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2017-03-31
|
Spin Seebeck effect in Y-type hexagonal ferrite thin films
|
Spin Seebeck effect (SSE) has been investigated in thin films of two
Y-hexagonal ferrites Ba$_2$Zn$_{2}$Fe$_{12}$O$_{22}$ (Zn2Y) and
Ba$_2$Co$_{2}$Fe$_{12}$O$_{22}$ (Co2Y) deposited by a spin-coating method on
SrTiO$_3$(111) substrate. The selected hexagonal ferrites are both
ferrimagnetic with similar magnetic moments at room temperature and both
exhibit easy magnetization plane normal to $c$-axis. Despite that, SSE signal
was only observed for Zn2Y, whereas no significant SSE signal was detected for
Co2Y. We tentatively explain this different behavior by a presence of two
different magnetic ions in Co2Y, whose random distribution over octahedral
sites interferes the long range ordering and enhances the Gilbert damping
constant. The temperature dependence of SSE for Zn2Y was measured and analyzed
with regard to the heat flux and temperature gradient relevant to the SSE
signal.
|
1703.10903v1
|
2017-06-02
|
Power Loss for a Periodically Driven Ferromagnetic Nanoparticle in a Viscous Fluid: the Finite Anisotropy Aspects
|
The joint magnetic and mechanical motion of a ferromagnetic nanoparticle in a
viscous fluid is considered within the dynamical approach. The equation based
on the total momentum conservation law is used for the description of the
mechanical rotation, while the modified Landau-Lifshitz-Gilbert equation is
utilized for the description of the internal magnetic dynamics. The exact
expressions for the particles trajectories and the power loss are obtained in
the linear approximation. The comparison with the results of other widespread
approaches, such as the model of fixed particle and the model of frozen
magnetic moment, is performed. It is established that in the small oscillations
mode the damping precession of the nanopartile magnetic moment is the main
channel of energy dissipation, but the motion of the nanoparticle easy axis can
significantly influence the value of the resulting power loss.
|
1706.00777v2
|
2017-06-07
|
Adiabatic and nonadiabatic spin torques induced by spin-triplet supercurrent
|
We study spin transfer torques induced by a spin-triplet supercurrent in a
magnet with the superconducting proximity effect. By a perturbative approach,
we show that spin-triplet correlations realize new types of torques, which are
analogous to the adiabatic and non-adiabatic ($\beta$) torques, without
extrinsic spin-flip scattering. Remarkable advantages compared to conventional
spin-transfer torques are highlighted in domain wall manipulation. Oscillatory
motions of a domain wall do not occur for a small Gilbert damping, and the
threshold current density to drive its motion becomes zero in the absence of
extrinsic pinning potentials due to the nonadiabatic torque controlled by the
triplet correlations.
|
1706.02296v2
|
2017-06-26
|
Perpendicular magnetic anisotropy in insulating ferrimagnetic gadolinium iron garnet thin films
|
We present experimental control of the magnetic anisotropy in a gadolinium
iron garnet (GdIG) thin film from in-plane to perpendicular anisotropy by
simply changing the sample temperature. The magnetic hysteresis loops obtained
by SQUID magnetometry measurements unambiguously reveal a change of the
magnetically easy axis from out-of-plane to in-plane depending on the sample
temperature. Additionally, we confirm these findings by the use of temperature
dependent broadband ferromagnetic resonance spectroscopy (FMR). In order to
determine the effective magnetization, we utilize the intrinsic advantage of
FMR spectroscopy which allows to determine the magnetic anisotropy independent
of the paramagnetic substrate, while magnetometry determines the combined
magnetic moment from film and substrate. This enables us to quantitatively
evaluate the anisotropy and the smooth transition from in-plane to
perpendicular magnetic anisotropy. Furthermore, we derive the temperature
dependent $g$-factor and the Gilbert damping of the GdIG thin film.
|
1706.08488v1
|
2017-09-07
|
Tunable spin pumping in exchange coupled magnetic trilayers
|
Magnetic thin films at ferromagnetic resonance (FMR) leak angular momentum,
which may be absorbed by adjacent layers. This phenomenon, known as spin
pumping, is manifested by an increase in the resonance linewidth ($\Delta H$),
and the closely related Gilbert damping. Another effect of this transfer of
spin currents is a dynamical and long-range coupling that can drive two
magnetic layers into a collective precession when their FMR frequencies
coincide. A collective behavior is also found in magnetic trilayers with
interlayer exchange coupling (IEC). In this study we investigate the interplay
between IEC and spin pumping, using Co/Cu/Py pseudo-spin values. We employ
broadband FMR spectroscopy to explore both the frequency and coupling-strength
dependence of $\Delta H$. Our observations show that there exists a cut-off
frequency, set by the IEC strength, below which the precession is truly
collective and the spin pumping is suppressed. These results demonstrate that
it is possible to control the spin pumping efficiency by varying the frequency
or the interlayer exchange coupling.
|
1709.02295v1
|
2017-10-30
|
Probe of Spin Dynamics in Superconducting NbN Thin Films via Spin Pumping
|
The emerging field of superconductor (SC) spintronics has attracted intensive
attentions recently. Many fantastic spin dependent properties in SC have been
discovered, including the observation of large magnetoresistance, long spin
lifetimes and the giant spin Hall effect in SC, as well as spin supercurrent in
Josephson junctions, etc. Regarding the spin dynamic in SC films, few studies
has been reported yet. Here, we report the investigation of the spin dynamics
in an s-wave superconducting NbN film via spin pumping from an adjacent
insulating ferromagnet GdN layer. A profound coherence peak of the Gilbert
damping is observed slightly below the superconducting critical temperature of
the NbN layer, which is consistent with recent theoretical studies. Our results
further indicate that spin pumping could be a powerful tool for investigating
the spin dynamics in 2D crystalline superconductors.
|
1710.10833v2
|
2017-11-17
|
Shot noise of charge and spin transport in a junction with a precessing molecular spin
|
Magnetic molecules and nanomagnets can be used to influence the electronic
transport in mesoscopic junction. In a magnetic field the precessional motion
leads to resonances in the dc- and ac-transport properties of a nanocontact, in
which the electrons are coupled to the precession. Quantities like the
dc-conductance or the ac-response provide valuable information like the level
structure and the coupling parameters. Here, we address the current noise
properties of such contacts. This encompasses the charge current and
spin-torque shot noise, which both show a step-like behavior as functions of
bias voltage and magnetic field. The charge current noise shows pronounced dips
around the steps, which we trace back to interference effects of electron in
quasienergy levels coupled by the molecular spin precession. We show that some
components of the noise of the spin-torque currents are directly related to the
Gilbert damping and, hence, are experimentally accessible. Our results show
that the noise characteristics allow to investigate in more detail the
coherence of spin transport in contacts containing magnetic molecules.
|
1711.06759v2
|
2018-02-05
|
Cooper-Pair Spin Current in a Strontium Ruthenate Heterostructure
|
It has been recognized that the condensation of spin-triplet Cooper pairs
requires not only the broken gauge symmetry but also the spin ordering as well.
One consequence of this is the possibility of the Cooper-pair spin current
analogous to the magnon spin current in magnetic insulators, the analogy also
extending to the existence of the Gilbert damping of the collective
spin-triplet dynamics. The recently fabricated heterostructure of the thin film
of the itinerant ferromagnet SrRuO3 on the bulk Sr2RuO4, the best-known
candidate material for the spin-triplet superconductor, offers a promising
platform for generating such spin current. We will show how such
heterostructure allows us to not only realize the long-range spin valve but
also electrically drive the collective spin mode of the spin-triplet order
parameter. Our proposal represents both a new realization of the spin
superfluidity and a transport signature of the spin-triplet superconductivity.
|
1802.01599v1
|
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
|
2018-02-12
|
Spin-orbit torque and spin pumping in YIG/Pt with interfacial insertion layers
|
We experimentally investigate spin-orbit torque and spin pumping in
Y$_3$Fe$_5$O$_{12}$(YIG)/Pt bilayers with ultrathin insertion layers at the
interface. An insertion layer of Cu suppresses both spin-orbit torque and spin
pumping, whereas an insertion layer of Ni$_{80}$Fe$_{20}$ (permalloy, Py)
enhances them, in a quantitatively consistent manner with the reciprocity of
the two spin transmission processes. However, we observe a large enhancement of
Gilbert damping with the insertion of Py that cannot be accounted for solely by
spin pumping, suggesting significant spin-memory loss due to the interfacial
magnetic layer. Our findings indicate that the magnetization at the YIG-metal
interface strongly influences the transmission and depolarization of pure spin
current.
|
1802.03865v3
|
2018-06-01
|
Dirac-Surface-State Modulated Spin Dynamics in a Ferrimagnetic Insulator at Room Temperature
|
This work demonstrates dramatically modified spin dynamics of magnetic
insulator (MI) by the spin-momentum locked Dirac surface states of the adjacent
topological insulator (TI) which can be harnessed for spintronic applications.
As the Bi-concentration x is systematically tuned in 5 nm thick (BixSb1-x)2Te3
TI film, the weight of the surface relative to bulk states peaks at x = 0.32
when the chemical potential approaches the Dirac point. At this concentration,
the Gilbert damping constant of the precessing magnetization in 10 nm thick
Y3Fe5O12 MI film in the MI/TI heterostructures is enhanced by an order of
magnitude, the largest among all concentrations. In addition, the MI acquires
additional strong magnetic anisotropy that favors the in-plane orientation with
similar Bi-concentration dependence. These extraordinary effects of the Dirac
surface states distinguish TI from other materials such as heavy metals in
modulating spin dynamics of the neighboring magnetic layer.
|
1806.00151v1
|
2018-08-23
|
Reduced thermal stability of antiferromagnetic nanostructures
|
Antiferromagnetic materials hold promising prospects in novel types of
spintronics applications. Assessing the stability of antiferromagnetic
nanostructures against thermal excitations is a crucial aspect of designing
devices with a high information density. Here we use theoretical calculations
and numerical simulations to determine the mean switching time of
antiferromagnetic nanoparticles in the superparamagnetic limit. It is
demonstrated that the thermal stability is drastically reduced compared to
ferromagnetic particles in the limit of low Gilbert damping, attributed to the
exchange enhancement of the attempt frequencies. It is discussed how the system
parameters have to be engineered in order to optimize the switching rates in
antiferromagnetic nanoparticles.
|
1808.07665v3
|
2018-09-17
|
On the speed of domain walls in thin nanotubes: the transition from the linear to the magnonic regime
|
Numerical simulations of domain wall propagation in thin nanotubes when an
external magnetic field is applied along the nanotube axis have shown an
unexpected behavior described as a transition from a linear to a magnonic
regime. As the applied magnetic field increases, the initial regime of linear
growth of the speed with the field is followed by a sudden change in slope
accompanied by the emission of spin waves. In this work an analytical formula
for the speed of the domain wall that explains this behavior is derived by
means of an asymptotic study of the Landau Lifshitz Gilbert equation for thin
nanotubes. We show that the dynamics can be reduced to a one dimensional
hyperbolic reaction diffusion equation, namely, the damped double Sine Gordon
equation, which shows the transition to the magnonic regime as the domain wall
speed approaches the speed of spin waves. This equation has been previously
found to describe domain wall propagation in weak ferromagnets with the
mobility proportional to the Dzyaloshinskii-Moriya interaction constant, for
Permalloy nanotubes the mobility is proportional to the nanotube radius.
|
1809.06278v3
|
2018-10-19
|
Magnon properties of random alloys
|
We study magnon properties in terms of spin stiffness, Curie temperatures and
magnon spectrum of Fe-Ni, Co-Ni and Fe-Co random alloys using a combination of
electronic structure calculations and atomistic spin dynamics simulations.
Influence of the disorder are studied in detail by use of large supercells with
random atomic arrangement. It is found that disorder affects the magnon
spectrum in vastly different ways depending on the system. Specifically, it is
more pronounced in Fe-Ni alloys compared to Fe-Co alloys. In particular, the
magnon spectrum at room temperature in Permalloy (Fe$_{20}$Ni$_{80}$) is found
to be rather diffuse in a large energy interval while in Fe$_{75}$Co$_{25}$ it
forms sharp branches. Fe-Co alloys are very interesting from a technological
point of view due to the combination of large Curie temperatures and very low
calculated Gilbert damping of $\sim$0.0007 at room temperature for Co
concentrations around 20--30\%.
|
1810.08487v1
|
2018-11-09
|
Switching of biaxial synthetic antiferromagnets: a micromagentic study
|
We simulate the switching behavior of nanoscale synthetic antiferromagnets
(SAFs), inspired by recent experimental progress in spin-orbit-torque switching
of crystal antiferromagnets. The SAF consists of two ferromagnetic thin films
with in-plane biaxial anisotropy and interlayer exchange coupling. Staggered
field-like Rashba spin-orbit torques from the opposite surfaces of the SAF
induce a canted net magnetization, which triggers an orthogonal torque that
drives 90$^\circ$ switching of the N\'eel vector. Such dynamics driven by the
field-like spin-orbit torque allows for faster switching with increased Gilbert
damping, without a significant detrimental increase of the threshold switching
current density. Our results point to the potential of SAFs as model systems,
based on simple ferromagnetic metals, to mimic antiferromagnetic device
physics.
|
1811.04094v2
|
2018-12-03
|
Microscopic theory of magnon-drag electron flow in ferromagnetic metals
|
A temperature gradient applied to a ferromagnetic metal induces not only
independent flows of electrons and magnons but also drag currents because of
their mutual interaction. In this paper, we present a microscopic study of the
electron flow induced by the drag due to magnons. The analysis is based on the
$s$-$d$ model, which describes conduction electrons and magnons coupled via the
$s$-$d$ exchange interaction. Magnetic impurities are introduced in the
electron subsystem as a source of spin relaxation. The obtained magnon-drag
electron current is proportional to the entropy of magnons and to $\alpha -
\beta$ (more precisely, to $1 - \beta/\alpha$), where $\alpha$ is the Gilbert
damping constant and $\beta$ is the dissipative spin-transfer torque parameter.
This result almost coincides with the previous phenomenological result based on
the magnonic spin-motive forces, and consists of spin-transfer and
momentum-transfer contributions, but with a slight disagreement in the former.
The result is interpreted in terms of the nonequilibrium spin chemical
potential generated by nonequilibrium magnons.
|
1812.00720v1
|
2019-01-17
|
Spin transport parameters of NbN thin films characterised by spin pumping experiments
|
We present measurements of ferromagnetic-resonance - driven spin pumping and
inverse spin-Hall effect in NbN/Y3Fe5O12 (YIG) bilayers. A clear enhancement of
the (effective) Gilbert damping constant of the thin-film YIG was observed due
to the presence of the NbN spin sink. By varying the NbN thickness and
employing spin-diffusion theory, we have estimated the room temperature values
of the spin diffusion length and the spin Hall angle in NbN to be 14 nm and
-1.1 10-2, respectively. Furthermore, we have determined the spin-mixing
conductance of the NbN/YIG interface to be 10 nm-2. The experimental
quantification of these spin transport parameters is an important step towards
the development of superconducting spintronic devices involving NbN thin films.
|
1901.05753v1
|
2019-02-12
|
Characterization of spin wave propagation in (111) YIG thin films with large anisotropy
|
We report on long-range spin wave (SW) propagation in nanometer-thick yttrium
iron garnet (YIG) film with an ultralow Gilbert damping. The knowledge of a
wavenumber value $|\vec{k}|$ is essential for designing SW devices. Although
determining the wavenumber $|\vec{k}|$ in experiments like Brillouin light
scattering spectroscopy is straightforward, quantifying the wavenumber in
all-electrical experiments has not been widely commented upon so far. We
analyze magnetostatic spin wave (SW) propagation in YIG films in order to
determine the SW wavenumber $|\vec{k}|$ excited by the coplanar waveguide. We
show that it is crucial to consider the influence of magnetic anisotropy fields
present in YIG thin films for precise determination of SW wavenumber. With the
proposed methods we find that experimentally derived values of $|\vec{k}|$ are
in perfect agreement with that obtained from electromagnetic simulation only if
anisotropy fields are included.
|
1902.04608v1
|
2019-03-20
|
Nonlinear magnetization dynamics driven by strong terahertz fields
|
We present a comprehensive experimental and numerical study of magnetization
dynamics triggered in a thin metallic film by single-cycle terahertz pulses of
$\sim20$ MV/m electric field amplitude and $\sim1$ ps duration. The
experimental dynamics is probed using the femtosecond magneto-optical Kerr
effect (MOKE), and it is reproduced numerically using macrospin simulations.
The magnetization dynamics can be decomposed in three distinct processes: a
coherent precession of the magnetization around the terahertz magnetic field,
an ultrafast demagnetization that suddenly changes the anisotropy of the film,
and a uniform precession around the equilibrium effective field that is relaxed
on the nanosecond time scale, consistent with a Gilbert damping process.
Macrospin simulations quantitatively reproduce the observed dynamics, and allow
us to predict that novel nonlinear magnetization dynamics regimes can be
attained with existing table-top terahertz sources.
|
1903.08395v2
|
2019-04-11
|
Measurement of spin mixing conductance in Ni$_{81}$Fe$_{19}$/$α$-W and Ni$_{81}$Fe$_{19}$/$β$-W heterostrucutures via ferromagnetic resonance
|
We present measurements of interfacial Gilbert damping due to the spin
pumping effect in Ni$_{81}$Fe$_{19}$/W heterostructures. Measurements were
compared for heterostructures in which the crystallographic phase of W, either
$\alpha$(bcc)-W or $\beta$(A15)-W, was enriched through deposition conditions
and characterized using X-ray diffraction (XRD) and high-resolution
cross-sectional transmission electron microscopy (HR-XTEM). Single phase
Ni$_{81}$Fe$_{19}$/$\alpha$-W heterostructures could be realized, but
heterostructures with $\beta$-W were realized as mixed $\alpha$-$\beta$ phase.
The spin mixing conductances (SMC) for W at interfaces with Ni$_{81}$Fe$_{19}$
were found to be significantly lower than those for similarly heavy metals such
as Pd and Pt, but comparable to those for Ta, and independent of enrichment in
the $\beta$ phase.
|
1904.05950v2
|
2019-05-26
|
Influence of field-like torque in synchronization of spin torque oscillators
|
The magnetization dynamics of two parallelly coupled spin torque oscillators,
destabilization of steady states and removal of multistability, are
investigated by taking into account the influence of field-like torque. It is
shown that the existence of such torque can cancel the effect of damping and
can, therefore, cause the oscillators to exhibit synchronized oscillations in
response to direct current. Further, our results show that the presence of
field-like torque enhances the power and Q-factor of the synchronized
oscillations. The validity of the above results is confirmed by numerical and
analytical studies based on the stochastic Landau-Lifshitz-Gilbert-Slonczewski
equation.
|
1905.10804v2
|
2019-05-30
|
Sub-nanosecond switching in a cryogenic spin-torque spin-valve memory element with a dilute permalloy free layer
|
We present a study of the pulsed current switching characteristics of
spin-valve nanopillars with in-plane magnetized dilute permalloy and undiluted
permalloy free layers in the ballistic regime at low temperature. The dilute
permalloy free layer device switches much faster: the characteristic switching
time for a permalloy free (Ni0.83Fe0.17) layer device is 1.18 ns, while that
for a dilute permalloy ([Ni0.83Fe0.17]0.6Cu0.4) free layer device is 0.475 ns.
A ballistic macrospin model can capture the data trends with a reduced spin
torque asymmetry parameter, reduced spin polarization and increased Gilbert
damping for the dilute permalloy free layer relative to the permalloy devices.
Our study demonstrates that reducing the magnetization of the free layer
increases the switching speed while greatly reducing the switching energy and
shows a promising route toward even lower power magnetic memory devices
compatible with superconducting electronics.
|
1905.13262v1
|
2019-06-17
|
Controlling acoustic waves using magnetoelastic Fano resonances
|
We propose and analyze theoretically a class of energy-efficient
magneto-elastic devices for analogue signal processing. The signals are carried
by transverse acoustic waves while the bias magnetic field controls their
scattering from a magneto-elastic slab. By tuning the bias field, one can alter
the resonant frequency at which the propagating acoustic waves hybridize with
the magnetic modes, and thereby control transmission and reflection
coefficients of the acoustic waves. The scattering coefficients exhibit
Breit-Wigner/Fano resonant behaviour akin to inelastic scattering in atomic and
nuclear physics. Employing oblique incidence geometry, one can effectively
enhance the strength of magnetoelastic coupling, and thus countermand the
magnetic losses due to the Gilbert damping. We apply our theory to discuss
potential benefits and issues in realistic systems and suggest further routes
to enhance performance of the proposed devices.
|
1906.07297v2
|
2019-07-05
|
Theory for shift current of bosons: Photogalvanic spin current in ferrimagnetic and antiferromagnetic insulators
|
We theoretically study the optical generation of dc spin current (i.e., a
spin-current solar cell) in ordered antiferromagnetic and ferrimagnetic
insulators, motivated by a recent study on the laser-driven spinon spin current
in noncentrosymmetric quantum spin chains [H. Ishizuka and M. Sato, Phys. Rev.
Lett. 122, 197702 (2019)]. Using a non-linear response theory for magnons, we
analyze the dc spin current generated by a linearly-polarized electromagnetic
wave (typically, terahertz or gigahertz waves). Considering noncentrosymmetric
two-sublattice magnets as an example, we find a finite dc spin current
conductivity at $T=0$, where no thermally-excited magnons exist; this is in
contrast to the case of the spinon spin current, in which the optical
transition of the Fermi degenerate spinons plays an essential role. We find
that the dc spin-current conductivity is insensitive to the Gilbert damping,
i.e., it may be viewed as a shift current carried by bosonic particles
(magnons). Our estimate shows that an electric-field intensity of
$E\sim10^4-10^6$ V/cm is sufficient for an observable spin current. Our theory
indicates that the linearly-polarized electromagnetic wave generally produces a
dc spin current in noncentrosymmetric magnetic insulators.
|
1907.02734v1
|
2019-07-10
|
Temperature dependence of magnetic resonance in ferrimagnetic GdFeCo alloys
|
We provide a macroscopic theory and experimental results for magnetic
resonances of antiferromagnetically-coupled ferrimagnets. Our theory, which
interpolates the dynamics of antiferromagnets and ferromagnets smoothly, can
describe ferrimagnetic resonances across the angular momentum compensation
point. We also present experimental results for spin-torque induced
ferrimagnetic resonance at several temperatures. The spectral analysis based on
our theory reveals that the Gilbert damping parameter, which has been
considered to be strongly temperature dependent, is insensitive to temperature.
We envision that our work will facilitate further investigation of
ferrimagnetic dynamics by providing a theoretical framework suitable for a
broad range of temperatures.
|
1907.04540v1
|
2019-07-11
|
Improving the Signal-to-noise Ratio for Heat-Assisted Magnetic Recording by Optimizing a High/Low Tc bilayer structure
|
We optimize the recording medium for heat-assisted magnetic recording by
using a high/low $T_{\mathrm{c}}$ bilayer structure to reduce AC and DC noise.
Compared to a former work, small Gilbert damping $\alpha=0.02$ is considered
for the FePt like hard magnetic material. Atomistic simulations are performed
for a cylindrical recording grain with diameter $d=5\,$nm and height $h=8\,$nm.
Different soft magnetic material compositions are tested and the amount of hard
and soft magnetic material is optimized. The results show that for a soft
magnetic material with $\alpha_{\mathrm{SM}}=0.1$ and
$J_{ij,\mathrm{SM}}=7.72\times 10^{-21}\,$J/link a composition with $50\%$ hard
and $50\%$ soft magnetic material leads to the best results. Additionally, we
analyse how much the areal density can be improved by using the optimized
bilayer structure compared to the pure hard magnetic recording material. It
turns out that the optimized bilayer design allows an areal density that is
$1\,$Tb/in$^2$ higher than that of the pure hard magnetic material while
obtaining the same SNR.
|
1907.05027v1
|
2019-07-19
|
A cryogenic memory element based on an anomalous Josephson junction
|
We propose a non-volatile memory element based on a lateral ferromagnetic
Josephson junction with spin-orbit coupling and out-of-plane magnetization. The
interplay between the latter and the intrinsic exchange field of the
ferromagnet leads to a magnetoelectric effect that couples the charge current
through the junction and its magnetization, such that by applying a current
pulse the direction of the magnetic moment in F can be switched. The two memory
states are encoded in the direction of the out-of-plane magnetization. With the
aim to determine the optimal working temperature for the memory element, we
explore the noise-induced effects on the averaged stationary magnetization by
taking into account thermal fluctuations affecting both the Josephson phase and
the magnetic moment dynamics. We investigate the switching process as a
function of intrinsic parameters of the ferromagnet, such as the Gilbert
damping and strength of the spin-orbit coupling, and proposed a non-destructive
readout scheme based on a dc-SQUID. Additionally, we analyze a way to protect
the memory state from external perturbations by voltage gating in systems with
a both linear-in-momentum Rashba and Dresselhaus spin-orbit coupling.
|
1907.08454v2
|
2019-07-23
|
Electron transport in high-entropy alloys: Al$_{x}$CrFeCoNi as a case study
|
The high-entropy alloys Al$_{x}$CrFeCoNi exist over a broad range of Al
concentrations ($0 < x < 2$). With increasing Al content their structure is
changed from the fcc to bcc phase. We investigate the effect of such structural
changes on transport properties including the residual resistivity and the
anomalous Hall resistivity. We have performed a detailed comparison of the
first-principles simulations with available experimental data. We show that the
calculated residual resistivities for all studied alloy compositions are in a
fair agreement with available experimental data as concerns both the
resistivity values and concentration trends. We emphasize that a good agreement
with experiment was obtained also for the anomalous Hall resistivity. We have
completed study by estimation of the anisotropic magnetoresistance,
spin-disorder resistivity, and Gilbert damping. The obtained results prove that
the main scattering mechanism is due to the intrinsic chemical disorder whereas
the effect of spin polarization on the residual resistivity is appreciably
weaker.
|
1907.09731v1
|
2019-09-11
|
Chaos in nanomagnet via feedback current
|
Nonlinear magnetization dynamics excited by spin-transfer effect with
feedback current is studied both numerically and analytically. The numerical
simulation of the Landau-Lifshitz-Gilbert equation indicates the positive
Lyapunov exponent for a certain range of the feedback rate, which identifies
the existence of chaos in a nanostructured ferromagnet. Transient behavior from
chaotic to steady oscillation is also observed in another range of the feedback
parameter. An analytical theory is also developed, which indicates the
appearance of multiple attractors in a phase space due to the feedback current.
An instantaneous imbalance between the spin-transfer torque and damping torque
causes a transition between the attractors, and results in the complex
dynamics.
|
1909.05315v2
|
2019-11-27
|
Ellipticity and Dissipation Effects in Magnon Spin Valves
|
We consider alignment-dependent spin and heat transport across a magnon spin
valve in the tunneling regime, i.e., a junction consisting of two weakly
coupled ferromagnetic insulators. We determine the difference in spin and heat
conductance between the parallel and antiparallel configuration of the
magnetization direction. The dependence of these conductances on both the
Gilbert damping and ellipticity is studied. We find that both magnon
ellipticity and dissipation open channels for magnons to tunnel through in the
antiparallel configuration. Our results highlight an important difference
between electronic and magnon spin transport in spin-valve structures and may
be important for the development of devices based on magnetic insulators.
|
1911.12017v2
|
2020-03-24
|
Spin-transfer torque driven intrinsic localized spin excitations in the presence of field-like torque
|
We study the existence of intrinsic localized one-spin excitation in the
Heisenberg one-dimensional ferromagnetic spin chain in the presence of
perpendicular and parallel external magnetic fields and current with
spin-transfer torque and field-like torque. The
Landau-Lifshitz-Gilbert-Slonczewski(LLGS) equation is exactly solved for the
one spin excitation in the absence of onsite anisotropy for the excitations of
spin with fields perpendicular and parallel to the chain. We show the removal
of damping in the spin excitations by appropriately introducing current and
also the enhancement of angular frequency of the oscillations due to field-like
torque in the case of both perpendicular and parallel field. The exactness of
the analytical results is verified by matching with numerical counterparts.
Further, we numerically confirm the existence of in-phase and anti-phase stable
synchronized oscillations for two spin-excitations in the presence of current
with perpendicular field and field-like torque.
|
2003.11023v2
|
2020-04-02
|
Stable solitons in a nearly PT-symmetric ferromagnet with spin-transfer torque
|
We consider the Landau-Lifshitz equation for the spin torque oscillator - a
uniaxial ferromagnet in an external magnetic field with polarised spin current
driven through it. In the absence of the Gilbert damping, the equation turns
out to be PT-symmetric. We interpret the PT-symmetry as a balance between gain
and loss - and identify the gaining and losing modes. In the vicinity of the
bifurcation point of a uniform static state of magnetisation, the PT-symmetric
Landau-Lifshitz equation with a small dissipative perturbation reduces to a
nonlinear Schr\"odinger equation with a quadratic nonlinearity. The analysis of
the Schr\"odinger dynamics demonstrates that the spin torque oscillator
supports stable magnetic solitons. The PT near-symmetry is crucial for the
soliton stability: the addition of a finite dissipative term to the
Landau-Lifshitz equation destabilises all solitons that we have found.
|
2004.01245v2
|
2020-05-11
|
Manipulating 1-dimensinal skyrmion motion by external magnetic field gradient
|
We have investigated an analytic formula of the 1-dimensional magnetic
skyrmion dynamics under external magnetic field gradient. We find excellent
agreement between the analytical model and micromagnetic simulation results for
various magnetic parameters such as the magnetic field gradient, Gilbert
damping constant. We also observe much faster velocity of the chiral domain
wall (DW) motion. The chiral DW is exist with smaller interfacial
Dzyaloshinskii-Moriya interaction energy density cases. These results provide
to develop efficient control of skyrmion for spintronic devices.
|
2005.05011v1
|
2020-07-08
|
Finite-frequency spin susceptibility and spin pumping in superconductors with spin-orbit relaxation
|
Static spin susceptibility of superconductors with spin-orbit relaxation has
been calculated in the seminal work of A.A. Abrikosov and L.P. Gor'kov [Sov.
Phys. JETP, {\bf 15}, 752 (1962)]. Surprisingly the generalization of this
result to finite frequencies has not been done despite being quite important
for the modern topic of superconducting spintronics. The present paper fills
this gap by deriving the analytical expression for spin susceptibility. The
time-dependent spin response is shown to be captured by the quasiclassical
Eilenberger equation with collision integrals corresponding to the ordinary and
spin-orbit scattering. Using the developed formalism we study the linear spin
pumping effect between the ferromagnet and the adjacent superconducting film.
The consequences for understanding recent experiments demonstrating the
modification of Gilbert damping by the superconducting correlations are
discussed.
|
2007.04372v2
|
2020-07-16
|
Thermal noise effects on the magnetization switching of a ferromagnetic anomalous Josephson junction
|
We discuss the effects of thermal noise on the magnetic response of a lateral
ferromagnetic Josephson junction with spin-orbit coupling and out-of-plane
magnetization. The direction of the magnetic moment in the ferromagnetic layer
can be inverted by using controlled current pulses. This phenomenon is due to
the magnetoelectric effect that couples the flowing charge current and the
magnetization of the ferromagnet. We investigate the magnetization reversal
effect versus intrinsic parameters of the ferromagnet, such as the Gilbert
damping and strength of the spin-orbit coupling. We estimate the magnetization
reversing time and find the optimal values of the parameters for fast
switching. With the aim of increasing the operation temperature we study the
effects induced by thermal fluctuations on the averaged stationary
magnetization, and find the conditions that make the system more robust against
noise.
|
2007.08414v3
|
2020-08-21
|
Integration and characterization of micron-sized YIG structures with very low Gilbert damping on arbitrary substrates
|
We present a novel process that allows the transfer of monocrystalline
yttrium-iron-garnet microstructures onto virtually any kind of substrate. The
process is based on a recently developed method that allows the fabrication of
freestanding monocrystalline YIG bridges on gadolinium-gallium-garnet. Here the
bridges' spans are detached from the substrate by a dry etching process and
immersed in a watery solution. Using drop casting the immersed YIG platelets
can be transferred onto the substrate of choice, where the structures finally
can be reattached and thus be integrated into complex devices or experimental
geometries. Using time resolved scanning Kerr microscopy and inductively
measured ferromagnetic resonance we can demonstrate that the structures retain
their excellent magnetic quality. At room temperature we find a ferromagnetic
resonance linewidth of $\mu_0\Delta H_{HWHM}\approx 195\,\mu T$ and we were
even able to inductively measure magnon spectra on a single micron-sized
yttrium-iron-garnet platelet at a temperature of 5 K. The process is flexible
in terms of substrate material and shape of the structure. In the future this
approach will allow for new types of spin dynamics experiments up to now
unthinkable.
|
2008.09390v1
|
2020-09-01
|
Quantum Brownian Motion for Magnets
|
Spin precession in magnetic materials is commonly modelled with the classical
phenomenological Landau-Lifshitz-Gilbert (LLG) equation. Based on a quantized
spin+environment Hamiltonian, we here derive a general spin operator equation
of motion that describes three-dimensional precession and damping and
consistently accounts for effects arising from memory, coloured noise and
quantum statistics. The LLG equation is recovered as its classical, Ohmic
approximation. We further introduce resonant Lorentzian system--reservoir
couplings that allow a systematic comparison of dynamics between Ohmic and
non--Ohmic regimes. Finally, we simulate the full non-Markovian dynamics of a
spin in the semi--classical limit. At low temperatures, our numerical results
demonstrate a characteristic reduction and flattening of the steady state spin
alignment with an external field, caused by the quantum statistics of the
environment. The results provide a powerful framework to explore general
three-dimensional dissipation in quantum thermodynamics.
|
2009.00600v2
|
2020-09-30
|
Quantum hydrodynamics of spin winding
|
An easy-plane spin winding in a quantum spin chain can be treated as a
transport quantity, which propagates along the chain but has a finite lifetime
due to phase slips. In a hydrodynamic formulation for the winding dynamics, the
quantum continuity equation acquires a source term due to the transverse
vorticity flow. The latter reflects the phase slips and generally compromises
the global conservation law. A linear-response formalism for the nonlocal
winding transport then reduces to a Kubo response for the winding flow along
the spin chain, in conjunction with the parasitic vorticity flow transverse to
it. One-dimensional topological hydrodynamics can be recovered when the
vorticity flow is asymptotically small. Starting with a microscopic spin-chain
formulation, we focus on the asymptotic behavior of the winding transport based
on the renormalized sine-Gordon equation, incorporating phase slips as well as
Gilbert damping. A generic electrical device is proposed to manifest this
physics. We thus suggest winding conductivity as a tangible concept that can
characterize low-energy dynamics in a broad class of quantum magnets.
|
2010.00144v1
|
2020-11-29
|
Cross-sublattice Spin Pumping and Magnon Level Attraction in van der Waals Antiferromagnets
|
We theoretically study spin pumping from a layered van der Waals
antiferromagnet in its canted ground state into an adjacent normal metal. We
find that the resulting dc spin pumping current bears contributions along all
spin directions. Our analysis allows for detecting intra- and cross-sublattice
spin-mixing conductances via measuring the two in-plane spin current
components. We further show that sublattice symmetry-breaking Gilbert damping
can be realized via interface engineering and induces a dissipative coupling
between the optical and acoustic magnon modes. This realizes magnon level
attraction and exceptional points in the system. Furthermore, the dissipative
coupling and cross-sublattice spin pumping contrive to produce an
unconventional spin current in the out-of-plane direction. Our findings provide
a route to extract the spin mixing conductance matrix and uncovers the unique
opportunities, such as level attraction, offered by van der Waals
antiferromagnet-normal metal hybrids.
|
2011.14314v1
|
2021-01-18
|
Topological electric driving of magnetization dynamics in insulators
|
Established forms of electromagnetic coupling are usually conservative (in
insulators) or dissipative (in metals and semiconductors). Here we point out
the possibility of nondissipative electric driving of magnetization dynamics,
if the valence electronic states have nontrivial topology in the combined space
of crystal momentum and magnetization configuration. We provide a hybrid
insulator system to demonstrate that the topology-based nonconservative
electrical generalized force is capable of supporting sustained magnetization
motion in the presence of Gilbert damping, with quantized and steady energy
pumping into magnetization motion from the electric field. We also generalize
our results to magnetic textures, and discuss electric field induced
Dzyaloshinskii-Moriya interaction which can be nonconservative.
|
2101.07164v3
|
2021-02-07
|
Spinterface Induced Modification in Magnetic Properties in Co40Fe40B20/Fullerene Bilayers
|
Organic semiconductor/ferromagnetic bilayer thin films can exhibit novel
properties due to the formation of the spinterface at the interface.
Buckminsterfullerene (C60) has been shown to exhibit ferromagnetism at the
interface when it is placed next to a ferromagnet (FM) such as Fe or Co.
Formation of spinterface occurs due to the orbital hybridization and spin
polarized charge transfer at the interface. In this work, we have demonstrated
that one can enhance the magnetic anisotropy of the low Gilbert damping alloy
CoFeB by introducing a C60 layer. We have shown that anisotropy increases by
increasing the thickness of C60 which might be a result of the formation of
spinterface. However, the magnetic domain structure remains same in the bilayer
samples as compared to the reference CoFeB film.
|
2102.03914v4
|
2021-02-15
|
Magnetodynamic properties of dipole-coupled 1D magnonic crystals
|
Magnonic crystals are magnetic metamaterials, that provide a promising way to
manipulate magnetodynamic properties by controlling the geometry of the
patterned structures. Here, we study the magnetodynamic properties of 1D
magnonic crystals consisting of parallel NiFe strips with different strip
widths and separations. The strips couple via dipole-dipole interactions. As an
alternative to experiments and/or micromagnetic simulations, we investigate the
accuracy of a simple macrospin model. For the case of simple strips, a model
with a single free parameter to account for an overestimation of the
out-of-plane demagnetization of the magnonic lattice is described. By adjusting
this parameter a good fit with experimental as well as micromagnetic results is
obtained. Moreover, the Gilbert damping is found independent of the lattice
constant however the inhomogeneous linewidth broadening found to increase with
decreasing stripe separation.
|
2102.07712v2
|
2021-05-24
|
Spin pumping of two-dimensional electron gas with Rashba and Dresselhaus spin-orbit interactions
|
We theoretically consider spin pumping in a junction between a ferromagnetic
insulator (FI) and a two-dimensional electron gas (2DEG) in which the Rashba
and Dresselhaus spin-orbit interactions coexist. Using second-order
perturbation theory, we derive an increase in linewidth in the case of an
interfacial exchange coupling in a ferromagnetic resonance (FMR) experiment. We
clarify how the enhancement of Gilbert damping depends on the resonant
frequency and spin orientation of the FI. We show that this setup of an FMR
experiment can provide information on the spin texture of 2DEG at the Fermi
surface.
|
2105.11193v3
|
2021-08-05
|
Spin-transfer torque driven localized spin excitations in the presence of field-like torque
|
We study the existence of localized one-spin excitation in the Heisenberg
one-dimensional ferromagnetic spin chain in the presence of perpendicular and
parallel external magnetic fields and current with spin-transfer torque and
field-like torque. The Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation is
exactly solved for the one spin excitation in the absence of onsite anisotropy
for the excitations of spin with fields perpendicular and parallel to the
chain. We show the removal of damping in the spin excitations by appropriately
introducing current and also the enhancement of angular frequency of the
oscillations due to field-like torque in the case of both perpendicular and
parallel field. The exactness of the analytical results is verified by matching
with numerical counterparts. Further, we numerically confirm the existence of
in-phase and anti-phase stable synchronized oscillations for two
spin-excitations in the presence of current with perpendicular field and
field-like torque. We also show that the one-spin excitation is stable against
thermal noise and gets only slightly modified against thermal fluctuations.
|
2108.02380v1
|
2021-09-07
|
Inertial spin dynamics in epitaxial cobalt films
|
We investigate the spin dynamics driven by terahertz magnetic fields in
epitaxial thin films of cobalt in its three crystalline phases. The terahertz
magnetic field generates a torque on the magnetization which causes it to
precess for about 1 ps, with a sub-picosecond temporal lag from the driving
force. Then, the magnetization undergoes natural damped THz oscillations at a
frequency characteristic of the crystalline phase. We describe the experimental
observations solving the inertial Landau-Lifshitz-Gilbert equation. Using the
results from the relativistic theory of magnetic inertia, we find that the
angular momentum relaxation time $\eta$ is the only material parameter needed
to describe all the experimental evidence. Our experiments suggest a
proportionality between $\eta$ and the strength of the magneto-crystalline
anisotropy.
|
2109.03076v2
|
2021-09-26
|
Transition state dynamics of a driven magnetic free layer
|
Magnetization switching in ferromagnetic structures is an important process
for technical applications such as data storage in spintronics, and therefore
the determination of the corresponding switching rates becomes essential. We
investigate a free-layer system in an oscillating external magnetic field
resulting in an additional torque on the spin. The magnetization dynamics
including inertial damping can be described by the phenomenological Gilbert
equation. The magnetization switching between the two stable orientations on
the sphere then requires the crossing of a potential region characterized by a
moving rank-1 saddle. We adopt and apply recent extensions of transition state
theory for driven systems to compute both the time-dependent and average
switching rates of the activated spin system in the saddle region.
|
2109.12605v1
|
2021-12-24
|
Skyrmion nucleation on the surface of a topological insulator
|
Skyrmion nucleation induced by spin-transfer torques at an interface of a
topological insulator and a ferromagnetic insulator is investigated. Due to
strong spin-orbit coupling on a surface of topological insulators, which
enhances the effect of spin torques, efficient manipulation of skyrmions is
expected, and therefore, topological insulators could provide the ideal
platform to achieve high-performance skyrmionic devices. Using micromagnetic
simulations and energetics, we evaluate properties of the skyrmion nucleation
on a surface of topological insulators, such as nucleation time, critical
electric field, and skyrmion numbers. We show that the nucleation time is
inversely proportional to the applied electric field. We also identify the
Gilbert damping and temperature dependencies of the critical field.
Furthermore, we analytically evaluate the effect of the Dzyaloshinskii-Moriya
interaction and demonstrate that the temperature dependence can be explained by
the reduction of a magnon excitation gap due to the self-energy corrections.
|
2112.12967v2
|
2021-12-10
|
Enhanced Planar Antenna Efficiency Through Magnetic Thin-Films
|
This work proposes to use magnetic material as the substrate of planar
antennas to overcome the platform effect caused by the conducting ground plane.
The upper bound of the radiation efficiency of an electric-current-driven
low-profile antenna is theoretically derived, which is inversely proportional
to the Gilbert damping factor of the magnetic material. Meanwhile, the
improvement of radiation due to the use of magnetic material is demonstrated by
a three-dimensional (3D) multiphysics and multiscale time-domain model. The
simulation results match the theoretical derivation, showing 25% radiation
efficiency from a planar antenna backed by a FeGaB thin film with 2.56 um
thickness. Furthermore, for conductive ferromagnetic materials, it is shown
that the eddy current loss can be well suppressed by laminating the thin film
into multiple layers. The radiation efficiency of the modeled antenna with a
conductive ferromagnetic substrate is improved from 2.2% to 11.8% by dividing
the substrate into 10 layers, with a ferromagnetic material fill factor of 93%.
|
2201.04932v1
|
2022-03-07
|
Ultrafast optical observation of spin-pumping induced dynamic exchange coupling in ferromagnetic semiconductor/metal bilayer
|
Spin angular momentum transfer in magnetic bilayers offers the possibility of
ultrafast and low-loss operation for next-generation spintronic devices. We
report the field- and temperature- dependent measurements on the magnetization
precessions in Co$_2$FeAl/(Ga,Mn)As by time-resolved magneto-optical Kerr
effect (TRMOKE). Analysis of the effective Gilbert damping and phase shift
indicates a clear signature of an enhanced dynamic exchange coupling between
the two ferromagnetic (FM) layers due to the reinforced spin pumping at
resonance. The temperature dependence of the dynamic exchange-coupling reveals
a primary contribution from the ferromagnetism in (Ga,Mn)As.
|
2203.03225v2
|
2022-04-21
|
Transport theory for topological Josephson junctions with a Majorana qubit
|
We construct a semiclassical theory for the transport of topological
junctions starting from a microscopic Hamiltonian that comprehensively includes
the interplay among the Majorana qubit, the Josephson phase, and the
dissipation process. With the path integral approach, we derive a set of
semiclassical equations of motion that can be used to calculate the time
evolution of the Josephson phase and the Majorana qubit. In the equations we
reveal rich dynamical phenomena such as the qubit induced charge pumping, the
effective spin-orbit torque, and the Gilbert damping. We demonstrate the
influence of these dynamical phenomena on the transport signatures of the
junction. We apply the theory to study the Shapiro steps of the junction, and
find the suppression of the first Shapiro step due to the dynamical feedback of
the Majorana qubit.
|
2204.09923v1
|
2022-04-22
|
A short-circuited coplanar waveguide for low-temperature single-port ferromagnetic resonance spectroscopy set-up to probe the magnetic properties of ferromagnetic thin films
|
A coplanar waveguide shorted in one end is proposed, designed, and
implemented successfully to measure the properties of magnetic thin films as a
part of the vector network analyzer ferromagnetic resonance (VNA-FMR)
spectroscopy set-up. Its simple structure, potential applications and easy
installation inside the cryostat chamber made it advantageous especially for
low-temperature measurements. It provides a wide band of frequencies in the
gigahertz range essential for FMR measurements. Our spectroscopy set-up with
short-circuited coplanar waveguide has been used to extract Gilbert damping
coefficient and effective magnetization values for standard ferromagnetic thin
films like Py and Co. The thickness and temperature dependent studies of those
magnetic parameters have also been done here for the afore mentioned magnetic
samples.
|
2204.10596v2
|
2022-11-04
|
Derivation of Interacting Two-Qubit Dynamics from Spin-Boson Model
|
We derive damping equations of motion for interacting two-spin states from a
spin-boson model in order to examine qubit dynamics in quantum computers. On
the basis of the composite operator method, we develop the Caldeira-Leggett
approach for open quantum systems so that the entanglement dynamics originated
from the two-spin correlation can be taken. We demonstrate numerical results
for time dependence on the two-spin dynamics. We find that the relaxation of
the total spin is described by a quantum version of the Landau-Lifshitz-Gilbert
equation for magnetic materials. We also find that a two-spin composite mode
keeps oscillation even after the total spin has been fully relaxed. We thus
conclude that the two-spin correlation due to the presence of the composite
mode is stable against dissipation. We consider the mechanism of why the
correlation is maintained.
|
2211.02490v1
|
2023-02-06
|
Global solutions of the Landau--Lifshitz--Baryakhtar equation
|
The Landau--Lifshitz--Baryakhtar (LLBar) equation is a generalisation of the
Landau--Lifshitz--Gilbert and the Landau--Lifshitz--Bloch equations which takes
into account contributions from nonlocal damping and is valid at moderate
temperature below the Curie temperature. Therefore, it is used to explain some
discrepancies between the experimental observations and the known theories in
various problems on magnonics and magnetic domain-wall dynamics. In this paper,
the existence and uniqueness of global weak, strong, and regular solutions to
LLBar equation are proven. H\"older continuity of the solution is also
discussed.
|
2302.02556v3
|
2023-03-22
|
Twisted bilayer graphene reveals its flat bands under spin pumping
|
The salient property of the electronic band structure of twisted bilayer
graphene (TBG), at the so-called magic angle (MA), is the emergence of flat
bands around the charge neutrality point. These bands are associated with the
observed superconducting phases and the correlated insulating states. Scanning
tunneling microscopy combined with angle resolved photoemission spectroscopy
are usually used to visualize the flatness of the band structure of TBG at the
MA. Here, we theoretically argue that spin pumping (SP) provides a direct probe
of the flat bands of TBG and an accurate determination of the MA. We consider a
junction separating a ferromagnetic insulator and a heterostructure of TBG
adjacent to a monolayer of a transition metal dichalcogenide. We show that the
Gilbert damping of the ferromagnetic resonance experiment, through this
junction, depends on the twist angle of TBG, and exhibits a sharp drop at the
MA. We discuss the experimental realization of our results which open the way
to a twist switchable spintronics in twisted van der Waals heterostructures.
|
2303.12380v2
|
2023-05-01
|
Coherent and incoherent magnons induced by strong ultrafast demagnetization in thin permalloy films
|
Understanding spin dynamics on femto- and picosecond timescales offers new
opportunities for faster and more efficient spintronic devices. Here, we
experimentally investigate the coherent spin dynamics after ultrashort laser
excitation by time-resolved magneto optical Kerr effect (TR-MOKE) in thin
Ni80Fe20 films. We provide a detailed study of the magnetic field and pump
fluence dependence of the coherent precessional dynamics. We show that the
coherent precession lifetime increases with the applied external magnetic field
which cannot be understood by viscous Gilbert damping of the coherent magnons.
Instead, it can be explained by nonlinear magnon interactions and by the change
in the fraction of incoherent magnons. This interpretation is in agreement with
the observed trends of the coherent magnon amplitude and lifetime as a function
of the exciting laser fluence. Our results provide a new insight into the
magnetization relaxation processes in ferromagnetic thin films, which is of
great importance for further spintronic applications.
|
2305.00814v2
|
2023-07-15
|
Switching current distributions in ferromagnetic anomalous Josephson junctions
|
We investigate the switching current distributions of ferromagnetic anomalous
Josephson junctions subjected to a linearly increasing bias current. Our study
uncovers a significant correlation between the position of the switching
current distributions and crucial system parameters, such as the strength of
the spin-orbit coupling and the Gilbert damping parameter. This indicates that
these parameters can be directly determined through experimental measurements.
By conducting a comprehensive analysis of the interplay among noise,
magnetization, phase dynamics, and the statistical properties of the switching
current distribution, we deepen our understanding of these intriguing cryogenic
spintronics devices. These findings hold potential for applications in the
field of quantum computing architectures and information processing
technologies.
|
2307.07751v2
|
2023-07-26
|
Oscillatory Edge Modes in Two Dimensional Spin-Torque Oscillator Arrays
|
Spin torque oscillators (STOs) are dissipative magnetic systems that provide
a natural platform for exploring non-Hermitian phenomena. We theoretically
study a two-dimensional (2d) array of STOs and show that its dynamics can be
mapped to a 2d, non-Hermitian Su-Schrieffer-Heeger (SSH) model. We calculate
the energy spectrum and identify the one-dimensional (1d) edge states of our
model, corresponding to auto-oscillation of STOs on the boundary of the system
while the bulk oscillators do not activate. We show that tuning the Gilbert
damping, injected spin current, and coupling between STOs allows for exploring
the edge state properties under different parameter regimes. Furthermore, this
system admits 1d edge states with non-uniform probability density, and we
explore their properties in systems of different sizes. Additional symmetry
analysis indicates that these states are not topologically protected but are
nevertheless confined to the edge of the system, as the bulk is protected by
PT-symmetry. These results indicate that 2d arrays of STOs may be useful to
explore novel edge state behavior in dissipative systems.
|
2307.13876v1
|
2024-03-25
|
Detection of spin pumping free of rectification and thermal artefacts in molecular-based ferromagnetic insulator V[TCNE]x~2
|
The molecular-based ferrimagnetic insulator V(TCNE)x has gained recent
interest for efficient spin-wave excitation due to its low Gilbert damping
ratio a=4E-5, and narrow ferromagnetic resonance linewidth f=1Oe. Here we
report a clean spin pumping signal detected on V(TCNE)x/metal bilayer
structures, free from spin rectification or thermal artifacts. On-chip coupling
of microwave power is achieved via a coplanar waveguide to measure the in-plane
angle-dependence of the inverse spin-Hall effect under ferromagnetic resonance
conditions with respect to a constant external magnetic field. A signature of
pure spin current from V(TCNE)x is observed in both platinum and permalloy
metal layers, demonstrating the utility of V(TCNE)x for magnon spintronics
studies in molecule/solid-state heterostructures.
|
2403.16429v2
|
2024-04-01
|
Harnessing Interlayer Magnetic Coupling for Efficient, Field-Free Current-Induced Magnetization Switching in a Magnetic Insulator
|
Owing to the unique features of low Gilbert damping, long spin-diffusion
lengths and zero Ohmic losses, magnetic insulators are promising candidate
materials for next-generation spintronic applications. However, due to the
localized magnetic moments and the complex metal-oxide interface between
magnetic insulators and heavy metals, spin-functional Dzyaloshinskii-Moriya
interactions or spin Hall and Edelstein effects are weak, which diminishes the
performance of these typical building blocks for spintronic devices. Here, we
exploit the exchange coupling between metallic and insulating magnets for
efficient electrical manipulation of heavy metal/magnetic insulator
heterostructures. By inserting a thin Co layer, we enhance the spin-orbit
torque efficiency by more than 20 times, which significantly reduces the
switching current density. Moreover, we demonstrate field-free current-induced
magnetization switching caused by a symmetry-breaking non-collinear magnetic
texture. Our work launches magnetic insulators as an alternative platform for
low-power spintronic devices.
|
2404.00845v1
|
2005-07-15
|
A Theory of Physical Quantum Computation: The Quantum Computer Condition
|
In this paper we present a new unified theoretical framework that describes
the full dynamics of quantum computation. Our formulation allows any questions
pertaining to the physical behavior of a quantum computer to be framed, and in
principle, answered. We refer to the central organizing principle developed in
this paper, on which our theoretical structure is based, as the *Quantum
Computer Condition* (QCC), a rigorous mathematical statement that connects the
irreversible dynamics of the quantum computing machine, with the reversible
operations that comprise the quantum computation intended to be carried out by
the quantum computing machine. Armed with the QCC, we derive a powerful result
that we call the *Encoding No-Go Theorem*. This theorem gives a precise
mathematical statement of the conditions under which fault-tolerant quantum
computation becomes impossible in the presence of dissipation and/or
decoherence. In connection with this theorem, we explicitly calculate a
universal critical damping value for fault-tolerant quantum computation. In
addition we show that the recently-discovered approach to quantum error
correction known as "operator quantum error-correction" (OQEC) is a special
case of our more general formulation. Our approach furnishes what we will refer
to as "operator quantum fault-tolerance" (OQFT). In particular, we show how the
QCC allows one to derive error thresholds for fault tolerance in a completely
general context. We prove the existence of solutions to a class of
time-dependent generalizations of the Lindblad equation. Using the QCC, we also
show that the seemingly different circuit, graph- (including cluster-) state,
and adiabatic paradigms for quantum computing are in fact all manifestations of
a single, universal paradigm for all physical quantum computation.
|
0507141v2
|
2018-12-22
|
Spin dynamics of $3d$ and $4d$ impurities embedded in prototypical topological insulators
|
Topological insulators are insulating bulk materials hosting conducting
surface states. Their magnetic doping breaks time-reversal symmetry and
generates numerous interesting effects such as dissipationless transport.
Nonetheless, their dynamical properties are still poorly understood. Here, we
perform a systematic investigation of transverse spin excitations of $3d$ and
$4d$ single impurities embedded in two prototypical topological insulators
(Bi$_2$Te$_3$ and Bi$_2$Se$_3$). The impurity-induced states within the bulk
gap of the topological insulators are found to have a drastic impact on the
spin excitation spectra, resulting in very high lifetimes reaching up to
${microseconds}$. An intuitive picture of the spin dynamics is obtained by
mapping onto a generalized Landau-Lifshitz-Gilbert phenomenological model. The
first quantity extracted from this mapping procedure is the magnetic anisotropy
energy, which is then compared to the one provided by the magnetic force
theorem. This uncovers some difficulties encountered with the latter, which can
provide erroneous results for impurities with a high density of states at the
Fermi energy. Moreover, the Gilbert damping and nutation tensors are obtained.
The nutation effects can lead to a non-negligible shift in the spin excitation
resonance in the high-frequency regime. Finally, we study the impact of the
surface state on the spin dynamics, which may be severely altered due to the
repositioning of the impurity-induced state in comparison to the bulk case. Our
systematic investigation of this series of magnetic impurities sheds light on
their spin dynamics within topological insulators, with implications for
available and future experimental studies as, for instance, on the viability of
using such impurities for solid-state qubits.
|
1812.09596v1
|
2013-08-16
|
Quantum Gilbert-Varshamov Bound Through Symplectic Self-Orthogonal Codes
|
It is well known that quantum codes can be constructed through classical
symplectic self-orthogonal codes. In this paper, we give a kind of
Gilbert-Varshamov bound for symplectic self-orthogonal codes first and then
obtain the Gilbert-Varshamov bound for quantum codes. The idea of obtaining the
Gilbert-Varshamov bound for symplectic self-orthogonal codes follows from
counting arguments.
|
1308.3578v1
|
2017-04-19
|
Refractive index of dense materials
|
We show that applying the Lorentz-Lorenz transformation to the refractive
index of metals, semiconductors and insulators allows for a less empirical
modeling of this refractive index.
|
1704.05718v1
|
1998-10-01
|
Finite temperature dynamics of vortices in the two dimensional anisotropic Heisenberg model
|
We study the effects of finite temperature on the dynamics of non-planar
vortices in the classical, two-dimensional anisotropic Heisenberg model with
XY- or easy-plane symmetry. To this end, we analyze a generalized
Landau-Lifshitz equation including additive white noise and Gilbert damping.
Using a collective variable theory with no adjustable parameters we derive an
equation of motion for the vortices with stochastic forces which are shown to
represent white noise with an effective diffusion constant linearly dependent
on temperature. We solve these stochastic equations of motion by means of a
Green's function formalism and obtain the mean vortex trajectory and its
variance. We find a non-standard time dependence for the variance of the
components perpendicular to the driving force. We compare the analytical
results with Langevin dynamics simulations and find a good agreement up to
temperatures of the order of 25% of the Kosterlitz-Thouless transition
temperature. Finally, we discuss the reasons why our approach is not
appropriate for higher temperatures as well as the discreteness effects
observed in the numerical simulations.
|
9810011v1
|
2004-07-21
|
A selfconsistent theory of current-induced switching of magnetization
|
A selfconsistent theory of the current-induced switching of magnetization
using nonequilibrium Keldysh formalism is developed for a junction of two
ferromagnets separated by a nonmagnetic spacer. It is shown that the
spin-transfer torques responsible for current-induced switching of
magnetization can be calculated from first principles in a steady state when
the magnetization of the switching magnet is stationary. The spin-transfer
torque is expressed in terms of one-electron surface Green functions for the
junction cut into two independent parts by a cleavage plane immediately to the
left and right of the switching magnet. The surface Green functions are
calculated using a tight-binding Hamiltonian with parameters determined from a
fit to an {\it ab initio} band structure.This treatment yields the spin
transfer torques taking into account rigorously contributions from all the
parts of the junction. To calculate the hysteresis loops of resistance versus
current, and hence to determine the critical current for switching, the
microscopically calculated spin-transfer torques are used as an input into the
phenomenological Landau-Lifshitz equation with Gilbert damping. The present
calculations for Co/Cu/Co(111) show that the critical current for switching is
$\approx 10^7A/cm^2$, which is in good agreement with experiment.
|
0407562v2
|
2006-02-24
|
Magnetization dynamics in dysprosium orthoferrites via inverse Faraday effect
|
The ultrafast non-thermal control of magnetization has recently become
feasible in canted antiferromagnets through photomagnetic instantaneous pulses
[A.V. Kimel {\it et al.}, Nature {\bf 435}, 655 (2005)]. In this experiment
circularly polarized femtosecond laser pulses set up a strong magnetic field
along the wave vector of the radiation through the inverse Faraday effect,
thereby exciting non-thermally the spin dynamics of dysprosium orthoferrites. A
theoretical study is performed by using a model for orthoferrites based on a
general form of free energy whose parameters are extracted from experimental
measurements. The magnetization dynamics is described by solving coupled
sublattice Landau-Lifshitz-Gilbert equations whose damping term is associated
with the scattering rate due to magnon-magnon interaction. Due to the inverse
Faraday effect and the non-thermal excitation, the effect of the laser is
simulated by magnetic field Gaussian pulses with temporal width of the order of
hundred femtoseconds. When the field is along the z-axis, a single resonance
mode of the magnetization is excited. The amplitude of the magnetization and
out-of-phase behavior of the oscillations for fields in z and -z directions are
in good agreement with the cited experiment. The analysis of the effect of the
temperature shows that magnon-magnon scattering mechanism affects the decay of
the oscillations on the picosecond scale. Finally, when the field pulse is
along the x-axis, another mode is excited, as observed in experiments. In this
case the comparison between theoretical and experimental results shows some
discrepancies whose origin is related to the role played by anisotropies in
orthoferrites.
|
0602593v1
|
2006-04-19
|
Stress - and Magneto-Impedance in Co71-xFexCr7Si8B14 (x = 0, 2) amorphous ribbons
|
Systematic measurements of stress impedance (SI) and magneto-impedance (MI)
have been carried out using Co-rich amorphous ribbons of nominal composition
Co71-xFexCr7Si8B14 (x = 0, 2) at various excitation frequencies and bias fields
and at room temperature. The impedance, Z, for both the samples was found to be
very sensitive functions of applied tensile stress (up to 100MPa) exhibiting a
maximum SI ratio as much as 80% at low frequency ~ 0.1MHz. The nature of
variation of impedance, Z, changes with the excitation frequency especially at
higher frequencies in MHz region where it exhibits a peak. Magnetization
measurements were also performed to observe the effects of applied stress and
magnetization decreases with the application of stress confirming the negative
magnetostriction co-efficient of both the samples. Both the samples exhibited
negative magneto-impedance when the variation of Z is observed with the applied
bias magnetic field, H. Maximum MI ratio as large as 99% has been observed for
both the samples at low fields ~ 27Oe. The impedance as functions of applied
magnetic field, Z(H), decreases with the application of stress thus making the
MI curves broader. Based on the electromagnetic screening and magnetization
dynamics and incorporating the Gilbert and the Bloch-Bloembergen damping and
stress dependent anisotropy, the SI has been calculated and is found to
describe well the stress and field dependence of impedance of the two samples.
|
0604438v2
|
2010-05-25
|
Structural, static and dynamic magnetic properties of CoMnGe thin films on a sapphire a-plane substrate
|
Magnetic properties of CoMnGe thin films of different thicknesses (13, 34,
55, 83, 100 and 200 nm), grown by RF sputtering at 400{\deg}C on single crystal
sapphire substrates, were studied using vibrating sample magnetometry (VSM) and
conventional or micro-strip line (MS) ferromagnetic resonance (FMR). Their
behavior is described assuming a magnetic energy density showing twofold and
fourfold in-plane anisotropies with some misalignment between their principal
directions. For all the samples, the easy axis of the fourfold anisotropy is
parallel to the c-axis of the substrate while the direction of the twofold
anisotropy easy axis varies from sample to sample and seems to be strongly
influenced by the growth conditions. Its direction is most probably monitored
by the slight unavoidable angle of miscut the Al2O3 substrate. The twofold
in-plane anisotropy field is almost temperature independent, in contrast with
the fourfold field which is a decreasing function of the temperature. Finally,
we study the frequency dependence of the observed line-width of the resonant
mode and we conclude to a typical Gilbert damping constant of 0.0065 for the
55-nm-thick film.
|
1005.4595v3
|
2011-06-22
|
Effect of spin diffusion on current generated by spin motive force
|
Spin motive force is a spin-dependent force on conduction electrons induced
by magnetization dynamics. In order to examine its effects on magnetization
dynamics, it is indispensable to take into account spin accumulation, spin
diffusion, and spin-flip scattering since the spin motive force is in general
nonuniform. We examine the effects of all these on the way the spin motive
force generates the charge and spin currents in conventional situations, where
the conduction electron spin relaxation dynamics is much faster than the
magnetization dynamics. When the spin-dependent electric field is spatially
localized, which is common in experimental situations, we find that the
conservative part of the spin motive force is unable to generate the charge
current due to the cancelation effect by the diffusion current. We also find
that the spin current is a nonlocal function of the spin motive force and can
be effectively expressed in terms of nonlocal Gilbert damping tensor. It turns
out that any spin independent potential such as Coulomb potential does not
affect our principal results. At the last part of this paper, we apply our
theory to current-induced domain wall motion.
|
1106.4389v2
|
2011-07-11
|
Spin and charge transport induced by gauge fields in a ferromagnet
|
We present a microscopic theory of spin-dependent motive force ("spin motive
force") induced by magnetization dynamics in a conducting ferromagnet, by
taking account of spin relaxation of conduction electrons. The theory is
developed by calculating spin and charge transport driven by two kinds of gauge
fields; one is the ordinary electromagnetic field $A^{\rm em}_{\mu}$, and the
other is the effective gauge field $A^{z}_{\mu}$ induced by dynamical magnetic
texture. The latter acts in the spin channel and gives rise to a spin motive
force. It is found that the current induced as a linear response to
$A^{z}_{\mu}$ is not gauge-invariant in the presence of spin-flip processes.
This fact is intimately related to the non-conservation of spin via Onsager
reciprocity, so is robust, but indicates a theoretical inconsistency. This
problem is resolved by considering the time dependence of spin-relaxation
source terms in the "rotated frame", as in the previous study on Gilbert
damping [J. Phys. Soc. Jpn. {\bf 76}, 063710 (2007)]. This effect restores the
gauge invariance while keeping spin non-conservation. It also gives a
dissipative spin motive force expected as a reciprocal to the dissipative spin
torque ("$\beta$-term").
|
1107.2165v3
|
2012-07-02
|
Establishing micromagnetic parameters of ferromagnetic semiconductor (Ga,Mn)As
|
(Ga,Mn)As is at the forefront of research exploring the synergy of magnetism
with the physics and technology of semiconductors, and has led to discoveries
of new spin-dependent phenomena and functionalities applicable to a wide range
of material systems. Its recognition and utility as an ideal model material for
spintronics research has been undermined by the large scatter in reported
semiconducting doping trends and micromagnetic parameters. In this paper we
establish these basic material characteristics by individually optimizing the
highly non-equilibrium synthesis for each Mn-doping level and by simultaneously
determining all micromagnetic parameters from one set of magneto-optical
pump-and-probe measurements. Our (Ga,Mn)As thin-film epilayers, spannig the
wide range of accessible dopings, have sharp thermodynamic Curie point
singularities typical of uniform magnetic systems. The materials show
systematic trends of increasing magnetization, carrier density, and Curie
temperature (reaching 188 K) with increasing doping, and monotonous doping
dependence of the Gilbert damping constant of ~0.1-0.01 and the spin stiffness
of ~2-3 meVnm^2. These results render (Ga,Mn)As well controlled degenerate
semiconductor with basic magnetic characteristics comparable to common band
ferromagnets.
|
1207.0310v1
|
2013-03-14
|
Spin-torque effects in thermally assisted magnetization reversal: Method of statistical moments
|
Thermal fluctuations of nanomagnets driven by spin-polarized currents are
treated via the Landau-Lifshitz-Gilbert equation generalized to include both
the random thermal noise field and the Slonczewski spin-transfer torque term.
By averaging this stochastic (Langevin) equation over its realizations, the
explicit infinite hierarchy of differential-recurrence relations for
statistical moments (averaged spherical harmonics) is derived for arbitrary
demagnetizing factors and magnetocrystalline anisotropy for the generic
nanopillar model of a spin-torque device comprising two ferromagnetic strata
representing the free and fixed layers and a nonmagnetic conducting spacer all
sandwiched between two ohmic contacts. The influence of thermal fluctuations
and spin-transfer torques on relevant switching characteristics, such as the
stationary magnetization, the magnetization reversal time, etc., is calculated
by solving the hierarchy for wide ranges of temperature, damping, external
magnetic field, and spin-polarized current indicating new spin-torque effects
in the thermally assisted magnetization reversal comprising several orders of
magnitude. In particular, a pronounced dependence of the switching
characteristics on the directions of the external magnetic field and the spin
polarization exists.
|
1303.3476v4
|
2013-05-03
|
Co2 FeAl thin films grown on MgO substrates: Correlation between static, dynamic and structural properties
|
Co2FeAl (CFA) thin films with thickness varying from 10 nm to 115 nm have
been deposited on MgO(001) substrates by magnetron sputtering and then capped
by Ta or Cr layer. X-rays diffraction (XRD) revealed that the cubic $[001]$ CFA
axis is normal to the substrate and that all the CFA films exhibit full
epitaxial growth. The chemical order varies from the $B2$ phase to the $A2$
phase when decreasing the thickness. Magneto-optical Kerr effect (MOKE) and
vibrating sample magnetometer measurements show that, depending on the field
orientation, one or two-step switchings occur. Moreover, the films present a
quadratic MOKE signal increasing with the CFA thickness, due to the increasing
chemical order. Ferromagnetic resonance, MOKE transverse bias initial inverse
susceptibility and torque (TBIIST) measurements reveal that the in-plane
anisotropy results from the superposition of a uniaxial and of a fourfold
symmetry term. The fourfold anisotropy is in accord with the crystal structure
of the samples and is correlated to the biaxial strain and to the chemical
order present in the films. In addition, a large negative perpendicular
uniaxial anisotropy is observed. Frequency and angular dependences of the FMR
linewidth show two magnon scattering and mosaicity contributions, which depend
on the CFA thickness. A Gilbert damping coefficient as low as 0.0011 is found.
|
1305.0714v1
|
2013-06-19
|
Asymmetric Ferromagnetic Resonance, Universal Walker Breakdown, and Counterflow Domain Wall Motion in the Presence of Multiple Spin-Orbit Torques
|
We study the motion of several types of domain wall profiles in spin-orbit
coupled magnetic nanowires and also the influence of spin-orbit interaction on
the ferromagnetic resonance of uniform magnetic films. We extend previous
studies by fully considering not only the field-like contribution from the
spin-orbit torque, but also the recently derived Slonczewski-like spin-orbit
torque. We show that the latter interaction affects both the domain wall
velocity and the Walker breakdown threshold non-trivially, which suggests that
it should be accounted in experimental data analysis. We find that the presence
of multiple spin-orbit torques may render the Walker breakdown to be universal
in the sense that the threshold is completely independent on the
material-dependent Gilbert damping, non-adiabaticity, and the chirality of the
domain wall. We also find that domain wall motion against the current injection
is sustained in the presence of multiple spin-orbit torques and that the wall
profile will determine the qualitative influence of these different types of
torques (e.g. field-like and Slonczewski-like). In addition, we consider a
uniform ferromagnetic layer under a current bias, and find that the resonance
frequency becomes asymmetric against the current direction in the presence of
Slonczewski-like spin-orbit coupling. This is in contrast with those cases
where such an interaction is absent, where the frequency is found to be
symmetric with respect to the current direction. This finding shows that
spin-orbit interactions may offer additional control over pumped and absorbed
energy in a ferromagnetic resonance setup by manipulating the injected current
direction.
|
1306.4680v1
|
2013-11-29
|
Magnon radiation by moving Abrikosov vortices in ferromagnetic superconductors and superconductor-ferromagnet multilayers
|
In systems combining type-II superconductivity and magnetism the
non-stationary magnetic field of moving Abrikosov vortices may excite spin
waves, or magnons. This effect leads to the appearance of an additional damping
force acting on the vortices. By solving the London and Landau-Lifshitz-Gilbert
equations we calculate the magnetic moment induced force acting on vortices in
ferromagnetic superconductors and superconductor/ferromagnet superlattices. If
the vortices are driven by a dc force, magnon generation due to the Cherenkov
resonance starts as the vortex velocity exceeds some threshold value. For an
ideal vortex lattice this leads to an anisotropic contribution to the
resistivity and to the appearance of resonance peaks on the current voltage
characteristics. For a disordered vortex array the current will exhibit a
step-like increase at some critical voltage. If the vortices are driven by an
ac force with a frequency \omega, the interaction with magnetic moments will
lead to a frequency-dependent magnetic contribution \eta_M to the vortex
viscosity. If \omega is below the ferromagnetic resonance frequency \omega_F,
vortices acquire additional inertia. For \omega > \omega_F dissipation is
enhanced due to magnon generation. The viscosity \eta_M can be extracted from
the surface impedance of the ferromagnetic superconductor. Estimates of the
magnetic force acting on vortices for the U-based ferromagnetic superconductors
and cuprate/manganite superlattices are given.
|
1311.7620v1
|
2014-03-03
|
Observations and Implications of Large-Amplitude Longitudinal Oscillations in a Solar Filament
|
On 20 August 2010 an energetic disturbance triggered large-amplitude
longitudinal oscillations in a nearby filament. The triggering mechanism
appears to be episodic jets connecting the energetic event with the filament
threads. In the present work we analyze this periodic motion in a large
fraction of the filament to characterize the underlying physics of the
oscillation as well as the filament properties. The results support our
previous theoretical conclusions that the restoring force of large-amplitude
longitudinal oscillations is solar gravity, and the damping mechanism is the
ongoing accumulation of mass onto the oscillating threads. Based on our
previous work, we used the fitted parameters to determine the magnitude and
radius of curvature of the dipped magnetic field along the filament, as well as
the mass accretion rate onto the filament threads. These derived properties are
nearly uniform along the filament, indicating a remarkable degree of
cohesiveness throughout the filament channel. Moreover, the estimated mass
accretion rate implies that the footpoint heating responsible for the thread
formation, according to the thermal nonequilibrium model, agrees with previous
coronal heating estimates. We estimate the magnitude of the energy released in
the nearby event by studying the dynamic response of the filament threads, and
discuss the implications of our study for filament structure and heating.
|
1403.0381v1
|
2015-01-16
|
Direct measurement of the magnetic anisotropy field in Mn--Ga and Mn--Co--Ga Heusler films
|
The static and dynamic magnetic properties of tetragonally distorted Mn--Ga
based alloys were investigated. Static properties are determined in magnetic
fields up to 6.5~T using SQUID magnetometry. For the pure Mn$_{1.6}$Ga film,
the saturation magnetisation is 0.36~MA/m and the coercivity is 0.29~T. Partial
substitution of Mn by Co results in Mn$_{2.6}$Co$_{0.3}$Ga$_{1.1}$. The
saturation magnetisation of those films drops to 0.2~MA/m and the coercivity is
increased to 1~T.
Time-resolved magneto-optical Kerr effect (TR-MOKE) is used to probe the
high-frequency dynamics of Mn--Ga. The ferromagnetic resonance frequency
extrapolated to zero-field is found to be 125~GHz with a Gilbert damping,
$\alpha$, of 0.019. The anisotropy field is determined from both SQUID and
TR-MOKE to be 4.5~T, corresponding to an effective anisotropy density of
0.81~MJ/m$^3$.
Given the large anisotropy field of the Mn$_{2.6}$Co$_{0.3}$Ga$_{1.1}$ film,
pulsed magnetic fields up to 60~T are used to determine the field strength
required to saturate the film in the plane. For this, the extraordinary Hall
effect was employed as a probe of the local magnetisation. By integrating the
reconstructed in--plane magnetisation curve, the effective anisotropy energy
density for Mn$_{2.6}$Co$_{0.3}$Ga$_{1.1}$ is determined to be 1.23~MJ/m$^3$.
|
1501.03973v1
|
2015-06-02
|
Respective influence of in-plane and out-of-plane spin-transfer torques in magnetization switching of perpendicular magnetic tunnel junctions
|
The relative contributions of in-plane (damping-like) and out-of-plane
(field-like) spin-transfer-torques in the magnetization switching of
out-of-plane magnetized magnetic tunnel junctions (pMTJ) has been theoretically
analyzed using the transformed Landau-Lifshitz (LL) equation with the STT
terms. It is demonstrated that in a pMTJ structure obeying macrospin dynamics,
the out-of-plane torque influences the precession frequency but it does not
contribute significantly to the STT switching process (in particular to the
switching time and switching current density), which is mostly determined by
the in-plane STT contribution. This conclusion is confirmed by finite
temperature and finite writing pulse macrospin simulations of the current-field
switching diagrams. It contrasts with the case of STT-switching in in-plane
magnetized MTJ in which the field-like term also influences the switching
critical current. This theoretical analysis was successfully applied to the
interpretation of voltage-field STT switching diagrams experimentally measured
on perpendicular MTJ pillars 36 nm in diameter, which exhibit macrospin-like
behavior. The physical nonequivalence of Landau and Gilbert dissipation terms
in presence of STT-induced dynamics is also discussed.
|
1506.00780v2
|
2015-08-28
|
Control of magnetic relaxation by electric-field-induced ferroelectric phase transition and inhomogeneous domain switching
|
Electric-field modulation of magnetism in strain-mediated multiferroic
heterostructures is considered a promising scheme for enabling memory and
magnetic microwave devices with ultralow power consumption. However, it is not
well understood how electric-field-induced strain influences magnetic
relaxation, an important physical process for device applications. Here we
investigate resonant magnetization dynamics in ferromagnet/ferrolectric
multiferroic heterostructures, FeGaB/PMN-PT and NiFe/PMN-PT, in two distinct
strain states provided by electric-field-induced ferroelectric phase
transition. The strain not only modifies magnetic anisotropy but also magnetic
relaxation. In FeGaB/PMN-PT, we observe a nearly two-fold change in intrinsic
Gilbert damping by electric field, which is attributed to strain-induced tuning
of spin-orbit coupling. By contrast, a small but measurable change in extrinsic
linewidth broadening is attributed to inhomogeneous ferroelastic domain
switching during the phase transition of the PMN-PT substrate.
|
1508.07290v2
|
2016-04-05
|
Homodyne-detected ferromagnetic resonance of in-plane magnetized nano-contacts: composite spin wave resonances and their excitation mechanism
|
This work provides a detailed investigation of the measured in-plane
field-swept homodyne-detected ferromagnetic resonance (FMR) spectra of an
extended Co/Cu/NiFe pseudo spin valve stack using a nanocontact (NC) geometry.
The magnetodynamics are generated by a pulse-modulated microwave current and
the resulting rectified dc mixing voltage, which appears across the NC at
resonance, is detected using a lock-in amplifier. Most notably, we find that
the measured spectra of the NiFe layer are composite in nature and highly
asymmetric, consistent with the broadband excitation of multiple modes.
Additionally, the data must be fit with two Lorentzian functions in order to
extract a reasonable value for the Gilbert damping of the NiFe. Aided by
micromagnetic simulations, we conclude that (i) for in-plane fields the rf
Oersted field in the vicinity of the NC plays the dominant role in generating
the observed spectra, (ii) in addition to the FMR mode, exchange dominated spin
waves are also generated, and (iii) the NC diameter sets the mean wavevector of
the exchange dominated spin wave, in good agreement with the dispersion
relation.
|
1604.01389v1
|
2016-08-29
|
Sub-micrometer yttrium iron garnet LPE films with low ferromagnetic resonance losses
|
Using liquid phase epitaxy (LPE) technique (111) yttrium iron garnet (YIG)
films with thicknesses of ~100 nm and surface roughnesses as low as 0.3 nm have
been grown as a basic material for spin-wave propagation experiments in
microstructured waveguides. The continuously strained films exhibit nearly
perfect crystallinity without significant mosaicity and with effective lattice
misfits of delta a(perpendicular)/a(substrate) ~10-4 and below. The
film/substrate interface is extremely sharp without broad interdiffusion layer
formation. All LPE films exhibit a nearly bulk-like saturation magnetization of
(1800+-20) Gs and an `easy cone' anisotropy type with extremely small in-plane
coercive fields <0.2 Oe. There is a rather weak in-plane magnetic anisotropy
with a pronounced six-fold symmetry observed for saturation field <1.5 Oe. No
significant out-of-plane anisotropy is observed, but a weak dependence of the
effective magnetization on the lattice misfit is detected. The narrowest
ferromagnetic resonance linewidth is determined to be 1.4 Oe @ 6.5 GHz which is
the lowest value reported so far for YIG films of 100 nm thicknesses and below.
The Gilbert damping coefficient for investigated LPE films is estimated to be
close to 1 x 10-4.
|
1608.08043v1
|
2017-01-10
|
Motion of skyrmions in nanowires driven by magnonic momentum-transfer forces
|
We study the motion of magnetic skyrmions in a nanowire induced by a
spin-wave current $J$ flowing out of a driving layer close to the edge of the
wire. By applying micromagnetic simulation and an analysis of the effective
Thiele equation, we find that the skyrmion trajectory is governed by an
interplay of both forces due to the magnon current and the wire boundary. The
skyrmion is attracted to the driving layer and is accelerated by the repulsive
force due to the wire boundary. We consider both cases of a driving
longitudinal and transverse to the nanowire, but a steady-state motion of the
skyrmion is only obtained for a transverse magnon current. For the latter case,
we find in the limit of low current densities $J$ the velocity-current relation
$v \sim J/\alpha$ where $v$ is the skyrmion velocity and $\alpha$ is the
Gilbert damping. For large $J$ in case of strong driving, the skyrmion is
pushed into the driving layer resulting in a drop of the skyrmion velocity and,
eventually, the destruction of the skyrmion.
|
1701.02430v2
|
2017-01-19
|
Ultrafast Electron-Lattice Coupling Dynamics in VO2 and V2O3 Thin Films
|
Ultrafast optical pump - optical probe and optical pump - terahertz probe
spectroscopy were performed on vanadium dioxide (VO2) and vanadium sesquioxide
(V2O3) thin films over a wide temperature range. A comparison of the
experimental data from these two different techniques and two different
vanadium oxides, in particular a comparison of the electronic oscillations
generated by the photoinduced longitudinal acoustic modulation, reveals the
strong electron-phonon coupling that exists in the metallic state of both
materials. The low energy Drude response of V2O3 appears more susceptible than
VO2 to ultrafast strain control. Additionally, our results provide a
measurement of the temperature dependence of the sound velocity in both
systems, revealing a four- to fivefold increase in VO2 and a three- to fivefold
increase in V2O3 across the phase transition. Our data also confirm
observations of strong damping and phonon anharmonicity in the metallic phase
of VO2, and suggest that a similar phenomenon might be at play in the metallic
phase of V2O3. More generally, our simple table-top approach provides relevant
and detailed information about dynamical lattice properties of vanadium oxides,
opening the way to similar studies in other complex materials.
|
1701.05531v1
|
2017-02-21
|
All-optical Detection of Spin Hall Angle in W/CoFeB/SiO2 Heterostructures by Varying Tungsten Layer Thickness
|
The development of advanced spintronics devices hinges on the efficient
generation and utilization of pure spin current. In materials with large
spin-orbit coupling, the spin Hall effect may convert charge current to pure
spin current and a large conversion efficiency, which is quantified by spin
Hall angle (SHA), is desirable for the realization of miniaturized and energy
efficient spintronic devices. Here, we report a giant SHA in beta-tungsten
(\b{eta}-W) thin films in Sub/W(t)/Co20Fe60B20(3 nm)/SiO2(2 nm)
heterostructures with variable W thickness. We employed an all-optical
time-resolved magneto-optical Kerr effect microscope for an unambiguous
determination of SHA using the principle of modulation of Gilbert damping of
the adjacent ferromagnetic layer by the spin-orbit torque from the W layer. A
non-monotonic variation of SHA with W layer thickness (t) is observed with a
maximum of about 0.4 at about t = 3 nm, followed by a sudden reduction to a
very low value at t = 6 nm. This variation of SHA with W-thickness correlates
well with the thickness dependent structural phase transition and resistivity
variation of W above the spin diffusion length of W, while below this length
the interfacial electronic effect at W/CoFeB influences the estimation of SHA.
|
1702.06258v1
|
2017-03-21
|
Annealing stability of magnetic tunnel junctions based on dual MgO free layers and [Co/Ni] based thin synthetic antiferromagnet fixed system
|
We study the annealing stability of bottom-pinned perpendicularly magnetized
magnetic tunnel junctions based on dual MgO free layers and thin fixed systems
comprising a hard [Co/Ni] multilayer antiferromagnetically coupled to thin a Co
reference layer and a FeCoB polarizing layer. Using conventional magnetometry
and advanced broadband ferromagnetic resonance, we identify the properties of
each sub-unit of the magnetic tunnel junction and demonstrate that this
material option can ensure a satisfactory resilience to the 400$^\circ$C
thermal annealing needed in solid-state magnetic memory applications. The dual
MgO free layer possesses an anneal-robust 0.4 T effective anisotropy and
suffers only a minor increase of its Gilbert damping from 0.007 to 0.010 for
the toughest annealing conditions. Within the fixed system, the ferro-coupler
and texture-breaking TaFeCoB layer keeps an interlayer exchange above 0.8
mJ/m$^2$, while the Ru antiferrocoupler layer within the synthetic
antiferromagnet maintains a coupling above -0.5 mJ/m$^2$. These two strong
couplings maintain the overall functionality of the tunnel junction upon the
toughest annealing despite the gradual degradation of the thin Co layer
anisotropy that may reduce the operation margin in spin torque memory
applications. Based on these findings, we propose further optimization routes
for the next generation magnetic tunnel junctions.
|
1703.07154v1
|
2017-08-03
|
Evolution of the interfacial perpendicular magnetic anisotropy constant of the Co$_2$FeAl/MgO interface upon annealing
|
We investigate thickness series of films of the Heusler alloy Co$_2$FeAl in
order to study the effect of annealing on the interface with a MgO layer and on
the bulk magnetic properties. Our results reveal that while the perpendicular
interface anisotropy constant $K^{\perp}_{\rm S}$ is zero for the as-deposited
samples, its value increases with annealing up to a value of $1.14\, \pm
\,0.07$~mJ/m$^2$ for the series annealed at 320$^{\rm o}$C and of $2.07\, \pm
\,0.7$~mJ/m$^2$ for the 450$^{\rm o}$C annealed series owing to a strong
modification of the interface during the thermal treatment. This large value
ensures a stabilization of a perpendicular magnetization orientation for a
thickness below 1.7~nm. The data additionally shows that the in-plane biaxial
anisotropy constant has a different evolution with thickness in as-deposited
and annealed systems. The Gilbert damping parameter $\alpha$ shows minima for
all series for a thickness of 40~nm and an absolute minimum value of
$2.8\pm0.1\cdot10^{-3}$. The thickness dependence is explained in terms of an
inhomogenous magnetization state generated by the interplay between the
different anisotropies of the system and by crystalline disorder.
|
1708.01126v2
|
2017-08-08
|
Spin-orbit-torque driven magnetoimpedance in Pt-layer/magnetic-ribbon heterostructures
|
When a flow of electron passes through a paramagnetic layer with strong
spin-orbit-coupling such as platinum (Pt), a net spin current is produced via
spin Hall effect (SHE). This spin current can exert a torque on the
magnetization of an adjacent ferromagnetic layer which can be probed via
magnetization dynamic response, e.g. spin-torque ferromagnetic resonance
(ST-FMR). Nevertheless, that effect in lower frequency magnetization dynamic
regime (MHz) where skin effect occurs in high permeability ferromagnetic
conductors namely the magneto-impedance (MI) effect can be fundamentally
important which has not been studied so far. Here, by utilizing the MI effect
in magnetic-ribbon/Pt heterostructure with high magnetic permeability that
allows the ac current effectively confined at the skin depth of ~100 nm
thickness, the effect of spin-orbit-torque (SOT) induced by the SHE probed via
MI measurement is investigated. We observed a systematic MI frequency shift
that increases by increasing the applied current amplitude and thickness of the
Pt layer (varying from 0 nm to 20 nm). In addition, the role of Pt layer in
ribbon/Pt heterostructure is evaluated with ferromagnetic resonance (FMR)
effect representing standard Gilbert damping increase as the result of presence
of the SHE. Our results unveil the role of SOT in dynamic control of the
transverse magnetic permeability probed with impedance spectroscopy as useful
and valuable technique for detection of future SHE devices.
|
1708.02402v2
|
2017-12-20
|
Second-harmonic magnetic response characterizing magnetite-based colloid
|
Nonlinear second-harmonic magnetic response (M2) was used to characterize an
aqueous colloidal solution of dextran-coated magnetite (Fe3O4) nanoparticles.
Data analysis with the formalism based on Gilbert-Landau-Lifshitz equation for
stochastic dynamics of superparamagnetic (SP) particles ensured extensive
quantifying of the system via a set of magnetic and magnetodynamic parameters,
such as the mean magnetic moment, the damping constant, the longitudinal
relaxation time, the magnetic anisotropy field and energy, and others. Combined
with transmission electron microscopy and dynamic light scattering, M2
technique allowed obtaining additional parameters, viz., the dextran-coating
thickness and the interparticle magnetic dipolar energy. Aggregated colloidal
nanoparticles were shown to be magnetically correlated inside the aggregate due
to magnetic dipole-dipole (d-d) coupling within the correlation radius ~50 nm.
With the d-d coupling account, the volume distribution of the aggregates
recovered from M2 measurements is well consistent with electron microscopy
results. From electron magnetic resonance, abrupt change of SP dynamics with
increasing external magnetic field was observed and explained. The presented
study exemplifies a novel M2-based procedure of comprehensive quantitative
characterization applicable for a wide variety of SP systems.
|
1712.07534v1
|
2018-02-09
|
Monocrystalline free standing 3D yttrium iron garnet magnon nano resonators
|
Nano resonators in which mechanical vibrations and spin waves can be coupled
are an intriguing concept that can be used in quantum information processing to
transfer information between different states of excitation. Until now, the
fabrication of free standing magnetic nanostructures which host long lived spin
wave excitatons and may be suitable as mechanical resonators seemed elusive. We
demonstrate the fabrication of free standing monocrystalline yttrium iron
garnet (YIG) 3D nanoresonators with nearly ideal magnetic properties. The
freestanding 3D structures are obtained using a complex lithography process
including room temperature deposition and lift-off of amorphous YIG and
subsequent crystallization by annealing. The crystallization nucleates from the
substrate and propagates across the structure even around bends over distances
of several micrometers to form e.g. monocrystalline resonators as shown by
transmission electron microscopy. Spin wave excitations in individual
nanostructures are imaged by time resolved scanning Kerr microscopy. The narrow
linewidth of the magnetic excitations indicates a Gilbert damping constant of
only $\alpha = 2.6 \times 10^{-4}$ rivalling the best values obtained for
epitaxial YIG thin film material. The new fabrication process represents a leap
forward in magnonics and magnon mechanics as it provides 3D YIG structures of
unprecedented quality. At the same time it demonstrates a completely new route
towards the fabrication of free standing crystalline nano structures which may
be applicable also to other material systems.
|
1802.03176v2
|
2018-11-30
|
Dynamical precession of spin in the two-dimensional spin-orbit coupled systems
|
We investigate the spin dynamics in the two-dimensional spin-orbit coupled
system subject to an in-plane ($x$-$y$ plane) constant electric field, which is
assumed to be turned on at the moment $t=0$. The equation of spin precession in
linear response to the switch-on of the electric field is derived in terms of
Heisenberg's equation by the perturbation method up to the first order of the
electric field. The dissipative effect, which is responsible for bringing the
dynamical response to an asymptotic result, is phenomenologically implemented
\`{a} la the Landau-Lifshitz-Gilbert equation by introducing damping terms upon
the equation of spin dynamics. Mediated by the dissipative effect, the
resulting spin dynamics asymptotes to a stationary state, where the spin and
the momentum-dependent effective magnetic field are aligned again and have
nonzero components in the out-of-plane ($z$) direction. In the linear response
regime, the asymptotic response obtained by the dynamical treatment is in full
agreement with the stationary response as calculated in the Kubo formula, which
is a time-independent approach treating the applied electric field as
completely time-independent. Our method provides a new perspective on the
connection between the dynamical and stationary responses.
|
1811.12626v2
|
2019-03-08
|
Spin-transfer torques for domain walls in antiferromagnetically coupled ferrimagnets
|
Antiferromagnetic materials are outstanding candidates for next generation
spintronic applications, because their ultrafast spin dynamics makes it
possible to realize several orders of magnitude higher-speed devices than
conventional ferromagnetic materials1. Though spin-transfer torque (STT) is a
key for electrical control of spins as successfully demonstrated in
ferromagnetic spintronics, experimental understanding of STT in
antiferromagnets has been still lacking despite a number of pertinent
theoretical studies2-5. Here, we report experimental results on the effects of
STT on domain-wall (DW) motion in antiferromagnetically-coupled ferrimagnets.
We find that non-adiabatic STT acts like a staggered magnetic field and thus
can drive DWs effectively. Moreover, the non-adiabaticity parameter {\beta} of
STT is found to be significantly larger than the Gilbert damping parameter
{\alpha}, challenging our conventional understanding of the non-adiabatic STT
based on ferromagnets as well as leading to fast current-induced
antiferromagnetic DW motion. Our study will lead to further vigorous
exploration of STT for antiferromagnetic spin textures for fundamental physics
on spin-charge interaction as wells for efficient electrical control of
antiferromagnetic devices.
|
1903.03251v1
|
2019-03-26
|
Engineering of spin mixing conductance in Ru/FeCo/Ru interfaces: Effect of Re Doping
|
We have deposited polycrystalline Re doped $(Fe_{65}Co_{35})_{100-x}Re_{x}$
(0 $\leq$ x $\leq$ 12.6 at\%) thin films grown under identical conditions and
sandwiched between thin layers of Ru in order to study the phenomenon of spin
pumping as a function of Re concentration. In-plane and out-of-plane
ferromagnetic resonance spectroscopy results show an enhancement of the Gilbert
damping with an increase in Re doping. We found evidence of an increase in the
real part of effective spin mixing conductance
[Re($g^{\uparrow\downarrow}_{eff}$)] with the increase in Re doping of 6.6
at\%, while a decrease is evident at higher Re doping. The increase in
Re($g^{\uparrow\downarrow}_{eff}$) can be linked to the Re doping induced
change of the interface electronic structure in the non-magnetic Ru layer and
the effect interfacial spin-orbit coupling has on the effective spin-mixing
conductance. The lowest and highest values of
Re($g^{\uparrow\downarrow}_{eff}$) are found to be 9.883(02) $nm^{-2}$ and
19.697(02) $nm^{-2}$ for 0 at\% and 6.6 at\% Re doping, respectively. The
saturation magnetization decreases with increasing Re doping, from 2.362(13) T
for the undoped film to 1.740(03) T for 12.6 at\% Re doping. This study opens a
new direction of tuning the spin-mixing conductance in magnetic
heterostructures by doping of the ferromagnetic layerr, which is essential for
the realization of energy efficient operation of spintronic devices.
|
1903.10966v2
|
2019-09-12
|
Spin Transport in Thick Insulating Antiferromagnetic Films
|
Spin transport of magnonic excitations in uniaxial insulating
antiferromagnets (AFs) is investigated. In linear response to spin biasing and
a temperature gradient, the spin transport properties of
normal-metal--insulating antiferromagnet--normal-metal heterostructures are
calculated. We focus on the thick-film regime, where the AF is thicker than the
magnon equilibration length. This regime allows the use of a drift-diffusion
approach, which is opposed to the thin-film limit considered by Bender {\it et
al.} 2017, where a stochastic approach is justified. We obtain the temperature-
and thickness-dependence of the structural spin Seebeck coefficient
$\mathcal{S}$ and magnon conductance $\mathcal{G}$. In their evaluation we
incorporate effects from field- and temperature-dependent spin conserving
inter-magnon scattering processes. Furthermore, the interfacial spin transport
is studied by evaluating the contact magnon conductances in a microscopic model
that accounts for the sub-lattice symmetry breaking at the interface. We find
that while inter-magnon scattering does slightly suppress the spin Seebeck
effect, transport is generally unaffected, with the relevant spin decay length
being determined by non-magnon-conserving processes such as Gilbert damping. In
addition, we find that while the structural spin conductance may be enhanced
near the spin flip transition, it does not diverge due to spin impedance at the
normal metal|magnet interfaces.
|
1909.05881v2
|
2019-12-16
|
Spin-current manipulation of photoinduced magnetization dynamics in heavy metal / ferromagnet double layer based nanostructures
|
Spin currents offer a way to control static and dynamic magnetic properties,
and therefore they are crucial for next-generation MRAM devices or spin-torque
oscillators. Manipulating the dynamics is especially interesting within the
context of photo-magnonics. In typical $3d$ transition metal ferromagnets like
CoFeB, the lifetime of light-induced magnetization dynamics is restricted to
about 1 ns, which e.g. strongly limits the opportunities to exploit the wave
nature in a magnonic crystal filtering device. Here, we investigate the
potential of spin-currents to increase the spin wave lifetime in a functional
bilayer system, consisting of a heavy metal (8 nm of $\beta$-Tantalum
(Platinum)) and 5 nm CoFeB. Due to the spin Hall effect, the heavy metal layer
generates a transverse spin current when a lateral charge current passes
through the strip. Using time-resolved all-optical pump-probe spectroscopy, we
investigate how this spin current affects the magnetization dynamics in the
adjacent CoFeB layer. We observed a linear spin current manipulation of the
effective Gilbert damping parameter for the Kittel mode from which we were able
to determine the system's spin Hall angles. Furthermore, we measured a strong
influence of the spin current on a high-frequency mode. We interpret this mode
an an exchange dominated higher order spin-wave resonance. Thus we infer a
strong dependence of the exchange constant on the spin current.
|
1912.07728v1
|
2020-01-09
|
Role of longitudinal fluctuations in L$1_0$ FePt
|
L$1_0$ FePt is a technologically important material for a range of novel data
storage applications. In the ordered FePt structure the normally non-magnetic
Pt ion acquires a magnetic moment, which depends on the local field originating
from the neighboring Fe atoms. In this work a model of FePt is constructed,
where the induced Pt moment is simulated by using combined longitudinal and
rotational spin dynamics. The model is parameterized to include a linear
variation of the moment with the exchange field, so that at the Pt site the
magnetic moment depends on the Fe ordering. The Curie temperature of FePt is
calculated and agrees well with similar models that incorporate the Pt dynamics
through an effective Fe-only Hamiltonian. By computing the dynamic correlation
function the anisotropy field and the Gilbert damping are extracted over a
range of temperatures. The anisotropy exhibits a power-law dependence with
temperature with exponent $n\approx2.1$. This agrees well with what observed
experimentally and it is obtained without including a two-ion anisotropy term
as in other approaches. Our work shows that incorporating longitudinal
fluctuations into spin dynamics calculations is crucial for understanding the
properties of materials with induced moments.
|
2001.03074v1
|
2020-05-07
|
Effect of interfacial oxidation layer in spin pumping experiments on Ni$_{80}$Fe$_{20}$/SrIrO$_3$ heterostructures
|
SrIrO$_3$ with its large spin-orbit coupling and low charge conductivity has
emerged as a potential candidate for efficient spin-orbit torque magnetization
control in spintronic devices. We here report on the influence of an
interfacial oxide layer on spin pumping experiments in Ni$_{80}$Fe$_{20}$
(NiFe)/SrIrO$_3$ bilayer heterostructures. To investigate this scenario we have
carried out broadband ferromagnetic resonance (BBFMR) measurements, which
indicate the presence of an interfacial antiferromagnetic oxide layer. We
performed in-plane BBFMR experiments at cryogenic temperatures, which allowed
us to simultaneously study dynamic spin pumping properties (Gilbert damping)
and static magnetic properties (such as the effective magnetization and
magnetic anisotropy). The results for NiFe/SrIrO$_3$ bilayer thin films were
analyzed and compared to those from a NiFe/NbN/SrIrO$_3$ trilayer reference
sample, where a spin-transparent, ultra-thin NbN layer was inserted to prevent
oxidation of NiFe. At low temperatures, we observe substantial differences in
the magnetization dynamics parameters of these samples, which can be explained
by an antiferromagnetic interfacial layer in the NiFe/SrIrO$_3$ bilayers.
|
2005.03727v1
|
2020-05-28
|
Hard antinodal gap revealed by quantum oscillations in the pseudogap regime of underdoped high-$T_{\rm c}$ superconductors
|
An understanding of the missing antinodal electronic excitations in the
pseudogap state is essential for uncovering the physics of the underdoped
cuprate high temperature superconductors. The majority of high temperature
experiments performed thus far, however, have been unable to discern whether
the antinodal states are rendered unobservable due to their damping, or whether
they vanish due to their gapping. Here we distinguish between these two
scenarios by using quantum oscillations to examine whether the small Fermi
surface pocket, found to occupy only 2% of the Brillouin zone in the underdoped
cuprates, exists in isolation against a majority of completely gapped density
of states spanning the antinodes, or whether it is thermodynamically coupled to
a background of ungapped antinodal states. We find that quantum oscillations
associated with the small Fermi surface pocket exhibit a signature sawtooth
waveform characteristic of an isolated two-dimensional Fermi surface pocket.
This finding reveals that the antinodal states are destroyed by a hard gap that
extends over the majority of the Brillouin zone, placing strong constraints on
a drastic underlying origin of quasiparticle disappearance over almost the
entire Brillouin zone in the pseudogap regime.
|
2005.14123v1
|
2020-06-01
|
Enhancement in Thermally Generated Spin Voltage at Pd/NiFe$_2$O$_4$ Interfaces by the Growth on Lattice-Matched Substrates
|
Efficient spin injection from epitaxial ferrimagnetic NiFe$_2$O$_4$ thin
films into a Pd layer is demonstrated via spin Seebeck effect measurements in
the longitudinal geometry. The NiFe$_2$O$_4$ films (60 nm to 1 $\mu$m) are
grown by pulsed laser deposition on isostructural spinel MgAl$_2$O$_4$,
MgGa$_2$O$_4$, and CoGa$_2$O$_4$ substrates with lattice mismatch varying
between 3.2% and 0.2%. For the thinner films ($\leq$ 330 nm), an increase in
the spin Seebeck voltage is observed with decreasing lattice mismatch, which
correlates well with a decrease in the Gilbert damping parameter as determined
from ferromagnetic resonance measurements. High resolution transmission
electron microscopy studies indicate substantial decrease of antiphase boundary
and interface defects that cause strain-relaxation, i.e., misfit dislocations,
in the films with decreasing lattice mismatch. This highlights the importance
of reducing structural defects in spinel ferrites for efficient spin injection.
It is further shown that angle-dependent spin Seebeck effect measurements
provide a qualitative method to probe for in-plane magnetic anisotropies
present in the films.
|
2006.00777v1
|
2020-06-10
|
Study of magnetic interface and its effect in Fe/NiFe bilayers of alternating order
|
We present a comprehensive study on the magnetization reversal in Fe/NiFe
bilayer system by alternating the order of the magnetic layers. All the samples
show growth-induced uniaxial magnetic anisotropy due to oblique angle
deposition technique. Strong interfacial exchange coupling between the Fe and
NiFe layers leads to the single-phase hysteresis loops in the bilayer system.
The strength of coupling being dependent on the interface changes upon
alternating the order of magnetic layers. The magnetic parameters such as
coercivity HC, and anisotropy field HK become almost doubled when NiFe layer is
grown over the Fe layers. This enhancement in the magnetic parameters is
primarily dependent on the increase of the thickness and magnetic moment of
Fe-NiFe interfacial layer as revealed from the polarized neutron reectivity
(PNR) data of the bilayer samples. The difference in the thickness and
magnetization of the Fe-NiFe interfacial layer indicates the modification of
the microstructure by alternating the order of the magnetic layers of the
bilayers. The interfacial magnetic moment increased by almost 18 % when NiFe
layer is grown over the Fe layer. In spite of the different values of
anisotropy fields and modified interfacial exchange coupling, the Gilbert
damping constant values of the ferromagnetic bilayers remain similar to single
NiFe layer.
|
2006.05756v1
|
2020-09-07
|
Spin pumping in d-wave superconductor/ferromagnet hybrids
|
Spin-pumping across ferromagnet/superconductor (F/S) interfaces has attracted
much attention lately. Yet the focus has been mainly on s-wave
superconductors-based systems whereas (high-temperature) d-wave superconductors
such as YBa2Cu3O7-d (YBCO) have received scarce attention despite their
fundamental and technological interest. Here we use wideband ferromagnetic
resonance to study spin-pumping effects in bilayers that combine a soft
metallic Ni80Fe20 (Py) ferromagnet and YBCO. We evaluate the spin conductance
in YBCO by analyzing the magnetization dynamics in Py. We find that the Gilbert
damping exhibits a drastic drop as the heterostructures are cooled across the
normal-superconducting transition and then, depending on the S/F interface
morphology, either stays constant or shows a strong upturn. This unique
behavior is explained considering quasiparticle density of states at the YBCO
surface, and is a direct consequence of zero-gap nodes for particular
directions in the momentum space. Besides showing the fingerprint of d-wave
superconductivity in spin-pumping, our results demonstrate the potential of
high-temperature superconductors for fine tuning of the magnetization dynamics
in ferromagnets using k-space degrees of freedom of d-wave/F interfaces.
|
2009.03196v3
|
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