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2022-03-10 | Accelerated gradient methods combining Tikhonov regularization with geometric damping driven by the Hessian | In a Hilbert setting, for convex differentiable optimization, we consider
accelerated gradient dynamics combining Tikhonov regularization with
Hessian-driven damping. The Tikhonov regularization parameter is assumed to
tend to zero as time tends to infinity, which preserves equilibria. The
presence of the Tikhonov regularization term induces a strong convexity
property which vanishes asymptotically. To take advantage of the exponential
convergence rates attached to the heavy ball method in the strongly convex
case, we consider the inertial dynamic where the viscous damping coefficient is
taken proportional to the square root of the Tikhonov regularization parameter,
and therefore also converges towards zero. Moreover, the dynamic involves a
geometric damping which is driven by the Hessian of the function to be
minimized, which induces a significant attenuation of the oscillations. Under
an appropriate tuning of the parameters, based on Lyapunov's analysis, we show
that the trajectories have at the same time several remarkable properties: they
provide fast convergence of values, fast convergence of gradients towards zero,
and strong convergence to the minimum norm minimizer. This study extends a
previous paper by the authors where similar issues were examined but without
the presence of Hessian driven damping. | 2203.05457v2 |
2022-08-25 | The Effect of Frequency Droop Damping on System Parameters and Battery Sizing During Load Change Condition | Inverter-based resources (IBR) have been widely studied for their advantages
on the current power systems. This increase in the penetration of renewable
energy has raised some concerns about the stability of the existing grid.
Historically, power systems are dominated by synchronous generators that can
easily react to system instability due to high inertia and damping
characteristics. However, with IBR, the control of the inverter plays a crucial
role in contributing to the system stability and enhancing the functionality of
the inverters. One of these novel control methods is droop control. Droop
characteristics are used to control voltage, frequency, and active and reactive
power. This paper presents the impact of frequency droop damping on system
frequency, real power, and the rate of change of frequency with distributed
energy resources. Also, battery sizing is suggested based on the results. The
results also show the need for optimal selection for the frequency droop
damping to fulfill the appropriate battery size in terms of cost and
performance. The simulations are carried out in an electromagnetic transient
program (EMTP) | 2208.12291v1 |
2023-07-28 | Premature jump-down mimicks nonlinear damping in nanoresonators | Recent experiments on nano-resonators in a bistable regime use the
`jump-down' point between states to infer mechanical properties of the membrane
or a load, but often suggest the presence of some nonlinear damping. Motivated
by such experiments, we develop a mechanical model of a membrane subject to a
uniform, oscillatory load and linear damping. We solve this model numerically
and compare its jump-down behaviour with standard asymptotic predictions for a
one-dimensional Duffing oscillator with strain stiffening. We show that the
axisymmetric, but spatially-varying, problem can be mapped to the Duffing
problem with coefficients determined rationally from the model's Partial
Differential Equations. However, we also show that jump-down happens earlier
than expected (i.e.~at lower frequency, and with a smaller oscillation
amplitude). Although this premature jump-down is often interpreted as the
signature of a nonlinear damping in experiments, its appearance in numerical
simulations with only linear damping suggests instead that indicate that the
limitations of asymptotic results may, at least sometimes, be the cause. We
therefore suggest that care should be exercised in interpreting the results of
nano-resonator experiments. | 2307.15656v1 |
2023-10-11 | Damping Density of an Absorptive Shoebox Room Derived from the Image-Source Method | The image-source method is widely applied to compute room impulse responses
(RIRs) of shoebox rooms with arbitrary absorption. However, with increasing RIR
lengths, the number of image sources grows rapidly, leading to slow
computation. In this paper, we derive a closed-form expression for the damping
density, which characterizes the overall multi-slope energy decay. The
omnidirectional energy decay over time is directly derived from the damping
density. The resulting energy decay model accurately matches the late
reverberation simulated via the image-source method. The proposed model allows
the fast stochastic synthesis of late reverberation by shaping noise with the
energy envelope. Simulations of various wall damping coefficients demonstrate
the model's accuracy. The proposed model consistently outperforms the energy
decay prediction accuracy compared to a state-of-the-art approximation method.
The paper elaborates on the proposed damping density's applicability to
modeling multi-sloped sound energy decay, predicting reverberation time in
non-diffuse sound fields, and fast frequency-dependent RIR synthesis. | 2310.07363v1 |
2003-11-17 | Cosmic Ray Scattering by Compressible Magnetohydrodynamic Turbulence | Recent advances in understanding of magnetohydrodynamic (MHD) turbulence call
for substantial revisions in the picture of cosmic ray transport. In this paper
we use recently obtained scaling laws for MHD modes to calculate the scattering
frequency for cosmic rays in the ISM. We consider gyroresonance with MHD modes
(Alfvenic, slow and fast) and transit-time damping (TTD) by fast modes. We
provide calculations of cosmic ray scattering for various phases of
interstellar medium with realistic interstellar turbulence driving that is
consistent with the velocity dispersions observed in diffuse gas. We account
for the turbulence cutoff arising from both collisional and collisionless
damping. We obtain analytical expressions for diffusion coefficients that enter
Fokker-Planck equation describing cosmic ray evolution. We calculate the
scattering rate and parallel spatial diffusion coefficients of cosmic rays for
both Alfvenic and fast modes. We conclude that fast modes provides the dominant
contribution to cosmic ray scattering for the typical interstellar conditions
in spite of the fact that fast modes are subjected to damping. We show that the
efficiency of the scattering depends on the plasma beta since it determines the
damping of the fast modes. We also show that the streaming instability is
modified in the presence of turbulence. | 0311369v1 |
2010-03-16 | Justification of the symmetric damping model of the dynamical Casimir effect in a cavity with a semiconductor mirror | A "microscopic" justification of the "symmetric damping" model of a quantum
oscillator with time-dependent frequency and time-dependent damping is given.
This model is used to predict results of experiments on simulating the
dynamical Casimir effect in a cavity with a photo-excited semiconductor mirror.
It is shown that the most general bilinear time-dependent coupling of a
selected oscillator (field mode) to a bath of harmonic oscillators results in
two equal friction coefficients for the both quadratures, provided all the
coupling coefficients are proportional to a single arbitrary function of time
whose duration is much shorter than the periods of all oscillators. The choice
of coupling in the rotating wave approximation form leads to the "mimimum
noise" model of the quantum damped oscillator, introduced earlier in a pure
phenomenological way. | 1003.3061v2 |
2022-07-13 | Energy decay for the time dependent damped wave equation | Energy decay is established for the damped wave equation on compact
Riemannian manifolds where the damping coefficient is allowed to depend on
time. Using a time dependent observability inequality, it is shown that the
energy of solutions decays at an exponential rate if the damping coefficient
satisfies a time dependent analogue of the classical geometric control
condition. Existing time dependent observability inequalities are improved by
removing technical assumptions on the permitted initial data. | 2207.06260v4 |
2022-10-10 | Finite time extinction for a critically damped Schr{ö}dinger equation with a sublinear nonlinearity | This paper completes some previous studies by several authors on the finite
time extinction for nonlinear Schr{\"o}dinger equation when the nonlinear
damping term corresponds to the limit cases of some ``saturating non-Kerr law''
$F(|u|^2)u=\frac{a}{\varepsilon+(|u|^2)^\alpha}u,$ with $a\in\mathbb{C},$
$\varepsilon\geqslant0,$ $2\alpha=(1-m)$ and $m\in[0,1).$ Here we consider the
sublinear case $0<m<1$ with a critical damped coefficient: $a\in\mathbb{C}$ is
assumed to be in the set $D(m)=\big\{z\in\mathbb{C}; \; \mathrm{Im}(z)>0 \text{
and } 2\sqrt{m}\mathrm{Im}(z)=(1-m)\mathrm{Re}(z)\big\}.$ Among other things,
we know that this damping coefficient is critical, for instance, in order to
obtain the monotonicity of the associated operator (see the paper by Liskevich
and Perel'muter [16] and the more recent study by Cialdea and Maz'ya [14]). The
finite time extinction of solutions is proved by a suitable energy method after
obtaining appropiate a priori estimates. Most of the results apply to
non-necessarily bounded spatial domains. | 2210.04493v4 |
2023-12-28 | Cause-effect relationship between model parameters and damping performance of hydraulic shock absorbers | Despite long-term research and development of modern shock absorbers, the
effect of variations of several crucial material and model parameters still
remains dubious. The goal of this work is therefore a study of the changes of
shock absorber dynamics with respect to typical parameter ranges in a realistic
model. We study the impact of shim properties, as well as geometric features
such as discharge coefficients and bleed orifice cross section. We derive
cause-effect relationships by nonlinear parameter fitting of the differential
equations of the model and show digressive and progressive quadratic damping
curves for shim number and thickness, sharp exponential curves for discharge
coefficients, and leakage width, as well as a linear decrease of damping
properties with bleed orifice area. Temperature increase affecting material
properties, such as density and viscosity of the mineral oil, is found to have
a mostly linear relationship with damping and pressure losses. Our results are
not only significant for the general understanding of shock absorber dynamics,
but also serve as a guidance for the development of specific models by
following the proposed methodology. | 2312.17175v1 |
2024-01-04 | A Pure Integral-Type PLL with a Damping Branch to Enhance the Stability of Grid-Tied Inverter under Weak Grids | In a phase-locked loop (PLL) synchronized inverter, due to the strong
nonlinear coupling between the PLL's parame-ters and the operation power angle,
the equivalent damping coefficient will quickly deteriorate while the power
angle is close to 90{\deg} under an ultra-weak grid, which causes the
synchronous instability. To address this issue, in this letter, a pure
integral-type phase-locked loop (IPLL) with a damping branch is proposed to
replace the traditional PI-type PLL. The equivalent damping coefficient of an
IPLL-synchronized inverter is decoupled with the steady-state power angle. As a
result, the IPLL-synchronized inverter can stably operate under an ultra-weak
grid when the equilibrium point exists. Finally, time-domain simulation results
verify the effectiveness and correctness of the proposed IPLL. | 2401.02202v1 |
2024-01-16 | Waves in strong centrifugal filed: dissipative gas | In the fast rotating gas (with the velocity typical for Iguassu gas
centrifuge) three families of linear waves exist with different polarizations
and law of dispersion. The energy of the waves is basically concentrated at the
axis of rotation in the rarefied region. Therefore these waves decay on the
distance comparable with the wavelength. There is only one type of waves
propagating strictly along the axis of rotation with the law of dispersion
similar to ordinary acoustic waves. These waves are interested for the physics
of gas centrifuges. The energy density of these waves concentrates at the wall
of the rotor. These waves have weak damping due to the molecular viscosity and
heat conductivity. The damping coefficient is determined for this type of waves
by numerical calculations. Analytical approximations for the damping
coefficient is defined as well. At the parameters typical for the Iguassu
centrifuge the damping is defined by interaction of the waves with the rotor
wall. | 2401.08240v1 |
2024-01-19 | Upper bound of the lifespan of the solution to the nonlinear fractional wave equations with time-dependent damping | In this paper, we study the Cauchy problem of the nonlinear wave equation
with fractional Laplacian and time-dependent damping. Firstly, we derive the
weighted Sobolev estimate of the solution operators for the linear wave
equation with the damping of constant coefficient, and prove the local
existence and uniqueness in the weighted Sobolev space for the power-type
nonlinearity and $b(t)\in L^\infty$, by the contraction mapping principle.
Secondly, we consider the case of the source nonlinearity $f(u)\approx |u|^p$.
In the subcritical and critical cases $1<p\leq p_c=1+\frac \sigma N$, based on
the blow-up result on the ordinary differential inequality, we could prove the
blow-up of the solution and obtain the upper bound of the lifespan. And the
upper bound of the lifespan in the critical case is independent on the
coefficient of the time-dependent damping and is completely new even if the
classical case $b(t)=1$. | 2401.10552v1 |
2014-01-08 | Dynamic exchange via spin currents in acoustic and optical modes of ferromagnetic resonance in spin-valve structures | Two ferromagnetic layers magnetically decoupled by a thick normal metal
spacer layer can be, nevertheless, dynamically coupled via spin currents
emitted by the spin-pump and absorbed through the spin-torque effects at the
neighboring interfaces. A decrease of damping in both layers due to a partial
compensation of the angular momentum leakage in each layer was previously
observed at the coincidence of the two ferromagnetic resonances. In case of
non-zero magnetic coupling, such a dynamic exchange will depend on the mutual
precession of the magnetic moments in the layers. A difference in the linewidth
of the resonance peaks is expected for the acoustic and optical regimes of
precession. However, the interlayer coupling hybridizes the resonance responses
of the layers and therefore can also change their linewidths. The interplay
between the two mechanisms has never been considered before. In the present
work, the joint influence of the hybridization and non-local damping on the
linewidth has been studied in weakly coupled NiFe/CoFe/Cu/CoFe/MnIr spin-valve
multilayers. It has been found that the dynamic exchange by spin currents is
different in the optical and acoustic modes, and this difference is dependent
on the interlayer coupling strength. In contrast to the acoustic precession
mode, the dynamic exchange in the optical mode works as an additional damping
source. A simulation in the framework of the Landau-Lifshitz-Gilbert formalism
for two ferromagnetic layers coupled magnetically and by spin currents has been
done to separate the effects of the non-local damping from the resonance modes
hybridization. In our samples both mechanisms bring about linewidth changes of
the same order of magnitude, but lead to a distinctly different angular
behavior. The obtained results are relevant for a broad class of coupled
magnetic multilayers with ballistic regime of the spin transport. | 1401.1672v1 |
2020-08-29 | Exploring a quantum-information-relevant magnonic material: Ultralow damping at low temperature in the organic ferrimagnet V[TCNE]x | Quantum information science and engineering requires novel low-loss magnetic
materials for magnon-based quantum-coherent operations. The search for low-loss
magnetic materials, traditionally driven by applications in microwave
electronics near room-temperature, has gained additional constraints from the
need to operate at cryogenic temperatures for many applications in quantum
information science and technology. Whereas yttrium iron garnet (YIG) has been
the material of choice for decades, the emergence of molecule-based materials
with robust magnetism and ultra-low damping has opened new avenues for
exploration. Specifically, thin-films of vanadium tetracyanoethylene (V[TCNE]x)
can be patterned into the multiple, connected structures needed for hybrid
quantum elements and have shown room-temperature Gilbert damping ({\alpha} = 4
\times 10^-5) that rivals the intrinsic (bulk) damping otherwise seen only in
highly-polished YIG spheres (far more challenging to integrate into arrays).
Here, we present a comprehensive and systematic study of the low-temperature
magnetization dynamics for V[TCNE]x thin films, with implications for their
application in quantum systems. These studies reveal a temperature-driven,
strain-dependent magnetic anisotropy that compensates the thin-film shape
anisotropy, and the recovery of a magnetic resonance linewidth at 5 K that is
comparable to room-temperature values (roughly 2 G at 9.4 GHz). We can account
for these variations of the V[TCNE]x linewidth within the context of scattering
from very dilute paramagnetic impurities, and anticipate additional linewidth
narrowing as the temperature is further reduced. | 2008.13061v3 |
2011-09-08 | On the attenuation coefficient of monomode periodic waveguides | It is widely accepted that, on ensemble average, the transmission T of guided
modes decays exponentially with the waveguide length L due to small
imperfections, leading to the important figure of merit defined as the
attenuation-rate coefficient alpha = -<ln(T)>/L. In this letter, we evidence
that the exponential-damping law is not valid in general for periodic monomode
waveguides, especially as the group velocity decreases. This result that
contradicts common beliefs and experimental practices aiming at measuring alpha
is supported by a theoretical study of light transport in the limit of very
small imperfections, and by numerical results obtained for two waveguide
geometries that offer contrasted damping behaviours. | 1109.1642v1 |
2011-12-31 | Stability of cnoidal waves in the parametrically driven nonlinear Schrödinger equation | The parametrically driven, damped nonlinear Schr\"odinger equation has two
cn- and two dn-wave solutions. We show that one pair of the cn and dn solutions
is unstable for any combination of the driver's strength, dissipation
coefficient and spatial period of the wave; this instability is against
periodic perturbations. The second dn-wave solution is shown to be unstable
against antiperiodic perturbations --- in a certain region of the parameter
space. We also consider quasiperiodic perturbations with long modulation
wavelength, in the limit where the driving strength is only weakly exceeding
the damping coefficient. | 1201.0263v1 |
2016-04-20 | Reconstruction for multiwave imaging in attenuating media with large damping coefficient | In this article we study the reconstruction problem in TAT/PAT on an
attenuating media. Namely, we prove a reconstruction procedure of the initial
condition for the damped wave equation via Neumann series that works for
arbitrary large smooth attenuation coefficients extending the result of Homan
in [1]. We also illustrate the theoretical result by including some numerical
experiments at the end of the paper. | 1604.06068v3 |
2016-09-20 | H{ö}lder stability in determining the potential and the damping coefficient in a wave equation | We improve the preceding results obtained by the first and the second authors
in [3]. They concern the stability issue of the inverse problem that consists
in determining the potential and the damping coefficient in a wave equation
from an initial-to-boundary operator. We partially modify the arguments in [3]
to show that actually we have H{\"o}lder stability instead of logarithmic
stability. | 1609.06102v1 |
2021-11-30 | Determining damping terms in fractional wave equations | This paper deals with the inverse problem of recovering an arbitrary number
of fractional damping terms in a wave equation. We develop several approaches
on uniqueness and reconstruction, some of them relying on Tauberian theorems on
the relation between the asymptotics of solutions in time and Laplace domain.
Also the possibility of additionally recovering space dependent coefficients or
initial data is discussed. The resulting methods for reconstructing
coefficients and fractional orders in these terms are tested numerically.
Additionally, we provide an analysis of the forward problem, a multiterm
fractional wave equation. | 2112.00080v2 |
2023-06-28 | Global solutions and blow-up for the wave equation with variable coefficients: II. boundary supercritical source | In this paper, we consider the wave equation with variable coefficients and
boundary damping and supercritical source terms. The goal of this work is
devoted to prove the local and global existence, and classify decay rate of
energy depending on the growth near zero on the damping term. Moreover, we
prove the blow-up of the weak solution with positive initial energy as well as
nonpositive initial energy. | 2306.15897v4 |
2023-07-29 | An inverse problem for the fractionally damped wave equation | We consider an inverse problem for a Westervelt type nonlinear wave equation
with fractional damping. This equation arises in nonlinear acoustic imaging,
and we show the forward problem is locally well-posed. We prove that the smooth
coefficient of the nonlinearity can be uniquely determined, based on the
knowledge of the source-to-solution map and a priori knowledge of the
coefficient in an arbitrarily small subset of the domain. Our approach relies
on a second order linearization as well as the unique continuation property of
the spectral fractional Laplacian. | 2307.16065v1 |
2011-11-09 | Stabilization by switching control methods | In this paper we consider some stabilization problems for the wave equation
with switching. We prove exponential stability results for appropriate damping
coefficients. The proof of the main results is based on D'Alembert formula and
some energy estimates. | 1111.2171v1 |
2015-03-31 | Existence of the global attractor for the plate equation with nonlocal nonlinearity in R^{n} | We consider Cauchy problem for the semilinear plate equation with nonlocal
nonlinearity. Under mild conditions on the damping coefficient, we prove that
the semigroup generated by this problem possesses a global attractor. | 1503.09123v1 |
2021-03-29 | Nonequilibrium Dynamics of the Chiral Quark Condensate under a Strong Magnetic Field | Strong magnetic fields impact quantum-chromodynamics (QCD) properties in
several situations; examples include the early universe, magnetars, and
heavy-ion collisions. These examples share a common trait: time evolution. A
prominent QCD property impacted by a strong magnetic field is the quark
condensate, an approximate order parameter of the QCD transition between a
high-temperature quark-gluon phase and a low-temperature hadronic phase. We use
the linear sigma model with quarks to address the quark condensate time
evolution under a strong magnetic field. We use the closed time path formalism
of nonequilibrium quantum field theory to integrate out the quarks and obtain a
mean-field Langevin equation for the condensate. The Langevin equation features
dissipation and noise kernels controlled by a damping coefficient. We compute
the damping coefficient for magnetic field and temperature values achieved in
peripheral relativistic heavy-ion collisions and solve the Langevin equation
for a temperature quench scenario. The magnetic field changes the dissipation
and noise pattern by increasing the damping coefficient compared to the
zero-field case. An increased damping coefficient increases fluctuations and
time scales controlling condensate's short-time evolution, a feature that can
impact hadron formation at the QCD transition. The formalism developed here can
be extended to include other order parameters, hydrodynamic modes, and system's
expansion to address magnetic field effects in complex settings as heavy-ion
collisions, the early universe, and magnetars. | 2103.15665v1 |
2003-04-18 | Elementary Excitations of Ferromagnetic Metal Nanoparticles | We present a theory of the elementary spin excitations in transition metal
ferromagnet nanoparticles which achieves a unified and consistent quantum
description of both collective and quasiparticle physics. The theory starts by
recognizing the essential role played by spin-orbit interactions in determining
the energies of ferromagnetic resonances in the collective excitation spectrum
and the strength of their coupling to low-energy particle-hole excitations. We
argue that a crossover between Landau-damped ferromagnetic resonance and
pure-state collective magnetic excitations occurs as the number of atoms in
typical transition metal ferromagnet nanoparticles drops below approximately
$10^4$, approximately where the single-particle level spacing, $\delta$,
becomes larger than, $\sqrt{\alpha} E_{\rm res}$, where $E_{\rm res}$ is the
ferromagnetic resonance frequency and $\alpha$ is the Gilbert damping
parameter. We illustrate our ideas by studying the properties of semi-realistic
model Hamiltonians, which we solve numerically for nanoparticles containing
several hundred atoms. For small nanoparticles, we find one isolated
ferromagnetic resonance collective mode below the lowest particle-hole
excitation energy, at $E_{\rm res} \approx 0.1$ meV. The spectral weight of
this pure excitation nearly exhausts the transverse dynamical susceptibility
spectral weight. As $\delta$ approaches $\sqrt{\alpha} E_{\rm res}$, the
ferromagnetic collective excitation is more likely to couple strongly with
discrete particle-hole excitations. In this regime the distinction between the
two types of excitations blurs. We discuss the significance of this picture for
the interpretation of recent single-electron tunneling experiments. | 0304427v1 |
2014-08-02 | Tunnel magnetoresistance and spin-transfer-torque switching in polycrystalline Co2FeAl full-Heusler alloy magnetic tunnel junctions on Si/SiO2 amorphous substrates | We studied polycrystalline B2-type Co2FeAl (CFA) full-Heusler alloy based
magnetic tunnel junctions (MTJs) fabricated on a Si/SiO2 amorphous substrate.
Polycrystalline CFA films with a (001) orientation, a high B2 ordering, and a
flat surface were achieved using a MgO buffer layer. A tunnel magnetoresistance
(TMR) ratio up to 175% was obtained for an MTJ with a CFA/MgO/CoFe structure on
a 7.5-nm-thick MgO buffer. Spin-transfer torque induced magnetization switching
was achieved in the MTJs with a 2-nm-thick polycrystalline CFA film as a
switching layer. Using a thermal activation model, the intrinsic critical
current density (Jc0) was determined to be 8.2 x 10^6 A/cm^2, which is lower
than 2.9 x 10^7 A/cm^2, the value for epitaxial CFA-MTJs [Appl. Phys. Lett.
100, 182403 (2012)]. We found that the Gilbert damping constant evaluated using
ferromagnetic resonance measurements for the polycrystalline CFA film was
~0.015 and was almost independent of the CFA thickness (2~18 nm). The low Jc0
for the polycrystalline MTJ was mainly attributed to the low damping of the CFA
layer compared with the value in the epitaxial one (~0.04). | 1408.0341v1 |
2018-02-20 | Ultrafast magnetization dynamics in pure and doped Heusler and inverse Heusler alloys | By using a multiscale approach based on first-principles density functional
theory combined with atomistic spin dynamics, we investigate the electronic
structure and magnetization dynamics of an inverse Heusler and a Heusler
compound and their alloys, i. e. Mn$_{2-x}Z_x$CoAl and Mn$_{2-x}Z_x$VAl, where
$Z$ = Mo, W, Os and Ru, respectively. A signature of the ferrimagnetic ordering
of Mn$_{2}$CoAl and Mn$_{2}$VAl Heusler alloys is reflected in the calculated
Heisenberg exchange constants. They decay very rapidly with the interatomic
distance and have short range, which is a consequence of the existence of the
finite gap in the minority spin band. The calculated Gilbert damping parameter
of both Mn$_2$CoAl and Mn$_2$VAl is high compared to other half-metals, but
interestingly in the particular case of the inverse Mn$_{2}$CoAl alloys and due
to the spin-gapless semiconducting property, the damping parameters decrease
with the doping concentration in clear contradiction to the general trend.
Atomistic spin dynamics simulations predict ultrafast magnetisation switching
in Mn$_{2}$CoAl and Mn$_{2}$VAl under the influence of an external magnetic
field, starting from a threshold field of $2\text{T}$. Our overall finding
extends with Heusler and inverse Heusler alloys, the class of materials that
exhibits laser induced magnetic switching. | 1802.07195v1 |
2018-04-10 | GONG Catalog of Solar Filament Oscillations Near Solar Maximum | We have catalogued 196 filament oscillations from the GONG $H{\alpha}$
network data during several months near the maximum of solar cycle 24 (January
- June 2014). Selected examples from the catalog are described in detail, along
with our statistical analyses of all events. Oscillations were classified
according to their velocity amplitude: 106 small-amplitude oscillations (SAOs),
with velocities $<10\mathrm{\, km \; s^{-1}}$, and 90 large-amplitude
oscillations (LAOs), with velocities $>10\mathrm{\, km \; s^{-1}}$. Both SAOs
and LAOs are common, with one event of each class every two days on the visible
side of the Sun. For nearly half of the events we identified their apparent
trigger. The period distribution has a mean value of 58$\pm$15 min for both
types of oscillations. The distribution of the damping time per period peaks at
$\tau/P=1.75$ and $1.25$ for SAOs and LAOs respectively. We confirmed that LAO
damping rates depend nonlinearly on the oscillation velocity. The angle between
the direction of motion and the filament spine has a distribution centered at
$27^\circ$ for all filament types. This angle agrees with the observed
direction of filament-channel magnetic fields, indicating that most of the
catalogued events are longitudinal (i.e., undergo field-aligned motions). We
applied seismology to determine the average radius of curvature in the magnetic
dips, $R\approx89$ Mm, and the average minimum magnetic-field strength,
$B\approx16$ G. The catalog is available to the community online, and is
intended to be expanded to cover at least 1 solar cycle. | 1804.03743v1 |
2018-10-16 | Spin-wave-induced lateral temperature gradient in a YIG thin film/GGG system excited in an ESR cavity | Lateral thermal gradient of an yttrium iron garnet (YIG) film under the
microwave application in the cavity of the electron spin resonance system (ESR)
was measured at room temperature by fabricating a Cu/Sb thermocouple onto it.
To date, thermal transport in YIG films caused by the Damon-Eshbach mode (DEM)
- the unidirectional spin-wave heat conveyer effect - was demonstrated only by
the excitation using coplanar waveguides. Here we show that effect exists even
under YIG excitation using the ESR cavity - tool often employed to realize spin
pumping. The temperature difference observed around the ferromagnetic resonance
(FMR) field under the 4 mW microwave power peaked at 13 mK. The observed
thermoelectric signal indicates the imbalance of the population between the
DEMs that propagate near the top and bottom surfaces of the YIG film. We
attribute the DEM population imbalance to the different magnetic damping near
the top and bottom YIG surfaces. Additionally, the spin wave dynamics of the
system were investigated using the micromagnetic simulations. The micromagnetic
simulations confirmed the existence of the DEM imbalance in the system with the
increased Gilbert damping at one of the YIG interfaces. The reported results
are indispensable for the quantitative estimation of the electromotive force in
the spin-charge conversion experiments using ESR cavities. | 1810.06875v1 |
2021-08-24 | Shape anisotropy effect on magnetization reversal induced by linear down chirp pulse | We investigate the influence of shape anisotropy on the magnetization
reversal of a single-domain magnetic nanoparticle driven by a circularly
polarized linear down-chirp microwave field pulse (DCMP). Based on the
Landau-Lifshitz-Gilbert equation, numerical results show that the three
controlling parameters of DCMP, namely, microwave amplitude, initial frequency
and chirp rate, decrease with the increase of shape anisotropy. For certain
shape anisotropy, the reversal time significantly reduces. These findings are
related to the competition of shape anisotropy and uniaxial magnetocrystalline
anisotropy and thus to the height of energy barrier which separates the two
stable states. The result of damping dependence of magnetization reversal
indicates that for a certain sample shape, there exists an optimal damping
situation at which magnetization is fastest. Moreover, it is also shown that
the required microwave field amplitude can be lowered by applying the
spin-polarized current simultaneously. The usage of an optimum combination of
both microwave field pulse and current is suggested to achieve cost efficiency
and faster switching. So these findings may provide the knowledge to fabricate
the shape of a single domain nanoparticle for the fast and power-efficient
magnetic data storage device. | 2108.10965v2 |
2021-11-23 | Resonant dynamics of skyrmion lattices in thin film multilayers: Localised modes and spin wave emission | The spectral signatures of magnetic skyrmions under microwave field
excitation are of fundamental interest and can be an asset for high frequency
applications. These topological solitons can be tailored in multilayered thin
films, but the experimental observation of their spin wave dynamics remains
elusive, in particular due to large damping. Here, we study Pt/FeCoB/AlO$_x$
multilayers hosting dense and robust skyrmion lattices at room temperature with
Gilbert damping of $\sim 0.02$. We use magnetic force microscopy to
characterise their static magnetic phases and broadband ferromagnetic resonance
to probe their high frequency response. Micromagnetic simulations reproduce the
experiments with accuracy and allow us to identify distinct resonant modes
detected in the skyrmion lattice phase. Low ($<$ 2 GHz) and intermediate
frequency ($2-8$ GHz) modes involve excitations localised to skyrmion edges in
conjunction with precession of the uniform background magnetisation, while a
high frequency ($>$ 12 GHz) mode corresponds to in-phase skyrmion core
precession emitting spin waves into uniform background with wavelengths in the
50--80 nm range commensurate with the lattice structure. These findings could
be instrumental in the investigation of room temperature wave scattering and
the implementation of novel microwave processing schemes in reconfigurable
arrays of solitons. | 2111.11797v2 |
2022-05-20 | Effects of Crystalline Disorder on Interfacial and Magnetic Properties of Sputtered Topological Insulator/Ferromagnet Heterostructures | Thin films of Topological insulators (TIs) coupled with ferromagnets (FMs)
are excellent candidates for energy-efficient spintronics devices. Here, the
effect of crystalline structural disorder of TI on interfacial and magnetic
properties of sputter-deposited TI/FM, Bi2Te3/Ni80Fe20, heterostructures is
reported. Ni and a smaller amount of Fe from Py was found to diffuse across the
interface and react with Bi2Te3. For highly crystalline c-axis oriented Bi2Te3
films, a giant enhancement in Gilbert damping is observed, accompanied by an
effective out-of-plane magnetic anisotropy and enhanced damping-like spin-orbit
torque (DL-SOT), possibly due to the topological surface states (TSS) of
Bi2Te3. Furthermore, a spontaneous exchange bias is observed in hysteresis loop
measurements at low temperatures. This is because of an antiferromagnetic
topological interfacial layer formed by reaction of the diffused Ni with Bi2Te3
which couples with the FM, Ni80Fe20. For increasing disorder of Bi2Te3, a
significant weakening of exchange interaction in the AFM interfacial layer is
found. These experimental results Abstract length is one paragraph. | 2205.09913v1 |
2022-12-24 | Anatomy of ultrafast quantitative magneto-acoustics in freestanding nickel thin films | We revisit the quantitative analysis of the ultrafast magneto-acoustic
experiment in a freestanding nickel thin film by Kim and Bigot [1] by applying
our recently proposed approach of magnetic and acoustic eigenmodes
decomposition by Vernik et al. [2]. We show that the application of our
modeling to the analysis of time-resolved reflectivity measurements allows for
the determination of amplitudes and lifetimes of standing perpendicular
acoustic phonon resonances with unprecedented accuracy. The acoustic damping is
found to scale as $\propto\omega^2$ for frequencies up to 80~GHz and the peak
amplitudes reach $10^{-3}$. The experimentally measured magnetization dynamics
for different orientations of an external magnetic field agrees well with
numerical solutions of magneto-elastically driven magnon harmonic oscillators.
Symmetry-based selection rules for magnon-phonon interactions predicted by our
modeling approach allow for the unambiguous discrimination between spatially
uniform and non-uniform modes, as confirmed by comparing the resonantly
enhanced magneto-elastic dynamics simultaneously measured on opposite sides of
the film. Moreover, the separation of time scales for (early) rising and (late)
decreasing precession amplitudes provide access to magnetic (Gilbert) and
acoustic damping parameters in a single measurement. | 2212.12673v1 |
2023-05-16 | Non-Hermitian Casimir Effect of Magnons | There has been a growing interest in non-Hermitian quantum mechanics. The key
concepts of quantum mechanics are quantum fluctuations. Quantum fluctuations of
quantum fields confined in a finite-size system induce the zero-point energy
shift. This quantum phenomenon, the Casimir effect, is one of the most striking
phenomena of quantum mechanics in the sense that there are no classical analogs
and has been attracting much attention beyond the hierarchy of energy scales,
ranging from elementary particle physics to condensed matter physics, together
with photonics. However, the non-Hermitian extension of the Casimir effect and
the application to spintronics have not yet been investigated enough, although
exploring energy sources and developing energy-efficient nanodevices are its
central issues. Here we fill this gap. By developing a magnonic analog of the
Casimir effect into non-Hermitian systems, we show that this non-Hermitian
Casimir effect of magnons is enhanced as the Gilbert damping constant (i.e.,
the energy dissipation rate) increases. When the damping constant exceeds a
critical value, the non-Hermitian Casimir effect of magnons exhibits an
oscillating behavior, including a beating one, as a function of the film
thickness and is characterized by the exceptional point. Our result suggests
that energy dissipation serves as a key ingredient of Casimir engineering. | 2305.09231v1 |
2008-02-21 | Gas Damping Coefficient Research for MEMS Comb Linear Vibration Gyroscope | Silicon-MEMS gyroscope is an important part of MEMS (Micro Electrical
Mechanical System). There are some disturb ignored in traditional gyroscope
that must be evaluated newly because of its smaller size (reach the level of
micron). In these disturb, the air pressure largely influences the performance
of MEMS gyroscope. Different air pressure causes different gas damping
coefficient for the MEMS comb linear vibration gyroscope and different gas
damping coefficient influences the quality factor of the gyroscope directive.
The quality factor influences the dynamic working bandwidth of the MEMS comb
linear vibration gyroscope, so it is influences the output characteristic of
the MEMS comb linear vibration gyroscope. The paper shows the relationship
between the air pressure and the output amplified and phase of the detecting
axis through analyzing the air pressure influence on the MEMS comb linear
vibration gyroscope. It discusses the influence on the frequency distribute and
quality factor of the MEMS comb linear vibration gyroscope for different air
pressure. | 0802.3048v1 |
2008-11-13 | Higher order energy decay rates for damped wave equations with variable coefficients | Under appropriate assumptions the energy of wave equations with damping and
variable coefficients $c(x)u_{tt}-\hbox{div}(b(x)\nabla u)+a(x)u_t =h(x)$ has
been shown to decay. Determining the rate of decay for the higher order
energies involving the $k$th order spatial and time derivatives has been an
open problem with the exception of some sparse results obtained for $k=1,2,3$.
We establish estimates that optimally relate the higher order energies with the
first order energy by carefully analyzing the effects of linear damping. The
results concern weighted (in time) and also pointwise (in time) energy decay
estimates. We also obtain $L^\infty$ estimates for the solution $u$. As an
application we compute explicit decay rates for all energies which involve the
dimension $n$ and the bounds for the coefficients $a(x)$ and $b(x)$ in the case
$c (x)=1$ and $h(x)=0.$ | 0811.2159v1 |
2009-05-13 | Time-dependent barrier passage of Two-dimensional non-Ohmic damping system | The time-dependent barrier passage of an anomalous damping system is studied
via the generalized Langevin equation (GLE) with non-Ohmic memory damping
friction tensor and corresponding thermal colored noise tensor describing a
particle passing over the saddle point of a two-dimensional quadratic potential
energy surface. The time-dependent passing probability and transmission
coefficient are analytically obtained by using of the reactive flux method. The
long memory aspect of friction is revealed to originate a non-monotonic
$\delta$(power exponent of the friction) dependence of the passing probability,
the optimal incident angle of the particle and the steady anomalous
transmission coefficient. In the long time limit a bigger steady transmission
coefficient is obtained which means less barrier recrossing than the
one-dimensional case. | 0905.2074v1 |
2011-07-11 | One-dimensional vertical dust strings in a glass box | The oscillation spectrum of a one-dimensional vertical dust string formed
inside a glass box on top of the lower electrode in a GEC reference cell was
studied. A mechanism for creating a single vertical dust string is described.
It is shown that the oscillation amplitudes, resonance frequencies, damping
coefficients, and oscillation phases of the dust particles separate into two
distinct groups. One group exhibits low damping coefficients, increasing
amplitudes and decreasing resonance frequencies for dust particles closer to
the lower electrode. The other group shows high damping coefficients but
anomalous resonance frequencies and amplitudes. At low oscillation frequencies,
the two groups are also separated by a {\pi}-phase difference. One possible
cause for the difference in behavior between the two groups is discussed. | 1107.2074v1 |
2016-02-24 | Pressure of a gas of underdamped active dumbbells | The pressure exerted on a wall by a gas at equilibrium does not depend on the
shape of the confining potential defining the wall. In contrast, it has been
shown recently [A.P. Solon et al., Nat. Phys. 11, 673 (2015)] that a gas of
overdamped active particles exerts on a wall a force that depends on the
confining potential, resulting in a net force on an asymmetric wall between two
chambers at equal densities. Here, considering a model of underdamped
self-propelled dumbbells in two dimensions, we study how the behavior of the
pressure depends on the damping coefficient of the dumbbells, thus exploring
inertial effects. We find in particular that the force exerted on a moving wall
between two chambers at equal density continuously vanishes at low damping
coefficient, and exhibits a complex dependence on the damping coefficient at
low density, when collisions are scarce. We further show that this behavior of
the pressure can to a significant extent be understood in terms of the
trajectories of individual particles close to and in contact with the wall. | 1602.07420v1 |
2016-03-31 | Recovery of time-dependent damping coefficients and potentials appearing in wave equations from partial data | We consider the inverse problem of determining a time-dependent damping
coefficient $a$ and a time-dependent potential $q$, appearing in the wave
equation $\partial_t^2u-\Delta_x u+a(t,x)\partial_tu+q(t,x)u=0$ in
$Q=(0,T)\times\Omega$, with $T>0$ and $\Omega$ a $ \mathcal C^2$ bounded domain
of $\mathbb R^n$, $n\geq2$, from partial observations of the solutions on
$\partial Q$. More precisely, we look for observations on $\partial Q$ that
allow to determine uniquely a large class of time-dependent damping
coefficients $a$ and time-dependent potentials $q$ without involving an
important set of data. We prove global unique determination of $a\in
W^{1,p}(Q)$, with $p>n+1$, and $q\in L^\infty(Q)$ from partial observations on
$\partial Q$. | 1603.09600v2 |
2020-05-04 | Remarks on asymptotic order for the linear wave equation with the scale-invariant damping and mass with $L^r$-data | In the present paper, we consider the linear wave equation with the
scale-invariant damping and mass. It is known that the global behavior of the
solution depends on the size of the coefficients in front of the damping and
mass at initial time $t=0$. Indeed, the solution satisfies the similar decay
estimate to that of the corresponding heat equation if it is large and to that
of the modified wave equation if it is small. In our previous paper, we obtain
the scattering result and its asymptotic order for the data in the energy space
$H^1\times L^2$ when the coefficients are in the wave regime. In fact, the
threshold of the coefficients relies on the spatial decay of the initial data.
Namely, it varies depending on $r$ when the initial data is in $L^r$ ($1\leq r
< 2$). In the present paper, we will show the scattering result and the
asymptotic order in the wave regime for $L^r$-data, which is wider than the
wave regime for the data in the energy space. Moreover, we give an improvement
of the asymptotic order obtained in our previous paper for the data in the
energy space. | 2005.01335v2 |
2023-06-18 | Partial data inverse problem for hyperbolic equation with time-dependent damping coefficient and potential | We study an inverse problem of determining a time-dependent damping
coefficient and potential appearing in the wave equation in a compact
Riemannian manifold of dimension three or higher. More specifically, we are
concerned with the case of conformally transversally anisotropic manifolds, or
in other words, compact Riemannian manifolds with boundary conformally embedded
in a product of the Euclidean line and a transversal manifold. With an
additional assumption of the attenuated geodesic ray transform being injective
on the transversal manifold, we prove that the knowledge of a certain partial
Cauchy data set determines time-dependent damping coefficient and potential
uniquely. | 2306.10442v2 |
2013-03-14 | Drag and Diffusion coefficients in extreme scenarios of temperature and chemical potential | A comparative study of high and zero temperature plasma for the case of
damping rate, drag and diffusion coefficients have been presented. In each of
these quantities, it is revealed how the magnetic interaction dominates over
the electric one at zero temperature unlike what happens at high temperature. | 1303.3353v1 |
2002-09-07 | Neural network analysis of the magnetization reversal in magnetic dot arrays | We simulated the remagnetization dynamics of the ultra-dense and ultra-thin
magnetic dot array system with dipole-dipole and exchange coupling
interactions. Within the proposed 2D XY superlattice model, the square dots are
modeled by the spatially modulated exchange-couplings. The dipole-dipole
interactions were approximated by the hierarchical sums and dynamics was
reduced to damping term of the Landau-Lifshitz-Gilbert equation. The simulation
of 40 000 spin system leads to nonequilibrium nonuniform configurations with
soliton-antisoliton pairs detected at intra-dot and inter-dot scales. The
classification of intra-dot magnetic configurations was performed using the
self-adaptive neural networks with varying number of neurons. | 0209186v1 |
2005-04-06 | Macrospin Models of Spin Transfer Dynamics | The current-induced magnetization dynamics of a spin valve are studied using
a macrospin (single domain) approximation and numerical solutions of a
generalized Landau-Lifshitz-Gilbert equation. For the purpose of quantitative
comparison with experiment [Kiselev {\it et al.} Nature {\bf 425}, 380 (2003)],
we calculate the resistance and microwave power as a function of current and
external field including the effects of anisotropies, damping, spin-transfer
torque, thermal fluctuations, spin-pumping, and incomplete absorption of
transverse spin current. While many features of experiment appear in the
simulations, there are two significant discrepancies: the current dependence of
the precession frequency and the presence/absence of a microwave quiet magnetic
phase with a distinct magnetoresistance signature. Comparison is made with
micromagnetic simulations designed to model the same experiment. | 0504142v1 |
2006-02-01 | Mapping Monte Carlo to Langevin dynamics: A Fokker-Planck approach | We propose a general method of using the Fokker-Planck equation (FPE) to link
the Monte-Carlo (MC) and the Langevin micromagnetic schemes. We derive the
drift and disusion FPE terms corresponding to the MC method and show that it is
analytically equivalent to the stochastic Landau-Lifshitz-Gilbert (LLG)
equation of Langevin-based micromagnetics. Subsequent results such as the time
quantification factor for the Metropolis MC method can be rigorously derived
from this mapping equivalence. The validity of the mapping is shown by the
close numerical convergence between the MC method and the LLG equation for the
case of a single magnetic particle as well as interacting arrays of particles.
We also found that our Metropolis MC is accurate for a large range of damping
factors $\alpha$, unlike previous time-quantified MC methods which break down
at low $\alpha$, where precessional motion dominates. | 0602011v2 |
2006-04-21 | Dynamic approach for micromagnetics close to the Curie temperature | In conventional micromagnetism magnetic domain configurations are calculated
based on a continuum theory for the magnetization which is assumed to be of
constant length in time and space. Dynamics is usually described with the
Landau-Lifshitz-Gilbert (LLG) equation the stochastic variant of which includes
finite temperatures. Using simulation techniques with atomistic resolution we
show that this conventional micromagnetic approach fails for higher
temperatures since we find two effects which cannot be described in terms of
the LLG equation: i) an enhanced damping when approaching the Curie temperature
and, ii) a magnetization magnitude that is not constant in time. We show,
however, that both of these effects are naturally described by the
Landau-Lifshitz-Bloch equation which links the LLG equation with the theory of
critical phenomena and turns out to be a more realistic equation for
magnetization dynamics at elevated temperatures. | 0604508v1 |
2007-02-20 | Spin dynamics in a superconductor / ferromagnet proximity system | The ferromagnetic resonance of thin sputtered Ni80Fe20 films grown on Nb is
measured. By varying the temperature and thickness of the Nb the role of the
superconductivity on the whole ferromagnetic layer in these heterostructures is
explored. The change in the spin transport properties below the superconducting
transition of the Nb is found to manifest itself in the Ni80Fe20 layer by a
sharpening in the resonance of the ferromagnet, or a decrease in the effective
Gilbert damping co-efficient. This dynamic proximity effect is in contrast to
low frequency studies in these systems, where the effect of the superconductor
is confined to a small region in the ferromagnet. We interpret this in terms of
the spin pumping model. | 0702461v1 |
2007-02-21 | Domain wall mobility, stability and Walker breakdown in magnetic nanowires | We present an analytical calculation of the velocity of a single 180 degree
domain wall in a magnetic structure with reduced thickness and/or lateral
dimension under the combined action of an external applied magnetic field and
an electrical current. As for the case of field-induced domain wall propagation
in thick films, two motion regimes with different mobilities are obtained,
below and far above the so-called Walker field. Additionally, for the case of
current induced motion, a Walker-like current density threshold can be defined.
When the dimensions of the system become comparable to the domain wall width,
the threshold field and current density, stating the wall's internal structure
stability, are reduced by the same geometrical demagnetising factor which
accounts for the confinement. This points out the fact that the velocity
dependence over an extended field/current range and the knowledge of the Walker
breakdown are mandatory to draw conclusions about the phenomenological Gilbert
damping parameter tuning the magnetisation dynamics. | 0702492v1 |
2001-01-09 | Hysteresis in layered spring magnets | This article addresses a problem of micromagnetics: the reversal of magnetic
moments in layered spring magnets. A one-dimensional model is used of a film
consisting of several atomic layers of a soft material on top of several atomic
layers of a hard material. Each atomic layer is taken to be uniformly
magnetized, and spatial inhomogeneities within an atomic layer are neglected.
The state of such a system is described by a chain of magnetic spin vectors.
Each spin vector behaves like a spinning top driven locally by the effective
magnetic field and subject to damping (Landau-Lifshitz-Gilbert equation). A
numerical integration scheme for the LLG equation is presented that is
unconditionally stable and preserves the magnitude of the magnetization vector
at all times. The results of numerical investigations for a bilayer in a
rotating in-plane magnetic field show hysteresis with a basic period of $2\pi$
at moderate fields and hysteresis with a basic period of $\pi$ at strong
fields. | 0101077v1 |
2005-01-01 | Equatorial and related non-equilibrium states in magnetization dynamics of ferromagnets: Generalization of Suhl's spin-wave instabilities | We investigate the nonlinear dynamics underlying the evolution of a 2-D
nanoscale ferromagnetic film with uniaxial anisotropy in the presence of
perpendicular pumping. Considering the associated Landau-Lifshitz spin
evolution equation with Gilbert damping together with Maxwell equation for the
demagnetization field, we study the dynamics in terms of the stereographic
variable. We identify several new fixed points for suitable choice of external
field in a rotating frame of reference. In particular, we identify explicit
equatorial and related fixed points of the spin vector in the plane transverse
to the anisotropy axis when the pumping frequency coincides with the amplitude
of the static parallel field. We then study the linear stability of these novel
fixed points under homogeneous and spin wave perturbations and obtain a
generalized Suhl's instability criterion, giving the condition for exponential
growth of P-modes under spin wave perturbations. Two parameter phase diagrams
(in terms of amplitudes of static parallel and oscillatory perpendicular
magnetic fields) for stability are obtained, which differ qualitatively from
those for the conventional ferromagnetic resonance near thermal equilibrium and
are amenable to experimental tests. | 0501002v2 |
2002-12-30 | Stochastic resonance in periodic potentials: realization in a dissipative optical lattice | We have observed the phenomenon of stochastic resonance on the Brillouin
propagation modes of a dissipative optical lattice. Such a mode has been
excited by applying a moving potential modulation with phase velocity equal to
the velocity of the mode. Its amplitude has been characterized by the
center-of-mass (CM) velocity of the atomic cloud. At Brillouin resonance, we
studied the CM-velocity as a function of the optical pumping rate at a given
depth of the potential wells. We have observed a resonant dependence of the CM
velocity on the optical pumping rate, corresponding to the noise strength. This
corresponds to the experimental observation of stochastic resonance in a
periodic potential in the low-damping regime. | 0212156v1 |
2007-05-03 | Planar spin-transfer device with a dynamic polarizer | In planar nano-magnetic devices magnetization direction is kept close to a
given plane by the large easy-plane magnetic anisotropy, for example by the
shape anisotropy in a thin film. In this case magnetization shows effectively
in-plane dynamics with only one angle required for its description. Moreover,
the motion can become overdamped even for small values of Gilbert damping. We
derive the equations of effective in-plane dynamics in the presence of
spin-transfer torques. The simplifications achieved in the overdamped regime
allow to study systems with several dynamic magnetic pieces (``free layers'').
A transition from a spin-transfer device with a static polarizer to a device
with two equivalent magnets is observed. When the size difference between the
magnets is less than critical, the device does not exhibit switching, but goes
directly into the ``windmill'' precession state. | 0705.0406v1 |
2007-05-03 | Effective attraction induced by repulsive interaction in a spin-transfer system | In magnetic systems with dominating easy-plane anisotropy the magnetization
can be described by an effective one dimensional equation for the in-plane
angle. Re-deriving this equation in the presence of spin-transfer torques, we
obtain a description that allows for a more intuitive understanding of
spintronic devices' operation and can serve as a tool for finding new dynamic
regimes. A surprising prediction is obtained for a planar ``spin-flip
transistor'': an unstable equilibrium point can be stabilized by a current
induced torque that further repels the system from that point. Stabilization by
repulsion happens due to the presence of dissipative environment and requires a
Gilbert damping constant that is large enough to ensure overdamped dynamics at
zero current. | 0705.0508v1 |
2007-06-21 | Spin pumping by a field-driven domain wall | We calculate the charge current in a metallic ferromagnet to first order in
the time derivative of the magnetization direction. Irrespective of the
microscopic details, the result can be expressed in terms of the conductivities
of the majority and minority electrons and the non-adiabatic spin transfer
torque parameter $\beta$. The general expression is evaluated for the specific
case of a field-driven domain wall and for that case depends strongly on the
ratio of $\beta$ and the Gilbert damping constant. These results may provide an
experimental method to determine this ratio, which plays a crucial role for
current-driven domain-wall motion. | 0706.3160v3 |
2007-09-18 | Theory of current-driven magnetization dynamics in inhomogeneous ferromagnets | We give a brief account of recent developments in the theoretical
understanding of the interaction between electric currents and inhomogeneous
ferromagnetic order parameters. We start by discussing the physical origin of
the spin torques responsible for this interaction and construct a
phenomenological description. We then consider the electric current-induced
ferromagnetic instability and domain-wall motion. Finally, we present a
microscopic justification of the phenomenological description of current-driven
magnetization dynamics, with particular emphasis on the dissipative terms, the
so-called Gilbert damping $\alpha$ and the $\beta$ component of the adiabatic
current-driven torque. | 0709.2937v2 |
2008-02-12 | Temperature dependent magnetization dynamics of magnetic nanoparticles | Recent experimental and theoretical studies show that the switching behavior
of magnetic nanoparticles can be well controlled by external time-dependent
magnetic fields. In this work, we inspect theoretically the influence of the
temperature and the magnetic anisotropy on the spin-dynamics and the switching
properties of single domain magnetic nanoparticles (Stoner-particles). Our
theoretical tools are the Landau-Lifshitz-Gilbert equation extended as to deal
with finite temperatures within a Langevine framework. Physical quantities of
interest are the minimum field amplitudes required for switching and the
corresponding reversal times of the nanoparticle's magnetic moment. In
particular, we contrast the cases of static and time-dependent external fields
and analyze the influence of damping for a uniaxial and a cubic anisotropy. | 0802.1740v1 |
2008-09-25 | The theory of magnetic field induced domain-wall propagation in magnetic nanowires | A global picture of magnetic domain wall (DW) propagation in a nanowire
driven by a magnetic field is obtained: A static DW cannot exist in a
homogeneous magnetic nanowire when an external magnetic field is applied. Thus,
a DW must vary with time under a static magnetic field. A moving DW must
dissipate energy due to the Gilbert damping. As a result, the wire has to
release its Zeeman energy through the DW propagation along the field direction.
The DW propagation speed is proportional to the energy dissipation rate that is
determined by the DW structure. An oscillatory DW motion, either the precession
around the wire axis or the breath of DW width, should lead to the speed
oscillation. | 0809.4311v1 |
2008-10-08 | Transverse spin diffusion in ferromagnets | We discuss the dissipative diffusion-type term of the form
$\mathbf{m}\times\nabla^2\partial_t\mathbf{m}$ in the phenomenological
Landau-Lifshitz equation of ferromagnetic precession, which describes enhanced
Gilbert damping of finite-momentum spin waves. This term arises physically from
itinerant-electron spin flows through a perturbed ferromagnetic configuration
and can be understood to originate in the ferromagnetic spin pumping in the
continuum limit. We develop a general phenomenology as well as provide
microscopic theory for the Stoner and s-d models of ferromagnetism, taking into
account disorder and electron-electron scattering. The latter is manifested in
our problem through the Coulomb drag between the spin bands. The spin diffusion
is identified in terms of the transverse spin conductivity, in analogy with the
Einstein relation in the kinetic theory. | 0810.1340v2 |
2008-10-16 | Interaction of reed and acoustic resonator in clarinetlike systems | Sound emergence in clarinetlike instruments is investigated in terms of
instability of the static regime. Various models of reed-bore coupling are
considered, from the pioneering work of Wilson and Beavers ["Operating modes of
the clarinet", J. Acoust. Soc. Am. 56, 653--658 (1974)] to more recent modeling
including viscothermal bore losses and vena contracta at the reed inlet. The
pressure threshold above which these models may oscillate as well as the
frequency of oscillation at threshold are calculated. In addition to Wilson and
Beavers' previous conclusions concerning the role of the reed damping in the
selection of the register the instrument will play on, the influence of the
reed motion induced flow is also emphasized, particularly its effect on playing
frequencies, contributing to reduce discrepancies between Wilson and Beavers'
experimental results and theory, despite discrepancies still remain concerning
the pressure threshold. Finally, analytical approximations of the oscillating
solution based on Fourier series expansion are obtained in the vicinity of the
threshold of oscillation. This allows to emphasize the conditions which
determine the nature of the bifurcation (direct or inverse) through which the
note may emerge, with therefore important consequences on the musical playing
performances. | 0810.2870v1 |
2008-11-21 | Spin Transfer Torque as a Non-Conservative Pseudo-Field | In this paper we show that the spin transfer torque can be described by a
pseudo magnetic field, proportional to the magnetic moment of the itinerant
electrons that enters the Landau-Lifshitz-Gilbert equation in the same way as
other external or internal magnetic fields. However, unlike an ordinary
magnetic field, which is always conservative in nature, the spin torque induced
pseudo field may have both conservative and non-conservative components. We
further show that the magnetic moment of itinerant electrons develops an
out-of-plane component only at non-equilibrium and this component is
responsible for the Slonczewski type switching that acts against the damping
and is always non-conservative. On the other hand, the in-plane components of
the pseudo field exist both at equilibrium and out-of-equilibrium, and are
responsible for the field like term. For tunnel based devices, this term
results in lower switching current for anti-parallel (AP) to parallel (P)
switching compared to P to AP, even when the torque magnitudes are completely
symmetric with voltage. | 0811.3472v1 |
2008-12-13 | Non-Adiabatic Spin Transfer Torque in Real Materials | The motion of simple domain walls and of more complex magnetic textures in
the presence of a transport current is described by the
Landau-Lifshitz-Slonczewski (LLS) equations. Predictions of the LLS equations
depend sensitively on the ratio between the dimensionless material parameter
$\beta$ which characterizes non-adiabatic spin-transfer torques and the Gilbert
damping parameter $\alpha$. This ratio has been variously estimated to be close
to 0, close to 1, and large compared to 1. By identifying $\beta$ as the
influence of a transport current on $\alpha$, we derive a concise, explicit and
relatively simple expression which relates $\beta$ to the band structure and
Bloch state lifetimes of a magnetic metal. Using this expression we demonstrate
that intrinsic spin-orbit interactions lead to intra-band contributions to
$\beta$ which are often dominant and can be (i) estimated with some confidence
and (ii) interpreted using the "breathing Fermi surface" model. | 0812.2570v1 |
2009-05-29 | Ferromagnetic resonance linewidth in ultrathin films with perpendicular magnetic anisotropy | Transition metal ferromagnetic films with perpendicular magnetic anisotropy
(PMA) have ferromagnetic resonance (FMR) linewidths that are one order of
magnitude larger than soft magnetic materials, such as pure iron (Fe) and
permalloy (NiFe) thin films. A broadband FMR setup has been used to investigate
the origin of the enhanced linewidth in Ni$|$Co multilayer films with PMA. The
FMR linewidth depends linearly on frequency for perpendicular applied fields
and increases significantly when the magnetization is rotated into the film
plane. Irradiation of the film with Helium ions decreases the PMA and the
distribution of PMA parameters. This leads to a great reduction of the FMR
linewidth for in-plane magnetization. These results suggest that fluctuations
in PMA lead to a large two magnon scattering contribution to the linewidth for
in-plane magnetization and establish that the Gilbert damping is enhanced in
such materials ($\alpha \approx 0.04$, compared to $\alpha \approx 0.002$ for
pure Fe). | 0905.4779v2 |
2009-10-01 | Spin motive forces and current fluctuations due to Brownian motion of domain walls | We compute the power spectrum of the noise in the current due to spin motive
forces by a fluctuating domain wall. We find that the power spectrum of the
noise in the current is colored, and depends on the Gilbert damping, the spin
transfer torque parameter $\beta$, and the domain-wall pinning potential and
magnetic anisotropy. We also determine the average current induced by the
thermally-assisted motion of a domain wall that is driven by an external
magnetic field. Our results suggest that measuring the power spectrum of the
noise in the current in the presence of a domain wall may provide a new method
for characterizing the current-to-domain-wall coupling in the system. | 0910.0163v1 |
2009-10-08 | Fast domain wall propagation under an optimal field pulse in magnetic nanowires | We investigate field-driven domain wall (DW) propagation in magnetic
nanowires in the framework of the Landau-Lifshitz-Gilbert equation. We propose
a new strategy to speed up the DW motion in a uniaxial magnetic nanowire by
using an optimal space-dependent field pulse synchronized with the DW
propagation. Depending on the damping parameter, the DW velocity can be
increased by about two orders of magnitude compared the standard case of a
static uniform field. Moreover, under the optimal field pulse, the change in
total magnetic energy in the nanowire is proportional to the DW velocity,
implying that rapid energy release is essential for fast DW propagation. | 0910.1477v2 |
2009-11-24 | Origin of adiabatic and non-adiabatic spin transfer torques in current-driven magnetic domain wall motion | A consistent theory to describe the correlated dynamics of quantum mechanical
itinerant spins and semiclassical local magnetization is given. We consider the
itinerant spins as quantum mechanical operators, whereas local moments are
considered within classical Lagrangian formalism. By appropriately treating
fluctuation space spanned by basis functions, including a zero-mode wave
function, we construct coupled equations of motion for the collective
coordinate of the center-of-mass motion and the localized zero-mode coordinate
perpendicular to the domain wall plane. By solving them, we demonstrate that
the correlated dynamics is understood through a hierarchy of two time scales:
Boltzmann relaxation time when a non-adiabatic part of the spin-transfer torque
appears, and Gilbert damping time when adiabatic part comes up. | 0911.4628v1 |
2010-01-26 | Strategies and tolerances of spin transfer torque switching | Schemes of switching nanomagnetic memories via the effect of spin torque with
various polarizations of injected electrons are studied. Simulations based on
macrospin and micromagnetic theories are performed and compared. We demonstrate
that switching with perpendicularly polarized current by short pulses and free
precession requires smaller time and energy than spin torque switching with
collinear in plane spin polarization; it is also found to be superior to other
kinds of memories. We study the tolerances of switching to the magnitude of
current and pulse duration. An increased Gilbert damping is found to improve
tolerances of perpendicular switching without increasing the threshold current,
unlike in plane switching. | 1001.4578v1 |
2010-03-31 | Magnonic Crystal with Two-Dimensional Periodicity as a Waveguide for Spin Waves | We describe a simple method of including dissipation in the spin wave band
structure of a periodic ferromagnetic composite, by solving the Landau-Lifshitz
equation for the magnetization with the Gilbert damping term. We use this
approach to calculate the band structure of square and triangular arrays of Ni
nanocylinders embedded in an Fe host. The results show that there are certain
bands and special directions in the Brillouin zone where the spin wave lifetime
is increased by more than an order of magnitude above its average value. Thus,
it may be possible to generate spin waves in such composites decay especially
slowly, and propagate especially large distances, for certain frequencies and
directions in ${\bf k}$-space. | 1003.6092v1 |
2010-07-20 | Precessing vortices and antivortices in ferromagnetic elements | A micromagnetic numerical study of the precessional motion of the vortex and
antivortex states in soft ferromagnetic circular nanodots is presented using
Landau-Lifshitz-Gilbert dynamics. For sufficiently small dot thickness and
diameter, the vortex state is metastable and spirals toward the center of the
dot when its initial displacement is smaller than a critical value. Otherwise,
the vortex spirals away from the center and eventually exits the dot which
remains in a state of in-plane magnetization (ground state). In contrast, the
antivortex is always unstable and performs damped precession resulting in
annihilation at the dot circumference. The vortex and antivortex frequencies of
precession are compared with the response expected on the basis of Thiele's
theory of collective coordinates. We also calculate the vortex restoring force
with an explicit account of the magnetostatic and exchange interaction on the
basis of the 'rigid' vortex and 'two-vortices side charges free' models and
show that neither model explains the vortex translation mode eigenfrequency for
nanodots of sufficiently small size. | 1007.3508v1 |
2010-10-07 | Power optimization for domain wall motion in ferromagnetic nanowires | The current mediated domain-wall dynamics in a thin ferromagnetic wire is
investigated. We derive the effective equations of motion of the domain wall.
They are used to study the possibility to optimize the power supplied by
electric current for the motion of domain walls in a nanowire. We show that a
certain resonant time-dependent current moving a domain wall can significantly
reduce the Joule heating in the wire, and thus it can lead to a novel proposal
for the most energy efficient memory devices. We discuss how Gilbert damping,
non-adiabatic spin transfer torque, and the presence of Dzyaloshinskii-Moriya
interaction can effect this power optimization. | 1010.1537v1 |
2011-03-30 | Spin motive forces due to magnetic vortices and domain walls | We study spin motive forces, i.e, spin-dependent forces, and voltages induced
by time-dependent magnetization textures, for moving magnetic vortices and
domain walls. First, we consider the voltage generated by a one-dimensional
field-driven domain wall. Next, we perform detailed calculations on
field-driven vortex domain walls. We find that the results for the voltage as a
function of magnetic field differ between the one-dimensional and vortex domain
wall. For the experimentally relevant case of a vortex domain wall, the
dependence of voltage on field around Walker breakdown depends qualitatively on
the ratio of the so-called $\beta$-parameter to the Gilbert damping constant,
and thus provides a way to determine this ratio experimentally. We also
consider vortices on a magnetic disk in the presence of an AC magnetic field.
In this case, the phase difference between field and voltage on the edge is
determined by the $\beta$ parameter, providing another experimental method to
determine this quantity. | 1103.5858v3 |
2011-07-04 | Influence of randomness and retardation on the FMR-linewidth | The theory predicts that the spin-wave lifetime $\tau_L$ and the linewidth of
ferromagnetic resonance $\Delta B$ can be governed by random fields and spatial
memory. To that aim the effective field around which the magnetic moments
perform a precession is superimposed by a stochastic time dependent magnetic
field with finite correlation time. The magnetization dynamics is altered by
inclusion of a spatial memory effect monitoring a non-local interaction of size
$\xi$. The underlying Landau-Lifshitz-Gilbert equation (LLG) is modified
accordingly. The stochastic LLG is equivalent to a Fokker-Planck equation which
enables to calculate the mean values of the magnetization vector. Within the
spin-wave approximation we present an analytical solution for the excitation
energy and its damping. The lifetime and the linewidth are analyzed depending
on the strength of the random field $D$ and its correlation time $\tau_c$ as
well as the retardation strength $\Gamma_0$ and the size $\xi$. Whereas
$\tau_L$ decreases with increasing $D$, retardation strength $\Gamma_0$ and
$\tau_c$, the lifetime is enhanced for growing width $\xi$ of the spatial
retardation kernel. In the same manner we calculate the experimentally
measurable linewidth $\Delta B$ is increased strongly when the correlation time
$\tau_c$ ranges in the nanosecond interval. | 1107.0638v1 |
2011-11-18 | Charge and Spin Transport in Magnetic Tunnel Junctions: Microscopic Theory | We study the charge and spin currents passing through a magnetic tunnel
junction (MTJ) on the basis of a tight-binding model. The currents are
evaluated perturbatively with respect to the tunnel Hamiltonian. The charge
current has the form $A[\bm M_1(t)\times\dot{\bm M}_1(t)]\cdot\bm M_2+B\dot{\bm
M}_1(t)\cdot\bm M_2$, where $\bm M_1(t)$ and $\bm M_2$ denote the directions of
the magnetization in the free layer and fixed layer, respectively. The constant
$A$ vanishes when one or both layers are insulators, {while the constant $B$
disappears when both layers are insulators or the same ferromagnets.} The first
term in the expression for charge current represents dissipation driven by the
effective electric field induced by the dynamic magnetization. In addition,
from an investigation of the spin current, we obtain the microscopic expression
for the enhanced Gilbert damping constant $\varDelta \alpha$. We show that
$\varDelta\alpha$ is proportional to the tunnel conductance and depends on the
bias voltage. | 1111.4295v2 |
2012-01-17 | Magnetic vortex echoes: application to the study of arrays of magnetic nanostructures | We propose the use of the gyrotropic motion of vortex cores in nanomagnets to
produce a magnetic echo, analogous to the spin echo in NMR. This echo occurs
when an array of nanomagnets, e.g., nanodisks, is magnetized with an in-plane
(xy) field, and after a time \tau a field pulse inverts the core magnetization;
the echo is a peak in M_{xy} at t=2\tau. Its relaxation times depend on the
inhomogeneity, on the interaction between the nanodots and on the Gilbert
damping constant \alpha. Its feasibility is demonstrated using micromagnetic
simulation. To illustrate an application of the echoes, we have determined the
inhomogeneity and measured the magnetic interaction in an array of nanodisks
separated by a distance d, finding a d^{-n} dependence, with n\approx 4. | 1201.3553v1 |
2012-02-15 | Current-induced motion of a transverse magnetic domain wall in the presence of spin Hall effect | We theoretically study the current-induced dynamics of a transverse magnetic
domain wall in bi-layer nanowires consisting of a ferromagnet on top of a
nonmagnet having strong spin-orbit coupling. Domain wall dynamics is
characterized by two threshold current densities, $J_{th}^{WB}$ and
$J_{th}^{REV}$, where $J_{th}^{WB}$ is a threshold for the chirality switching
of the domain wall and $J_{th}^{REV}$ is another threshold for the reversed
domain wall motion caused by spin Hall effect. Domain walls with a certain
chirality may move opposite to the electron-flow direction with high speed in
the current range $J_{th}^{REV} < J < J_{th}^{WB}$ for the system designed to
satisfy the conditions $J_{th}^{WB} > J_{th}^{REV}$ and \alpha > \beta, where
\alpha is the Gilbert damping constant and \beta is the nonadiabaticity of spin
torque. Micromagnetic simulations confirm the validity of analytical results. | 1202.3450v1 |
2012-04-23 | Rotating skyrmion lattices by spin torques and field or temperature gradients | Chiral magnets like MnSi form lattices of skyrmions, i.e. magnetic whirls,
which react sensitively to small electric currents j above a critical current
density jc. The interplay of these currents with tiny gradients of either the
magnetic field or the temperature can induce a rotation of the magnetic pattern
for j>jc. Either a rotation by a finite angle of up to 15 degree or -- for
larger gradients -- a continuous rotation with a finite angular velocity is
induced. We use Landau-Lifshitz-Gilbert equations extended by extra damping
terms in combination with a phenomenological treatment of pinning forces to
develop a theory of the relevant rotational torques. Experimental neutron
scattering data on the angular distribution of skyrmion lattices suggests that
continuously rotating domains are easy to obtain in the presence of remarkably
small currents and temperature gradients. | 1204.5051v1 |
2012-07-09 | Thermal vortex dynamics in thin circular ferromagnetic nanodisks | The dynamics of gyrotropic vortex motion in a thin circular nanodisk of soft
ferromagnetic material is considered. The demagnetization field is calculated
using two-dimensional Green's functions for the thin film problem and fast
Fourier transforms. At zero temperature, the dynamics of the
Landau-Lifshitz-Gilbert equation is simulated using fourth order Runge-Kutta
integration. Pure vortex initial conditions at a desired position are obtained
with a Lagrange multipliers constraint. These methods give accurate estimates
of the vortex restoring force constant $k_F$ and gyrotropic frequency, showing
that the vortex core motion is described by the Thiele equation to very high
precision. At finite temperature, the second order Heun algorithm is applied to
the Langevin dynamical equation with thermal noise and damping. A spontaneous
gyrotropic motion takes place without the application of an external magnetic
field, driven only by thermal fluctuations. The statistics of the vortex radial
position and rotational velocity are described with Boltzmann distributions
determined by $k_F$ and by a vortex gyrotropic mass $m_G=G^2/k_F$,
respectively, where $G$ is the vortex gyrovector. | 1207.2192v2 |
2013-02-19 | Chirality Sensitive Domain Wall Motion in Spin-Orbit Coupled Ferromagnets | Using the Lagrangian formalism, we solve analytically the equations of motion
for current-induced domain-wall dynamics in a ferromagnet with Rashba
spin-orbit coupling. An exact solution for the domain wall velocity is
provided, including the effect of non-equilibrium conduction electron
spin-density, Gilbert damping, and the Rashba interaction parameter. We
demonstrate explicitly that the influence of spin-orbit interaction can be
qualitatively different from the role of non-adiabatic spin-torque in the sense
that the former is sensitive to the chirality of the domain wall whereas the
latter is not: the domain wall velocity shows a reentrant behavior upon
changing the chirality of the domain wall. This could be used to experimentally
distinguish between the spin-orbit and non-adiabatic contribution to the wall
speed. A quantitative estimate for the attainable domain wall velocity is
given, based on an experimentally relevant set of parameters for the system. | 1302.4744v1 |
2013-12-17 | Control of the in-plane anisotropy in off-stoichiometric NiMnSb | NiMnSb is a ferromagnetic half-metal which, because of its rich anisotropy
and very low Gilbert damping, is a promising candidate for applications in
information technologies. We have investigated the in-plane anisotropy
properties of thin, MBE-grown NiMnSb films as a function of their Mn
concentration. Using ferromagnetic resonance (FMR) to determine the uniaxial
and four-fold anisotropy fields, 2KU/Ms and 2K1/Ms, we find that a small
variation in composition is sufficient to change the film from primarily
four-fold to primarily uniaxial behavior, allowing for continuous tuning of the
anisotropy. This provides valuable flexibility in designing new device
geometries. | 1312.4781v2 |
2014-05-09 | Current-induced magnetization dynamics in two magnetic insulators separated by a normal metal | We study the dynamics of spin valves consisting of two layers of magnetic
insulators separated by a normal metal in the macrospin model. A current
through the spacer generates a spin Hall current that can actuate the
magnetization via the spin-transfer torque. We derive expressions for the
effective Gilbert damping and the critical currents for the onset of
magnetization dynamics including the effects of spin pumping that can be tested
by ferromagnetic resonance experiments. The current generates an amplitude
asymmetry between the in-phase and out-of-phase modes. We briefly discuss
superlattices of metals and magnetic insulators. | 1405.2267v1 |
2014-05-25 | Spin Hall phenomenology of magnetic dynamics | We study the role of spin-orbit interactions in the coupled magnetoelectric
dynamics of a ferromagnetic film coated with an electrical conductor. While the
main thrust of this work is phenomenological, several popular simple models are
considered microscopically in some detail, including Rashba and Dirac
two-dimensional electron gases coupled to a magnetic insulator, as well as a
diffusive spin Hall system. We focus on the long-wavelength magnetic dynamics
that experiences current-induced torques and produces fictitious electromotive
forces. Our phenomenology provides a suitable framework for analyzing
experiments on current-induced magnetic dynamics and reciprocal charge pumping,
including the effects of magnetoresistance and Gilbert-damping anisotropies,
without a need to resort to any microscopic considerations or modeling.
Finally, some remarks are made regarding the interplay of spin-orbit
interactions and magnetic textures. | 1405.6354v2 |
2014-08-21 | Brownian motion of massive skyrmions forced by spin polarized currents | We report on the thermal effects on the motion of current-driven massive
magnetic skyrmions. The reduced equation for the motion of skyrmion has the
form of a stochastic generalized Thiele's equation. We propose an ansatz for
the magnetization texture of a non-rigid single skyrmion that depends linearly
with the velocity. By utilizing this ansatz it is is found that the mass of
skyrmion is closely related to intrinsic skyrmion parameters, such as Gilbert
damping, skyrmion-charge and dissipative force. We have found an exact
expression for the average drift velocity as well as the mean-square velocity
of the skyrmion. The longitudinal and transverse mobility of skyrmions for
small spin-velocity of electrons is also determined and found to be independent
of the skyrmion mass. | 1408.4861v2 |
2014-11-11 | Capturing of a Magnetic Skyrmion with a Hole | Magnetic whirls in chiral magnets, so-called skyrmions, can be manipulated by
ultrasmall current densities. Here we study both analytically and numerically
the interactions of a single skyrmion in two dimensions with a small hole in
the magnetic layer. Results from micromagnetic simulations are in good
agreement with effective equations of motion obtained from a generalization of
the Thiele approach. Skyrmion-defect interactions are described by an effective
potential with both repulsive and attractive components. For small current
densities a previously pinned skyrmion stays pinned whereas an unpinned
skyrmion moves around the impurities and never gets captured. For higher
current densities, j_c1 < j < j_c2, however, single holes are able to capture
moving skyrmions. The maximal cross section is proportional to the skyrmion
radius and to Sqrt(alpha), where alpha is the Gilbert damping. For j > j_c2 all
skyrmions are depinned. Small changes of the magnetic field strongly change the
pinning properties, one can even reach a regime without pinning, j_c2=0. We
also show that a small density of holes can effectively accelerate the motion
of the skyrmion and introduce a Hall effect for the skyrmion. | 1411.2857v1 |
2014-12-01 | Dissipation due to pure spin-current generated by spin pumping | Based on spin-dependent transport theory and thermodynamics, we develop a
generalized theory of the Joule heating in the presence of a spin current.
Along with the conventional Joule heating consisting of an electric current and
electrochemical potential, it is found that the spin current and spin
accumulation give an additional dissipation because the spin-dependent
scatterings inside bulk and ferromagnetic/nonmagnetic interface lead to a
change of entropy. The theory is applied to investigate the dissipation due to
pure spin-current generated by spin pumping across a
ferromagnetic/nonmagnetic/ferromagnetic multilayer. The dissipation arises from
an interface because the spin pumping is a transfer of both the spin angular
momentum and the energy from the ferromagnet to conduction electrons near the
interface. It is found that the dissipation is proportional to the enhancement
of the Gilbert damping constant by spin pumping. | 1412.0688v1 |
2015-01-30 | Head-to-Head Domain Wall Structures in Wide Permalloy Strips | We analyze the equilibrium micromagnetic domain wall structures encountered
in Permalloy strips of a wide range of thicknesses and widths, with strip
widths up to several micrometers. By performing an extensive set of
micromagnetic simulations, we show that the equilibrium phase diagram of the
domain wall structures exhibits in addition to the previously found structures
(symmetric and asymmetric transverse walls, vortex wall) also double vortex and
triple vortex domain walls for large enough strip widths and thicknesses. Also
several metastable domain wall structures are found for wide and/or thick
strips. We discuss the details of the relaxation process from random
magnetization initial states towards the stable domain wall structure, and show
that our results are robust with respect to changes of e.g. the magnitude of
the Gilbert damping constant and details of the initial conditions. | 1501.07731v1 |
2015-02-19 | Characterization of spin relaxation anisotropy in Co using spin pumping | Ferromagnets are believed to exhibit strongly anisotropic spin relaxation,
with relaxation lengths for spin longitudinal to magnetization significantly
longer than those for spin transverse to magnetization. Here we characterize
the anisotropy of spin relaxation in Co using the spin pumping contribution to
Gilbert damping in noncollinearly magnetized Py$_{1-x}$Cu$_{x}$/Cu/Co trilayer
structures. The static magnetization angle between Py$_{1-x}$Cu$_{x}$ and Co,
adjusted under field bias perpendicular to film planes, controls the
projections of longitudinal and transverse spin current pumped from
Py$_{1-x}$Cu$_{x}$ into Co. We find nearly isotropic absorption of pure spin
current in Co using this technique; fits to a diffusive transport model yield
the longitudinal spin relaxation length $< 2$ nm in Co. The longitudinal spin
relaxation lengths found are an order of magnitude smaller than those
determined by current-perpendicular-to-planes giant magnetoresistance
measurements, but comparable with transverse spin relaxation lengths in Co
determined by spin pumping. | 1502.05687v3 |
2015-04-01 | Multiscale modeling of ultrafast element-specific magnetization dynamics of ferromagnetic alloys | A hierarchical multiscale approach to model the magnetization dynamics of
ferromagnetic ran- dom alloys is presented. First-principles calculations of
the Heisenberg exchange integrals are linked to atomistic spin models based
upon the stochastic Landau-Lifshitz-Gilbert (LLG) equation to calculate
temperature-dependent parameters (e.g., effective exchange interactions,
damping param- eters). These parameters are subsequently used in the
Landau-Lifshitz-Bloch (LLB) model for multi-sublattice magnets to calculate
numerically and analytically the ultrafast demagnetization times. The developed
multiscale method is applied here to FeNi (permalloy) as well as to copper-
doped FeNi alloys. We find that after an ultrafast heat pulse the Ni sublattice
demagnetizes faster than the Fe sublattice for the here-studied FeNi-based
alloys. | 1504.00199v1 |
2015-05-04 | High-topological-number magnetic skyrmions and topologically protected dissipative structure | The magnetic skyrmion with the topological number of unity ($Q=1$) is a
well-known nanometric swirling spin structure in the nonlinear $\sigma$ model
with the Dzyaloshinskii-Moriya interaction. Here, we show that magnetic
skyrmion with the topological number of two ($Q=2$) can be created and
stabilized by applying vertical spin-polarized current though it cannot exist
as a static stable excitation. Magnetic skyrmion with $Q=2$ is a nonequilibrium
dynamic object, subsisting on a balance between the energy injection from the
current and the energy dissipation by the Gilbert damping. Once it is created,
it becomes a topologically protected object against fluctuations of various
variables including the injected current itself. Hence, we may call it a
topologically protected dissipative structure. We also elucidate the nucleation
and destruction mechanisms of the magnetic skyrmion with $Q=2$ by studying the
evolutions of the magnetization distribution, the topological charge density as
well as the energy density. Our results will be useful for the study of the
nontrivial topology of magnetic skyrmions with higher topological numbers. | 1505.00522v2 |
2015-08-06 | Large spin-wave bullet in a ferrimagnetic insulator driven by spin Hall effect | Due to its transverse nature, spin Hall effects (SHE) provide the possibility
to excite and detect spin currents and magnetization dynamics even in magnetic
insulators. Magnetic insulators are outstanding materials for the investigation
of nonlinear phenomena and for novel low power spintronics applications because
of their extremely low Gilbert damping. Here, we report on the direct imaging
of electrically driven spin-torque ferromagnetic resonance (ST-FMR) in the
ferrimagnetic insulator Y$_3$Fe$_5$O$_{12}$ based on the excitation and
detection by SHEs. The driven spin dynamics in Y$_3$Fe$_5$O$_{12}$ is directly
imaged by spatially-resolved microfocused Brillouin light scattering (BLS)
spectroscopy. Previously, ST-FMR experiments assumed a uniform precession
across the sample, which is not valid in our measurements. A strong spin-wave
localization in the center of the sample is observed indicating the formation
of a nonlinear, self-localized spin-wave `bullet'. | 1508.01427v1 |
2015-12-02 | Bose-Einstein Condensation of Magnons Pumped by the Bulk Spin Seebeck Effect | We propose inducing Bose-Einstein condensation of magnons in a magnetic
insulator by a heat flow oriented toward its boundary. At a critical heat flux,
the oversaturated thermal gas of magnons accumulated at the boundary
precipitates the condensate, which then grows gradually as the thermal bias is
dialed up further. The thermal magnons thus pumped by the magnonic bulk (spin)
Seebeck effect must generally overcome both the local Gilbert damping
associated with the coherent magnetic dynamics as well as the radiative
spin-wave losses toward the magnetic bulk, in order to achieve the threshold of
condensation. We quantitatively estimate the requisite bias in the case of the
ferrimagnetic yttrium iron garnet, discuss different physical regimes of
condensation, and contrast it with the competing (so-called Doppler-shift) bulk
instability. | 1512.00557v1 |
2016-01-10 | Interfacial Dzyaloshinskii-Moriya interaction, surface anisotropy energy,and spin pumping at spin orbit coupled Ir/Co interface | The interfacial Dzyaloshinskii-Moriya interaction (iDMI), surface anisotropy
energy, and spin pumping at the Ir/Co interface are experimentally investigated
by performing Brillouin light scattering. Contrary to previous reports, we
suggest that the sign of the iDMI at the Ir/Co interface is the same as in the
case of the Pt/Co interface. We also find that the magnitude of the iDMI energy
density is relatively smaller than in the case of the Pt/Co interface, despite
the large strong spin-orbit coupling (SOC) of Ir. The saturation magnetization
and the perpendicular magnetic anisotropy (PMA) energy are significantly
improved due to a strong SOC. Our findings suggest that an SOC in an Ir/Co
system behaves in different ways for iDMI and PMA. Finally, we determine the
spin pumping effect at the Ir/Co interface, and it increases the Gilbert
damping constant from 0.012 to 0.024 for 1.5 nmthick Co. | 1601.02210v3 |
2016-02-23 | Relaxation of a classical spin coupled to a strongly correlated electron system | A classical spin which is antiferromagnetically coupled to a system of
strongly correlated conduction electrons is shown to exhibit unconventional
real-time dynamics which cannot be described by Gilbert damping. Depending on
the strength of the local Coulomb interaction, the two main electronic
dissipation channels, transport of excitations via correlated hopping and via
excitations of correlation-induced magnetic moments, become active on largely
different time scales. We demonstrate that this can lead to a prethermalization
scenario which so far has been observed in purely electronic systems only and
which is governed here by proximity to the divergent magnetic time scale in the
infinite-U limit. | 1602.07317v2 |
2016-05-12 | Classical limit of Rabi nutations in spins of ferromagnets | Rabi oscillations describe the interaction of a two-level system with a
rotating electromagnetic field. As such, they serve as the principle method for
manipulating quantum bits. By using a combination of femtosecond laser pulses
and microwave excitations, we have observed the classical form of Rabi
nutations in a ferromagnetic system whose equations of motion mirror the case
of a precessing quantum two-level system. Key to our experiments is the
selection of a subset of spins that is in resonance with the microwave
excitation and whose coherence time is thereby extended. Taking advantage of
Gilbert damping, the relaxation times are further increased such that
mode-locking takes place. The observation of such Rabi nutations is the first
step towards potential applications based on phase-coherent spin manipulation
in ferromagnets. | 1605.03996v1 |
2016-06-07 | The temperature dependence of FeRh's transport properties | The finite-temperature transport properties of FeRh compounds are
investigated by first-principles Density Functional Theory-based calculations.
The focus is on the behavior of the longitudinal resistivity with rising
temperature, which exhibits an abrupt decrease at the metamagnetic transition
point, $T = T_m$ between ferro- and antiferromagnetic phases. A detailed
electronic structure investigation for $T \geq 0$ K explains this feature and
demonstrates the important role of (i) the difference of the electronic
structure at the Fermi level between the two magnetically ordered states and
(ii) the different degree of thermally induced magnetic disorder in the
vicinity of $T_m$, giving different contributions to the resistivity. To
support these conclusions, we also describe the temperature dependence of the
spin-orbit induced anomalous Hall resistivity and Gilbert damping parameter.
For the various response quantities considered the impact of thermal lattice
vibrations and spin fluctuations on their temperature dependence is
investigated in detail. Comparison with corresponding experimental data finds
in general a very good agreement. | 1606.02072v1 |
2016-09-05 | Coarsening dynamics of topological defects in thin Permalloy films | We study the dynamics of topological defects in the magnetic texture of
rectangular Permalloy thin film elements during relaxation from random
magnetization initial states. Our full micromagnetic simulations reveal complex
defect dynamics during relaxation towards the stable Landau closure domain
pattern, manifested as temporal power-law decay, with a system-size dependent
cut-off time, of various quantities. These include the energy density of the
system, and the number densities of the different kinds of topological defects
present in the system. The related power-law exponents assume non-trivial
values, and are found to be different for the different defect types. The
exponents are robust against a moderate increase in the Gilbert damping
constant and introduction of quenched structural disorder. We discuss details
of the processes allowed by conservation of the winding number of the defects,
underlying their complex coarsening dynamics. | 1609.01094v1 |
2016-09-27 | Anomalous Feedback and Negative Domain Wall Resistance | Magnetic induction can be regarded as a negative feedback effect, where the
motive-force opposes the change of magnetic flux that generates the
motive-force. In artificial electromagnetics emerging from spintronics,
however, this is not necessarily the case. By studying the current-induced
domain wall dynamics in a cylindrical nanowire, we show that the spin
motive-force exerting on electrons can either oppose or support the applied
current that drives the domain wall. The switching into the anomalous feedback
regime occurs when the strength of the dissipative torque {\beta} is about
twice the value of the Gilbert damping constant {\alpha}. The anomalous
feedback manifests as a negative domain wall resistance, which has an analogy
with the water turbine. | 1609.08250v1 |
2016-10-04 | Magnetomechanical coupling and ferromagnetic resonance in magnetic nanoparticles | We address the theory of the coupled lattice and magnetization dynamics of
freely suspended single-domain nanoparticles. Magnetic anisotropy generates
low-frequency satellite peaks in the microwave absorption spectrum and a
blueshift of the ferromagnetic resonance (FMR) frequency. The low-frequency
resonances are very sharp with maxima exceeding that of the FMR, because their
magnetic and mechanical precessions are locked, thereby suppressing Gilbert
damping. Magnetic nanoparticles can operate as nearly ideal motors that convert
electromagnetic into mechanical energy. The Barnett/Einstein-de Haas effect is
significant even in the absence of a net rotation. | 1610.01072v2 |
2016-10-05 | Finite-dimensional colored fluctuation-dissipation theorem for spin systems | When nano-magnets are coupled to random external sources, their magnetization
becomes a random variable, whose properties are defined by an induced
probability density, that can be reconstructed from its moments, using the
Langevin equation, for mapping the noise to the dynamical degrees of freedom.
When the spin dynamics is discretized in time, a general
fluctuation-dissipation theorem, valid for non-Markovian noise, can be
established, even when zero modes are present. We discuss the subtleties that
arise, when Gilbert damping is present and the mapping between noise and spin
degrees of freedom is non--linear. | 1610.01622v1 |
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 |
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