FMR) technique. This ratio can further increase to 0.14 by
inducing dopants, like Al and CoSi, into PtSn4. The dopants can also decrease
(Al doping) or increase (CoSi doping) the resistivity of PtSn4. The work
proposed a way to enhance the spin-orbit coupling (SOC) in amorphous TSs with
dopants. | 2202.01384v1 |
2022-02-04 | A Magnetoelectric Memory Device Based on Pseudo-Magnetization | We propose a new type of magnetoelectric memory device that stores magnetic
easy-axis information or pseudo-magnetization, rather than a definite
magnetization direction, in piezoelectric/ferromagnetic (PE/FM)
heterostructures. Theoretically, we show how a PE/FM combination can lead to
non-volatility in pseudo-magnetization exhibiting ferroelectric-like behavior.
The pseudo-magnetization can be manipulated by extremely low voltages
especially when the FM is a low-barrier nanomagnet. Using a circuit model
benchmarked against experiments, we determine the switching energy, delay,
switching probability and retention time of the envisioned 1T/1C memory device
in terms of magnetic and circuit parameters and discuss its thermal stability
in terms of a key parameter called back-voltage vm which is an electrical
measure of the strain-induced magnetic field. Taking advantage of ferromagnetic
resonance (FMR) measurements, we experimentally extract values for vm in CoFeB
films and circular nano-magnets deposited on Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT)
which agree well with the theoretical values. Our experimental findings indeed
indicate the feasibility of the proposed novel device and confirm the assumed
parameters in our modeling effort. | 2202.02203v2 |
2016-03-09 | Evolution of perpendicular magnetized tunnel junctions upon annealing | We study the evolution of perpendicularly magnetized tunnel junctions under
300 to 400 $^{\circ}$C annealing. The hysteresis loops do not evolve much
during annealing and they are not informative of the underlying structural
evolutions. These evolutions are better revealed by the frequencies of the
ferromagnetic resonance eigenmodes of the tunnel junction. Their modeling
provides the exchange couplings and the layers' anisotropies within the stack
which can serve as a diagnosis of the tunnel junction state after each
annealing step. The anisotropies of the two CoFeB-based parts and the two
Co/Pt-based parts of the tunnel junction decay at different rates during
annealing. The ferromagnet exchange coupling through the texture-breaking Ta
layer fails above 375$^{\circ}$C. The Ru spacer meant to promote a synthetic
antiferromagnet behavior is also insufficiently robust to annealing. Based on
these evolutions we propose optimization routes for the next generation tunnel
junctions. | 1603.02824v1 |
2017-06-02 | Dzyaloshinskii Moriya interaction across antiferromagnet / ferromagnet interface | The antiferromagnet (AFM) / ferromagnet (FM) interfaces are of central
importance in recently developed pure electric or ultrafast control of FM
spins, where the underlying mechanisms remain unresolved. Here we report the
direct observation of Dzyaloshinskii Moriya interaction (DMI) across the AFM/FM
interface of IrMn/CoFeB thin films. The interfacial DMI is quantitatively
measured from the asymmetric spin wave dispersion in the FM layer using
Brillouin light scattering. The DMI strength is enhanced by a factor of 7 with
increasing IrMn layer thickness in the range of 1- 7.5 nm. Our findings provide
deeper insight into the coupling at AFM/FM interface and may stimulate new
device concepts utilizing chiral spin textures such as magnetic skyrmions in
AFM/FM heterostructures. | 1706.00535v1 |
2017-06-26 | Enhancement of thermovoltage and tunnel magneto-Seebeck effect in CoFeB based magnetic tunnel junctions by variation of the MgAl$_2$O$_4$ and MgO barrier thickness | We investigate the influence of the barrier thickness of
Co$_{40}$Fe$_{40}$B$_{20}$ based magnetic tunnel junctions on the laser-induced
tunnel magneto-Seebeck effect. Varying the barrier thickness from 1nm to 3nm,
we find a distinct maximum in the tunnel magneto-Seebeck effect for 2.6nm
barrier thickness. This maximum is independently measured for two barrier
materials, namely MgAl$_2$O$_4$ and MgO. Additionally, samples with an
MgAl$_2$O$_4$ barrier exhibit a high thermovoltage of more than 350$\mu$V in
comparison to 90$\mu$V for the MTJs with MgO barrier when heated with the
maximum laser power of 150mW. Our results allow for the fabrication of improved
stacks when dealing with temperature differences across magnetic tunnel
junctions for future applications in spin caloritronics, the emerging research
field that combines spintronics and themoelectrics. | 1706.08287v1 |
2018-04-11 | Broadband voltage rectifier induced by linear bias dependence in CoFeB/MgO magnetic tunnel junctions | In this paper, the perpendicular magnetic anisotropy (PMA) is tailored by
changing the thickness of the free layer with the objective of producing MTJ
nano-pillars with smooth linear resistance dependence with both in-plane
magnetic field and DC bias. We furthermore demonstrate how this linear bias
dependence can be used to create a zero-threshold broadband voltage rectifier,
a feature which is important for rectification in wireless charging and energy
harvesting applications. By carefully balancing the amount of PMA acting in the
free layer the measured RF to DC voltage conversion efficiency can be made as
large as 11%. | 1804.04104v1 |
2018-04-14 | Correlation between Dzyaloshinskii Moriya interaction and spin mixing conductance at antiferromagnet / ferromagnet interface | The rich interaction phenomena at antiferromagnet (AFM)/ ferromagnet (FM)
interfaces are key ingredients in AFM spintronics, where many underlying
mechanisms remain unclear. Here we report a correlation observed between
interfacial Dzyaloshinskii-Moriya interaction (DMI) Ds and effective spin
mixing conductance g at IrMn/CoFeB interface. Both Ds and g are quantitatively
determined with Brillouin light scattering measurements, and increase with IrMn
thickness in the range of 2.5~7.5 nm. Such correlation likely originates from
the AFM-states-mediated spin-flip transitions in FM, which promote both
interfacial DMI and spin pumping effect. Our findings provide deeper insight
into the AFM-FM interfacial coupling for future spintronic design. | 1804.05151v2 |
2018-04-18 | Ferromagnetic resonance linewidth in coupled layers with easy-plane and perpendicular magnetic anisotropies | Magnetic bilayers with different magnetic anisotropy directions are
interesting for spintronic appli- cations as they offer the possibility to
engineer tilted remnant magnetization states. We investigate the ferromagnetic
resonance (FMR) linewidth of modes associated with two interlayer exchange-
coupled ferromagnetic layers, the first a CoNi multilayer with a perpendicular
magnetic anisotropy, and the second a CoFeB layer with an easy-plane
anisotropy. For antiferromagnetic interlayer ex- change coupling, elevated FMR
linewidths are observed below a characteristic field. This is in contrast to
what is found in uncoupled, ferromagnetically coupled and single ferromagnetic
layers in which the FMR linewidth increases monotonically with field. We show
that the characteristic field at which there is a dramatic increase in FMR
linewidth can be understood using a macrospin model with Heisenberg-type
exchange coupling between the layers. | 1804.06796v1 |
2019-03-01 | Individual skyrmion manipulation by local magnetic field gradients | Magnetic skyrmions are topologically protected spin textures, stabilised in
systems with strong Dzyaloshinskii-Moriya interaction (DMI). Several studies
have shown that electrical currents can move skyrmions efficiently through
spin-orbit torques. While promising for technological applications,
current-driven skyrmion motion is intrinsically collective and accompanied by
undesired heating effects. Here we demonstrate a new approach to control
individual skyrmion positions precisely, which relies on the magnetic
interaction between sample and a magnetic force microscopy (MFM) probe. We
investigate perpendicularly magnetised X/CoFeB/MgO multilayers, where for X = W
or Pt the DMI is sufficiently strong to allow for skyrmion nucleation in an
applied field. We show that these skyrmions can be manipulated individually
through the local field gradient generated by the scanning MFM probe with an
unprecedented level of accuracy. Furthermore, we show that the probe stray
field can assist skyrmion nucleation. Our proof-of-concepts results offer
current-free paradigms to efficient individual skyrmion control. | 1903.00367v1 |
2019-03-04 | Thermoelectric microscopy of magnetic skyrmions | The magnetic skyrmion is a nanoscale topological object characterized by the
winding of the magnetic moments, appearing in magnetic materials with broken
inversion symmetry. Because of its low current threshold for driving, the
skyrmions have been intensely studied toward novel storage applications by
using electron-beam, X-ray, and visible light microscopies. Here, we show that
the skyrmions can be imaged via thermoelectric signals of spin-caloritronic
phenomena in combination with focused heating. We applied a local temperature
gradient to a CoFeB/Ta/W multilayer film with scanning a heating position and,
by exploiting a Hall bar structure, mapped the magnitude as well as the
direction of the resultant thermoelectric current distributions, which link to
skyrmions' inner magnetic textures. This method also enables the observation of
skyrmion dynamics under driving current pulses, being a useful imaging
technique for the development of skyrmion devices. | 1903.01037v2 |
2019-03-08 | MFM and FORC+ study of switching mechanism in Co$_{25}$Pd$_{75}$ films | Recent research on CoPd alloys with perpendicular magnetic anisotropy (PMA)
has suggested that they might be useful as the pinning layer in CoFeB/MgO-based
perpendicular magnetic tunnel junctions (pMTJ's) for various spintronic
applications such as spin-torque transfer random access memory (STT-RAM). We
have previously studied the effect of seed layer and composition on the
structure (by XRD, SEM, AFM and TEM) and performance (coercivity) of these CoPd
films. These films do not switch coherently, so the coercivity is determined by
the details of the switching mechanism, which was not studied in our previous
paper. In the present paper, we show that information can be obtained about the
switching mechanism from magnetic force microscopy (MFM) together with first
order reversal curves (FORC), despite the fact that MFM can only be used at
zero field. We find that these films switch by a mechanism of domain nucleation
and dendritic growth into a labyrinthine structure, after which the unreversed
domains gradually shrink to small dots and then disappear. | 1903.03568v2 |
2020-03-26 | Bipolar spin Hall nano-oscillators | We demonstrate a novel type of spin Hall nano-oscillator (SHNO) that allows
for efficient tuning of magnetic auto-oscillations over an extended range of
gigahertz frequencies, using bipolar direct currents at constant magnetic
fields. This is achieved by stacking two distinct ferromagnetic layers with a
platinum interlayer. In this device, the orientation of the spin polarised
electrons accumulated at the top and bottom interfaces of the platinum layer is
switched upon changing the polarity of the direct current. As a result, the
effective anti-damping required to drive large amplitude auto-oscillations can
appear either at the top or bottom magnetic layer. Tuning of the
auto-oscillation frequencies by several gigahertz can be obtained by combining
two materials with sufficiently different saturation magnetization. Here we
show that the combination of NiFe and CoFeB can result in 3 GHz shifts in the
auto-oscillation frequencies. Bipolar SHNOs as such may bring enhanced
synchronisation capabilities to neuromorphic computing applications. | 2003.11776v1 |
2020-06-02 | Spin pumping and inverse spin Hall effect in iridium oxide | Large charge-to-spin conversion (spin Hall angle) and spin Hall conductivity
are prerequisites for development of next generation power efficient spintronic
devices. In this context, heavy metals (e.g. Pt, W etc.), topological
insulators, antiferromagnets are usually considered because they exhibit high
spin-orbit coupling (SOC). In addition to the above materials, 5d transition
metal oxide e.g. Iridium Oxide (IrO 2 ) is a potential candidate which exhibits
high SOC strength. Here we report a study of spin pumping and inverse spin Hall
effect (ISHE), via ferromagnetic resonance (FMR), in IrO 2 /CoFeB system. We
identify the individual contribution of spin pumping and other spin
rectification effects in the magnetic layer, by investigating the in-plane
angular dependence of ISHE signal. Our analysis shows significant contribution
of spin pumping effect to the ISHE signal. We show that polycrystalline IrO 2
thin film exhibits high spin Hall conductivity and spin Hall angle which are
comparable to the values of Pt. | 2006.01865v2 |
2020-06-03 | Giant voltage control of spin Hall nano-oscillator damping | Spin Hall nano-oscillators (SHNOs) are emerging spintronic devices for
microwave signal generation and oscillator based neuromorphic computing
combining nano-scale footprint, fast and ultra-wide microwave frequency
tunability, CMOS compatibility, and strong non-linear properties providing
robust large-scale mutual synchronization in chains and two-dimensional arrays.
While SHNOs can be tuned via magnetic fields and the drive current, neither
approach is conducive for individual SHNO control in large arrays. Here, we
demonstrate electrically gated W/CoFeB/MgO nano-constrictions in which the
voltage-dependent perpendicular magnetic anisotropy, tunes the frequency and,
thanks to nano-constriction geometry, drastically modifies the spin-wave
localization in the constriction region resulting in a giant 42 % variation of
the effective damping over four volts. As a consequence, the SHNO threshold
current can be strongly tuned. Our demonstration adds key functionality to
nano-constriction SHNOs and paves the way for energy-efficient control of
individual oscillators in SHNO chains and arrays for neuromorphic computing. | 2006.02151v1 |
2020-07-04 | Entropy-reduced retention times in magnetic memory elements: A case of the Meyer-Neldel Compensation Rule | We compute mean waiting times between thermally-activated magnetization
reversals in a nanodisk with parameters similar to a free CoFeB layer used in
magnetic random access memories. By combining Langer's theory and forward flux
sampling simulations, we show that the Arrhenius prefactor can take values up
to 10$^{21}$ Hz, orders of magnitude beyond the value of 10$^{9}$ Hz typically
assumed, and varies drastically as a function of material parameters. We show
that the prefactor behaves like an exponential of the activation energy, which
highlights a case of the Meyer-Neldel compensation rule. This suggests that
modeling information retention times with a barrier-independent prefactor in
such magnetic storage elements is not justified. | 2007.02152v1 |