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2020-02-20 | Stoner-Wohlfarth switching of the condensate magnetization in a dipolar spinor gas and the metrology of excitation damping | We consider quasi-one-dimensional dipolar spinor Bose-Einstein condensates in
the homogeneous-local-spin-orientation approximation, that is with
unidirectional local magnetization. By analytically calculating the exact
effective dipole-dipole interaction, we derive a Landau-Lifshitz-Gilbert
equation for the dissipative condensate magnetization dynamics, and show how it
leads to the Stoner-Wohlfarth model of a uni-axial ferro-magnetic particle,
where the latter model determines the stable magnetization patterns and
hysteresis curves for switching between them. For an external magnetic field
pointing along the axial, long direction, we analytically solve the
Landau-Lifshitz-Gilbert equation. The solution explicitly demonstrates that the
magnetic dipole-dipole interaction {\it accelerates} the dissipative dynamics
of the magnetic moment distribution and the associated dephasing of the
magnetic moment direction. Under suitable conditions, dephasing of the
magnetization direction due to dipole-dipole interactions occurs within time
scales up to two orders of magnitude smaller than the lifetime of currently
experimentally realized dipolar spinor condensates, e.g., produced with the
large magnetic-dipole-moment atoms ${}^{166} \textrm{Er}$. This enables
experimental access to the dissipation parameter $\Gamma$ in the
Gross-Pitaevski\v\i~mean-field equation, for a system currently lacking a
complete quantum kinetic treatment of dissipative processes and, in particular,
an experimental check of the commonly used assumption that $\Gamma$ is a single
scalar independent of spin indices. | 2002.08723v2 |
2022-06-20 | First-principles calculation of the parameters used by atomistic magnetic simulations | While the ground state of magnetic materials is in general well described on
the basis of spin density functional theory (SDFT), the theoretical description
of finite-temperature and non-equilibrium properties require an extension
beyond the standard SDFT. Time-dependent SDFT (TD-SDFT), which give for example
access to dynamical properties are computationally very demanding and can
currently be hardly applied to complex solids. Here we focus on the alternative
approach based on the combination of a parameterized phenomenological spin
Hamiltonian and SDFT-based electronic structure calculations, giving access to
the dynamical and finite-temperature properties for example via spin-dynamics
simulations using the Landau-Lifshitz-Gilbert (LLG) equation or Monte Carlo
simulations. We present an overview on the various methods to calculate the
parameters of the various phenomenological Hamiltonians with an emphasis on the
KKR Green function method as one of the most flexible band structure methods
giving access to practically all relevant parameters. Concerning these, it is
crucial to account for the spin-orbit coupling (SOC) by performing relativistic
SDFT-based calculations as it plays a key role for magnetic anisotropy and
chiral exchange interactions represented by the DMI parameters in the spin
Hamiltonian. This concerns also the Gilbert damping parameters characterizing
magnetization dissipation in the LLG equation, chiral multispin interaction
parameters of the extended Heisenberg Hamiltonian, as well as spin-lattice
interaction parameters describing the interplay of spin and lattice dynamics
processes, for which an efficient computational scheme has been developed
recently by the present authors. | 2206.09969v1 |
2023-09-25 | Ultrafast Demagnetization through Femtosecond Generation of Non-thermal Magnons | Ultrafast laser excitation of ferromagnetic metals gives rise to correlated,
highly non-equilibrium dynamics of electrons, spins and lattice, which are,
however, poorly described by the widely-used three-temperature model (3TM).
Here, we develop a fully ab-initio parameterized out-of-equilibrium theory
based on a quantum kinetic approach--termed (N+2) temperature model--that
describes magnon occupation dynamics due to electron-magnon scattering. We
apply this model to perform quantitative simulations on the ultrafast,
laser-induced generation of magnons in iron and demonstrate that on these
timescales the magnon distribution is non-thermal: predominantly high-energy
magnons are created, while the magnon occupation close to the center of the
Brillouin zone even decreases, due to a repopulation towards higher energy
states via a so-far-overlooked scattering term. We demonstrate that the simple
relation between magnetization and temperature computed at equilibrium does not
hold in the ultrafast regime and that the 3TM greatly overestimates the
demagnetization. The ensuing Gilbert damping becomes strongly magnon wavevector
dependent and requires a description beyond the conventional
Landau-Lifshitz-Gilbert spin dynamics. Our ab-initio-parameterized calculations
show that ultrafast generation of non-thermal magnons provides a sizable
demagnetization within 200fs in excellent comparison with experimentally
observed laser-induced demagnetizations. Our investigation emphasizes the
importance of non-thermal magnon excitations for the ultrafast demagnetization
process. | 2309.14167v3 |
2023-12-12 | Sliding Dynamics of Current-Driven Skyrmion Crystal and Helix in Chiral Magnets | The skyrmion crystal (SkX) and helix (HL) phases, present in typical chiral
magnets, can each be considered as forms of density waves but with distinct
topologies. The SkX exhibits gyrodynamics analogous to electrons under a
magnetic field, while the HL state resembles topological trivial spin density
waves. However, unlike the charge density waves, the theoretical analysis of
the sliding motion of SkX and HL remains unclear, especially regarding the
similarities and differences in sliding dynamics between these two spin density
waves. In this work, we systematically explore the sliding dynamics of SkX and
HL in chiral magnets in the limit of large current density. We demonstrate that
the sliding dynamics of both SkX and HL can be unified within the same
theoretical framework as density waves, despite their distinct microscopic
orders. Furthermore, we highlight the significant role of gyrotropic sliding
induced by impurity effects in the SkX state, underscoring the impact of
nontrivial topology on the sliding motion of density waves. Our theoretical
analysis shows that the effect of impurity pinning is much stronger in HL
compared with SkX, i.e., $\chi^{SkX}/\chi^{HL}\sim \alpha^2$ ($\chi^{SkX}$,
$\chi^{HL}$: susceptibility to the impurity potential, $\alpha$ ($\ll 1$) is
the Gilbert damping). Moreover, the velocity correction is mostly in the
transverse direction to the current in SkX. These results are further
substantiated by realistic Landau-Lifshitz-Gilbert simulations. | 2312.07116v2 |
2000-03-29 | Disagreement between correlations of quantum mechanics and stochastic electrodynamics in the damped parametric oscillator | Intracavity and external third order correlations in the damped nondegenerate
parametric oscillator are calculated for quantum mechanics and stochastic
electrodynamics (SED), a semiclassical theory. The two theories yield greatly
different results, with the correlations of quantum mechanics being cubic in
the system's nonlinear coupling constant and those of SED being linear in the
same constant. In particular, differences between the two theories are present
in at least a mesoscopic regime. They also exist when realistic damping is
included. Such differences illustrate distinctions between quantum mechanics
and a hidden variable theory for continuous variables. | 0003131v1 |
2012-12-18 | Using the mobile phone acceleration sensor in Physics experiments: free and damped harmonic oscillations | The mobile acceleration sensor has been used to in Physics experiments on
free and damped oscillations. Results for the period, frequency, spring
constant and damping constant match very well to measurements obtained by other
methods. The Accelerometer Monitor application for Android has been used to get
the outputs of the sensor. Perspectives for the Physics laboratory have also
been discussed. | 1212.4403v1 |
2014-03-19 | The effects of time-dependent dissipation on the basins of attraction for the pendulum with oscillating support | We consider a pendulum with vertically oscillating support and time-dependent
damping coefficient which varies until reaching a finite final value. The sizes
of the corresponding basins of attraction are found to depend strongly on the
full evolution of the dissipation. In order to predict the behaviour of the
system, it is essential to understand how the sizes of the basins of attraction
for constant dissipation depend on the damping coefficient. For values of the
parameters in the perturbation regime, we characterise analytically the
conditions under which the attractors exist and study numerically how the sizes
of their basins of attraction depend on the damping coefficient. Away from the
perturbation regime, a numerical study of the attractors and the corresponding
basins of attraction for different constant values of the damping coefficient
produces a much more involved scenario: changing the magnitude of the
dissipation causes some attractors to disappear either leaving no trace or
producing new attractors by bifurcation, such as period doubling and
saddle-node bifurcation. For an initially non-constant damping coefficient,
both increasing and decreasing to some finite final value, we numerically
observe that, when the damping coefficient varies slowly from a finite initial
value to a different final value, without changing the set of attractors, the
slower the variation the closer the sizes of the basins of attraction are to
those they have for constant damping coefficient fixed at the initial value. If
during the variation of the damping coefficient attractors appear or disappear,
remarkable additional phenomena may occur. For instance, a fixed point
asymptotically may attract the entire phase space, up to a zero measure set,
even though no attractor with such a property exists for any value of the
damping coefficient between the extreme values. | 1403.4996v1 |
1995-09-06 | Fermi Liquid Damping and NMR Relaxation in Superconductors | Electron collisions for a two dimensional Fermi liquid (FL) are shown to give
a quasiparticle damping with interesting frequency and temperature variations
in the BCS superconducting state. The spin susceptibility which determines the
structure of the damping is analyzed in the normal state for a Hubbard model
with a constant on--site Coulomb repulsion. This is then generalized to the
superconducting state by including coherence factors and self energy and vertex
corrections. Calculations of the NMR relaxation rate reveal that the FL damping
structure can reduce the Hebel--Slichter peak, in agreement with data on the
organic superconductor (MDT-TTF)$_2$AuI$_2$. However, the strongly suppressed
FL damping in the superconducting state does not eliminate the Hebel-Slichter
peak, and thus suggests that other mechanisms are needed to explain the NMR
data on (TMTSF)$_2$ClO$_4$, the BEDT organic compounds, and cuprate
superconductors. Predictions of the temperature variation of the damping and
the spin response are given over a wide frequency range as a guide to
experimental probes of the symmetry of the superconducting pairs. | 9509028v1 |
2002-07-26 | Landau damping of partially incoherent Langmuir waves | It is shown that partial incoherence, in the form of stochastic phase noise,
of a Langmuir wave in an unmagnetized plasma gives rise to a Landau-type
damping. Starting from the Zakharov equations, which describe the nonlinear
interaction between Langmuir and ion-acoustic waves, a kinetic equation is
derived for the plasmons by introducing the Wigner-Moyal transform of the
complex Langmuir wave field. This equation is then used to analyze the
stability properties of small perturbations on a stationary solution consisting
of a constant amplitude wave with stochastic phase noise. The concomitant
dispersion relation exhibits the phenomenon of Landau-like damping. However,
this damping differs from the classical Landau damping in which a Langmuir
wave, interacting with the plasma electrons, loses energy. In the present
process, the damping is non-dissipative and is caused by the resonant
interaction between an instantaneously-produced disturbance, due to the
parametric interactions, and a partially incoherent Langmuir wave, which can be
considered as a quasi-particle composed of an ensemble of partially incoherent
plasmons. | 0207050v1 |
2017-07-30 | Blow-up for semilinear damped wave equations with sub-Strauss exponent in the scattering case | It is well-known that the critical exponent for semilinear damped wave
equations is Fujita exponent when the damping is effective. Lai, Takamura and
Wakasa in 2017 have obtained a blow-up result not only for super-Fujita
exponent but also for the one closely related to Strauss exponent when the
damping is scaling invariant and its constant is relatively small,which has
been recently extended by Ikeda and Sobajima. Introducing a multiplier for the
time-derivative of the spatial integral of unknown functions, we succeed in
employing the technics on the analysis for semilinear wave equations and
proving a blow-up result for semilinear damped wave equations with sub-Strauss
exponent when the damping is in the scattering range. | 1707.09583v3 |
2020-05-15 | Slow magnetosonic wave absorption by pressure induced ionization-recombination dissipation | A new mechanisms for damping of slow magnetosonic waves (SMW) by pressure
induced oscillations of the ionization degree is proposed. An explicit formula
for the damping rate is quantitatively derived. Physical conditions where the
new mechanism will dominate are briefly discussed. The ionization-recombination
damping is frequency independent and has no hydrodynamic interpretation.
Roughly speaking large area of partially ionized plasma are damper for basses
of SMW while usual MHD mechanisms operate as a low pass filter. The derived
damping rate is proportional to the square of the sine between the constant
magnetic field and the wave-vector. Angular distribution of the spectral
density of SMW and Alfv\'en waves (AW) created by turbulent regions and passing
through large regions of partially ionized plasma is qualitatively considered.
The calculated damping rate is expressed by the electron impact cross section
of the hydrogen atom and in short all details of the proposed damping
mechanisms are well studied. | 2005.07730v1 |
2011-01-17 | Steiner Ratio for Manifolds | The Steiner ratio characterizes the greatest possible deviation of the length
of a minimal spanning tree from the length of the minimal Steiner tree. In this
paper, estimates of the Steiner ratio on Riemannian manifolds are obtained. As
a corollary, the Steiner ratio for flat tori, flat Klein bottles, and
projective plane of constant positive curvature are computed. Steiner ratio -
Steiner problem - Gilbert--Pollack conjecture - surfaces of constant curvature | 1101.3144v1 |
2016-12-30 | Spectroscopic evidence of Alfvén wave damping in the off-limb solar corona | We investigate off-limb active region and quiet Sun corona using
spectroscopic data. Active region is clearly visible in several spectral lines
formed in the temperature range of 1.1--2.8 MK. We derive electron number
density using line ratio method, and non-thermal velocity in the off-limb
region up to the distance of 140 Mm. We compare density scale heights derived
from several spectral line pairs with expected scale heights as per hydrostatic
equilibrium model. Using several isolated and unblended spectral line profiles,
we estimate non-thermal velocities in active region and quiet Sun. Non-thermal
velocities obtained from warm lines in active region first show increase and
later show either decrease or almost constant value with height in the far
off-limb region, whereas hot lines show consistent decrease. However, in the
quiet Sun region, non-thermal velocities obtained from various spectral lines
show either gradual decrease or remain almost constant with height. Using these
obtained parameters, we further calculate Alfv\'en wave energy flux in the both
active and quiet Sun regions. We find significant decrease in wave energy
fluxes with height, and hence provide evidence of Alfv\'en wave damping.
Furthermore, we derive damping lengths of Alfv\'en waves in the both regions
and find them to be in the range of 25-170 Mm. Different damping lengths
obtained at different temperatures may be explained as either possible
temperature dependent damping or measurements obtained in different coronal
structures formed at different temperatures along the line-of-sight.
Temperature dependent damping may suggest some role of thermal conduction in
the damping of Alfv\'en waves in the lower corona. | 1612.09551v2 |
1997-06-30 | Damped Lyman Alpha Systems at High Redshift and Models of Protogalactic Disks | We employ observationally determined intrinsic velocity widths and column
densities of damped Lyman-alpha systems at high redshift to investigate the
distribution of baryons in protogalaxies within the context of a standard cold
dark matter model. We proceed under the assumption that damped Lyman alpha
systems represent a population of cold, rotationally supported, protogalactic
disks and that the abundance of protogalactic halos is well approximated by a
cold dark matter model with critical density and vanishing cosmological
constant. Using conditional cross sections to observe a damped system with a
given velocity width and column density, we compare observationally inferred
velocity width and column density distributions to the corresponding
theoretically determined distributions for a variety of disk parameters and CDM
normalizations. In general, we find that the observations can not be reproduced
by the models for most disk parameters and CDM normalizations. Whereas the
column density distribution favors small disks with large neutral gas fraction,
the velocity width distribution favors large and thick disks with small neutral
gas fraction. The possible resolutions of this problem in the context of this
CDM model may be: (1) an increased contribution of rapidly rotating disks
within massive dark matter halos to damped Lyman-alpha absorption or (2) the
abandoning of simple disk models within this CDM model for damped Lyman-alpha
systems at high redshift. Here the first possibility may be achieved by
supposing that damped Lya system formation only occurs in halos with fairly
large circular velocities and the second possibility may result from a large
contribution of mergers and double-disks to damped Lya absorption at high
redshift. | 9706290v1 |
2019-01-24 | Generalization of Stokes-Einstein relation to coordinate dependent damping and diffusivity: An apparent conflict | Brownian motion with coordinate dependent damping and diffusivity is
ubiquitous. Understanding equilibrium of a Brownian particle with coordinate
dependent diffusion and damping is a contentious area. In this paper, we
present an alternative approach based on already established methods to this
problem. We solve for the equilibrium distribution of the over-damped dynamics
using Kramers-Moyal expansion. We compare this with the over-damped limit of
the generalized Maxwell-Boltzmann distribution. We show that the equipartition
of energy helps recover the Stokes-Einstein relation at constant diffusivity
and damping of the homogeneous space. However, we also show that, there exists
no homogeneous limit of coordinate dependent diffusivity and damping with
respect to the applicability of Stokes-Einstein relation when it does not hold
locally. In the other scenario where the Stokes-Einstein relation holds
locally, one needs to impose a restriction on the local maximum velocity of the
Brownian particle to make the modified Maxwell-Boltzmann distribution coincide
with the modified Boltzmann distribution in the over-damped limit. | 1901.08358v4 |
1996-11-25 | Damping rates of hard momentum particles in a cold ultrarelativistic plasma | We compute the damping rates of one-particle excitations in a cold
ultrarelativistic plasma to leading order in the coupling constant e for three
types of interaction: Yukawa coupling to a massless scalar boson, QED and QCD.
Damping rates of charged particles in QED and QCD are of order e^3 mu, while
damping rates of other particles are of order e^4 mu or e^4 mu log(1/e). We
find that the damping rate of an electron or of a quark is constant far from
the Fermi surface, and decreases linearly with the excitation energy close to
the Fermi surface. This unusual behavior is attributed to the long-range
magnetic interactions. | 9611415v2 |
2011-06-23 | Ratchet effect on a relativistic particle driven by external forces | We study the ratchet effect of a damped relativistic particle driven by both
asymmetric temporal bi-harmonic and time-periodic piecewise constant forces.
This system can be formally solved for any external force, providing the
ratchet velocity as a non-linear functional of the driving force. This allows
us to explicitly illustrate the functional Taylor expansion formalism recently
proposed for this kind of systems. The Taylor expansion reveals particularly
useful to obtain the shape of the current when the force is periodic, piecewise
constant. We also illustrate the somewhat counterintuitive effect that
introducing damping may induce a ratchet effect. When the force is symmetric
under time-reversal and the system is undamped, under symmetry principles no
ratchet effect is possible. In this situation increasing damping generates a
ratchet current which, upon increasing the damping coefficient eventually
reaches a maximum and decreases toward zero. We argue that this effect is not
specific of this example and should appear in any ratchet system with tunable
damping driven by a time-reversible external force. | 1106.4861v1 |
2012-10-20 | Radiative damping of surface plasmon resonance in spheroidal metallic nanoparticle embedded in a dielectric medium | The local field approach and kinetic equation method is applied to calculate
the surface plasmon radiative damping in a spheroidal metal nanoparticle
embedded in any dielectric media. The radiative damping of the surface plasmon
resonance as a function of the particle radius, shape, dielectric constant of
the surrounding medium and the light frequency is studied in detail. It is
found that the radiative damping grows quadratically with the particle radius
and oscillates with altering both the particle size and the dielectric constant
of a surrounding medium. Much attention is paid to the electron
surface-scattering contribution to the plasmon decay. All calculations of the
radiative damping are illustrated by examples on the Au and Na nanoparticles. | 1210.5647v1 |
2015-05-25 | New Explicit Binary Constant Weight Codes from Reed-Solomon Codes | Binary constant weight codes have important applications and have been
studied for many years. Optimal or near-optimal binary constant weight codes of
small lengths have been determined. In this paper we propose a new construction
of explicit binary constant weight codes from $q$-ary Reed-Solomon codes. Some
of our binary constant weight codes are optimal or new. In particular new
binary constant weight codes $A(64, 10, 8) \geq 4108$ and $A(64, 12, 8) \geq
522$ are constructed. We also give explicitly constructed binary constant
weight codes which improve Gilbert and Graham-Sloane lower bounds in some range
of parameters. An extension to algebraic geometric codes is also presented. | 1505.06524v4 |
2002-08-08 | Cosmology with the Sunyaev-Zel'dovich Effect | The Sunyaev-Zel'dovich effect (SZE) provides a unique way to map the
large-scale structure of the universe as traced by massive clusters of
galaxies. As a spectral distortion of the cosmic microwave background, the SZE
is insensitive to the redshift of the galaxy cluster, making it well-suited for
studies of clusters at all redshifts, and especially at reasonably high
redshifts (z > 1) where the abundance of clusters is critically dependent on
the underlying cosmology. Recent high signal-to-noise detections of the SZE
have enabled interesting constraints on the Hubble constant and the matter
density of the universe using small samples of galaxy clusters. Upcoming SZE
surveys are expected to find hundreds to thousands of new galaxy clusters, with
a mass selection function that is remarkably uniform with redshift. In this
review we provide an overview of the SZE and its use for cosmological studies
with emphasis on the cosmology that can, in principle, be extracted from SZE
survey yields. We discuss the observational and theoretical challenges that
must be met before precise cosmological constraints can be extracted from the
survey yields. | 0208192v1 |
2004-04-19 | Asymptotic Improvement of the Gilbert-Varshamov Bound on the Size of Binary Codes | Given positive integers $n$ and $d$, let $A_2(n,d)$ denote the maximum size
of a binary code of length $n$ and minimum distance $d$. The well-known
Gilbert-Varshamov bound asserts that $A_2(n,d) \geq 2^n/V(n,d-1)$, where
$V(n,d) = \sum_{i=0}^{d} {n \choose i}$ is the volume of a Hamming sphere of
radius $d$. We show that, in fact, there exists a positive constant $c$ such
that $$ A_2(n,d) \geq c \frac{2^n}{V(n,d-1)} \log_2 V(n,d-1) $$ whenever $d/n
\le 0.499$. The result follows by recasting the Gilbert- Varshamov bound into a
graph-theoretic framework and using the fact that the corresponding graph is
locally sparse. Generalizations and extensions of this result are briefly
discussed. | 0404325v1 |
2010-08-12 | Magnetization dynamics in the inertial regime: nutation predicted at short time scales | The dynamical equation of the magnetization has been reconsidered with
enlarging the phase space of the ferromagnetic degrees of freedom to the
angular momentum. The generalized Landau-Lifshitz-Gilbert equation that
includes inertial terms, and the corresponding Fokker-Planck equation, are then
derived in the framework of mesoscopic non-equilibrium thermodynamics theory. A
typical relaxation time $\tau$ is introduced describing the relaxation of the
magnetization acceleration from the inertial regime towards the precession
regime defined by a constant Larmor frequency. For time scales larger than
$\tau$, the usual Gilbert equation is recovered. For time scales below $\tau$,
nutation and related inertial effects are predicted. The inertial regime offers
new opportunities for the implementation of ultrafast magnetization switching
in magnetic devices. | 1008.2177v1 |
2011-09-12 | Externally-driven transmission and collisions of domain walls in ferromagnetic wires | Analytical multi-domain solutions to the dynamical (Landau-Lifshitz-Gilbert)
equation of a one-dimensional ferromagnet including an external magnetic field
and spin-polarized electric current are found using the Hirota bilinearization
method. A standard approach to solve the Landau-Lifshitz equation (without the
Gilbert term) is modified in order to treat the dissipative dynamics. I
establish the relations between the spin interaction parameters (the constants
of exchange, anisotropy, dissipation, external-field intensity, and
electric-current intensity) and the domain-wall parameters (width and velocity)
and compare them to the results of the Walker approximation and micromagnetic
simulations. The domain-wall motion driven by a longitudinal external field is
analyzed with especial relevance to the field-induced collision of two domain
walls. I determine the result of such a collision (which is found to be the
elastic one) on the domain-wall parameters below and above the Walker breakdown
(in weak- and strong-field regimes). Single-domain-wall dynamics in the
presence of an external transverse field is studied with relevance to the
challenge of increasing the domain-wall velocity below the breakdown. | 1109.2465v1 |
2015-01-31 | Bases and Structure Constants of Generalized Splines with Integer Coefficients on Cycles | An integer generalized spline is a set of vertex labels on an edge-labeled
graph that satisfy the condition that if two vertices are joined by an edge,
the vertex labels are congruent modulo the edge label. Foundational work on
these objects comes from Gilbert, Polster, and Tymoczko, who generalize ideas
from geometry/topology (equivariant cohomology rings) and algebra (algebraic
splines) to develop the notion of generalized splines. Gilbert, Polster, and
Tymoczko prove that the ring of splines on a graph can be decomposed in terms
of splines on its subgraphs (in particular, on trees and cycles), and then
fully analyze splines on trees. Following Handschy-Melnick-Reinders and Rose,
we analyze splines on cycles, in our case integer generalized splines. The
primary goal of this paper is to establish two new bases for the module of
integer generalized splines on cycles: the triangulation basis and the King
basis. Unlike bases in previous work, we are able to characterize each basis
element completely in terms of the edge labels of the underlying cycle. As an
application we explicitly construct the multiplication table for the ring of
integer generalized splines in terms of the King basis. | 1502.00176v1 |
2022-11-22 | Generalized Random Gilbert-Varshamov Codes: Typical Error Exponent and Concentration Properties | We find the exact typical error exponent of constant composition generalized
random Gilbert-Varshamov (RGV) codes over DMCs channels with generalized
likelihood decoding. We show that the typical error exponent of the RGV
ensemble is equal to the expurgated error exponent, provided that the RGV
codebook parameters are chosen appropriately. We also prove that the random
coding exponent converges in probability to the typical error exponent, and the
corresponding non-asymptotic concentration rates are derived. Our results show
that the decay rate of the lower tail is exponential while that of the upper
tail is double exponential above the expurgated error exponent. The explicit
dependence of the decay rates on the RGV distance functions is characterized. | 2211.12238v1 |
2023-01-05 | Improved Gilbert-Varshamov bounds for hopping cyclic codes and optical orthogonal codes | Hopping cyclic codes (HCCs) are (non-linear) cyclic codes with the additional
property that the $n$ cyclic shifts of every given codeword are all distinct,
where $n$ is the code length. Constant weight binary hopping cyclic codes are
also known as optical orthogonal codes (OOCs). HCCs and OOCs have various
practical applications and have been studied extensively over the years.
The main concern of this paper is to present improved Gilbert-Varshamov type
lower bounds for these codes, when the minimum distance is bounded below by a
linear factor of the code length. For HCCs, we improve the previously best
known lower bound of Niu, Xing, and Yuan by a linear factor of the code length.
For OOCs, we improve the previously best known lower bound of Chung, Salehi,
and Wei, and Yang and Fuja by a quadratic factor of the code length. As
by-products, we also provide improved lower bounds for frequency hopping
sequences sets and error-correcting weakly mutually uncorrelated codes. Our
proofs are based on tools from probability theory and graph theory, in
particular the McDiarmid's inequality on the concentration of Lipschitz
functions and the independence number of locally sparse graphs. | 2301.02042v1 |
2006-01-18 | Expressions for frictional and conservative force combinations within the dissipative Lagrange-Hamilton formalism | Dissipative Lagrangians and Hamiltonians having Coulomb, viscous and
quadratic damping,together with gravitational and elastic terms are presented
for a formalism that preserves the Hamiltonian as a constant of the motion.
Their derivations are also shown. The resulting L's and H's may prove useful in
exploring new types of damped quantum systems. | 0601133v1 |
2010-03-28 | Damped wave dynamics for a complex Ginzburg-Landau equation with low dissipation | We consider a complex Ginzburg-Landau equation, corresponding to a
Gross-Pitaevskii equation with a small dissipation term. We study an asymptotic
regime for long-wave perturbations of constant maps of modulus one. We show
that such solutions never vanish and we derive a damped wave dynamics for the
perturbation. | 1003.5375v1 |
2011-11-20 | Null controllability of the structurally damped wave equation with moving point control | We investigate the internal controllability of the wave equation with
structural damping on the one dimensional torus. We assume that the control is
acting on a moving point or on a moving small interval with a constant
velocity. We prove that the null controllability holds in some suitable Sobolev
space and after a fixed positive time independent of the initial conditions. | 1111.4655v1 |
2013-09-19 | Compressible Euler equation with damping on Torus in arbitrary dimensions | We study the exponential stability of constant steady state of isentropic
compressible Euler equation with damping on $\mathbb T^n$. The local existence
of solutions is based on semigroup theory and some commutator estimates. We
propose a new method instead of energy estimates to study the stability, which
works equally well for any spatial dimensions. | 1309.5059v3 |
2018-09-26 | Permutation-invariant constant-excitation quantum codes for amplitude damping | The increasing interest in using quantum error correcting codes in practical
devices has heightened the need for designing quantum error correcting codes
that can correct against specialized errors, such as that of amplitude damping
errors which model photon loss. Although considerable research has been devoted
to quantum error correcting codes for amplitude damping, not so much attention
has been paid to having these codes simultaneously lie within the decoherence
free subspace of their underlying physical system. One common physical system
comprises of quantum harmonic oscillators, and constant-excitation quantum
codes can be naturally stabilized within them. The purpose of this paper is to
give constant-excitation quantum codes that not only correct amplitude damping
errors, but are also immune against permutations of their underlying modes. To
construct such quantum codes, we use the nullspace of a specially constructed
matrix based on integer partitions. | 1809.09801v4 |
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 |
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 |
2017-06-15 | Generalized Voltage-based State-Space Modelling of Modular Multilevel Converters with Constant Equilibrium in Steady-State | This paper demonstrates that the sum and difference of the upper and lower
arm voltages are suitable variables for deriving a generalized state-space
model of an MMC which settles at a constant equilibrium in steady-state
operation, while including the internal voltage and current dynamics. The
presented modelling approach allows for separating the multiple frequency
components appearing within the MMC as a first step of the model derivation, to
avoid variables containing multiple frequency components in steady-state. On
this basis, it is shown that Park transformations at three different
frequencies ($+\omega$, $-2\omega$ and $+3\omega$) can be applied for deriving
a model formulation where all state-variables will settle at constant values in
steady-state, corresponding to an equilibrium point of the model. The resulting
model is accurately capturing the internal current and voltage dynamics of a
three-phase MMC, independently from how the control system is implemented. The
main advantage of this model formulation is that it can be linearised, allowing
for eigenvalue-based analysis of the MMC dynamics. Furthermore, the model can
be utilized for control system design by multi-variable methods requiring any
stable equilibrium to be defined by a fixed operating point. Time-domain
simulations in comparison to an established average model of the MMC, as well
as results from a detailed simulation model of an MMC with 400 sub-modules per
arm, are presented as verification of the validity and accuracy of the
developed model. | 1706.04959v1 |
1992-04-06 | Comment on ``High Temperature Fermion Propagator -- Resummation and Gauge Dependence of the Damping Rate'' | Baier et al. have reported the damping rate of long-wavelength fermionic
excitations in high-temperature QED and QCD to be gauge-fixing-dependent even
within the resummation scheme due to Braaten and Pisarski. It is shown that
this problem is caused by the singular nature of the on-shell expansion of the
fermion self-energy in the infra-red. Its regularization reveals that the
alleged gauge dependence pertains to the residue rather than the pole of the
fermion propagator, so that in particular the damping constant comes out
gauge-independent, as it should. | 9204210v1 |
2003-07-02 | Harmonic Oscillator Potential to describe Internal Dissipation | Assuming that a constant potential energy function has meaning for a
dissipated harmonic oscillator, then an important issue is the time dependence
of the turning points. Turning point studies demonstrate that the common model
of external (viscous) damping fails to properly describe those many systems
where structural (internal friction) damping is the most important source of
dissipation. For internal friction damping, the better model of potential
energy is one in which the function is not stationary. | 0307016v1 |
2009-12-16 | Toward a dynamical shift condition for unequal mass black hole binary simulations | Moving puncture simulations of black hole binaries rely on a specific gauge
choice that leads to approximately stationary coordinates near each black hole.
Part of the shift condition is a damping parameter, which has to be properly
chosen for stable evolutions. However, a constant damping parameter does not
account for the difference in mass in unequal mass binaries. We introduce a
position dependent shift damping that addresses this problem. Although the
coordinates change, the changes in the extracted gravitational waves are small. | 0912.3125v1 |
2010-03-09 | Damping of Nanomechanical Resonators | We study the transverse oscillatory modes of nanomechanical silicon nitride
strings under high tensile stress as a function of geometry and mode index m <=
9. Reproducing all observed resonance frequencies with classical elastic theory
we extract the relevant elastic constants. Based on the oscillatory local
strain we successfully predict the observed mode-dependent damping with a
single frequency independent fit parameter. Our model clarifies the role of
tensile stress on damping and hints at the underlying microscopic mechanisms. | 1003.1868v1 |
2011-10-12 | Acceleration Control in Nonlinear Vibrating Systems based on Damped Least Squares | A discrete time control algorithm using the damped least squares is
introduced for acceleration and energy exchange controls in nonlinear vibrating
systems. It is shown that the damping constant of least squares and sampling
time step of the controller must be inversely related to insure that vanishing
the time step has little effect on the results. The algorithm is illustrated on
two linearly coupled Duffing oscillators near the 1:1 internal resonance. In
particular, it is shown that varying the dissipation ratio of one of the two
oscillators can significantly suppress the nonlinear beat phenomenon. | 1110.2811v2 |
2012-03-21 | Approximate rogue wave solutions of the forced and damped Nonlinear Schrödinger equation for water waves | We consider the effect of the wind and the dissipation on the nonlinear
stages of the modulational instability. By applying a suitable transformation,
we map the forced/damped Nonlinear Schr\"odinger (NLS) equation into the
standard NLS with constant coefficients. The transformation is valid as long as
|{\Gamma}t| \ll 1, with {\Gamma} the growth/damping rate of the waves due to
the wind/dissipation. Approximate rogue wave solutions of the equation are
presented and discussed. The results shed some lights on the effects of wind
and dissipation on the formation of rogue waves. | 1203.4735v1 |
2014-10-05 | Ultimate limit of field confinement by surface plasmon polaritons | We show that electric field confinement in surface plasmon polaritons
propagating at the metal/dielectric interfaces enhances the loss due to Landau
damping and which effectively limits the degree of confinement itself. We prove
that Landau damping and associated with it surface collision damping follow
directly from Lindhard formula for the dielectric constant of free electron gas
Furthermore, we demonstrate that even if all the conventional loss mechanisms,
caused by phonons, electron-electron, and interface roughness scattering, were
eliminated, the maximum attainable degree of confinement and the loss
accompanying it would not change significantly compared to the best existing
plasmonic materials, such as silver. | 1410.1226v1 |
2016-04-18 | Parameter Estimation of Gaussian-Damped Sinusoids from a Geometric Perspective | The five parameter gaussian damped sinusoid equation is a reasonable model
for betatron motion with chromatic decoherence of the proton bunch centroid
signal in the ring at the Spallation Neutron Source. A geometric method for
efficiently fitting this equation to the turn by turn signals to extract the
betatron tune and damping constant will be presented. This method separates the
parameters into global and local parameters and allows the use of vector
arithmetic to eliminate the local parameters from the parameter search space.
Furthermore, this method is easily generalized to reduce the parameter search
space for a larger class of problems. | 1604.05167v1 |
2016-07-13 | Optimal decay rate for the wave equation on a square with constant damping on a strip | We consider the damped wave equation with Dirichlet boundary conditions on
the unit square. We assume the damping to be a characteristic function of a
strip. We prove the exact $t^{-4/3}$-decay rate for the energy of classical
solutions. This answers a question of Anantharaman and L\'eautaud (2014). | 1607.03633v2 |
2016-09-20 | Global existence and asymptotic behavior of solutions to the Euler equations with time-dependent damping | We study the isentropic Euler equations with time-dependent damping, given by
$\frac{\mu}{(1+t)^\lambda}\rho u$. Here, $\lambda,\mu$ are two non-negative
constants to describe the decay rate of damping with respect to time. We will
investigate the global existence and asymptotic behavior of small data
solutions to the Euler equations when $0<\lambda<1,0<\mu$ in multi-dimensions
$n\geq 1$. The asymptotic behavior will coincide with the one that obtained by
many authors in the case $\lambda=0$. We will also show that the solution can
only decay polynomially in time while in the three dimensions, the vorticity
will decay exponentially fast. | 1609.06286v1 |
2018-06-08 | Brownian motion of magnetic domain walls and skyrmions, and their diffusion constants | Extended numerical simulations enable to ascertain the diffusive behavior at
finite temperatures of chiral walls and skyrmions in ultra-thin model Co layers
exhibiting symmetric - Heisenberg - as well as antisymmetric -
Dzyaloshinskii-Moriya - exchange interactions. The Brownian motion of walls and
skyrmions is shown to obey markedly different diffusion laws as a function of
the damping parameter. Topology related skyrmion diffusion suppression with
vanishing damping parameter, albeit already documented, is shown to be
restricted to ultra-small skyrmion sizes or, equivalently, to ultra-low damping
coefficients, possibly hampering observation. | 1806.03172v1 |
2017-09-24 | Suppression of Recurrence in the Hermite-Spectral Method for Transport Equations | We study the unphysical recurrence phenomenon arising in the numerical
simulation of the transport equations using Hermite-spectral method. From a
mathematical point of view, the suppression of this numerical artifact with
filters is theoretically analyzed for two types of transport equations. It is
rigorously proven that all the non-constant modes are damped exponentially by
the filters in both models, and formally shown that the filter does not affect
the damping rate of the electric energy in the linear Landau damping problem.
Numerical tests are performed to show the effect of the filters. | 1709.08194v1 |
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 |
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 |
2019-11-21 | Low damping and microstructural perfection of sub-40nm-thin yttrium iron garnet films grown by liquid phase epitaxy | The field of magnon spintronics is experiencing an increasing interest in the
development of solutions for spin-wave-based data transport and processing
technologies that are complementary or alternative to modern CMOS
architectures. Nanometer-thin yttrium iron garnet (YIG) films have been the
gold standard for insulator-based spintronics to date, but a potential process
technology that can deliver perfect, homogeneous large-diameter films is still
lacking. We report that liquid phase epitaxy (LPE) enables the deposition of
nanometer-thin YIG films with low ferromagnetic resonance losses and
consistently high magnetic quality down to a thickness of 20 nm. The obtained
epitaxial films are characterized by an ideal stoichiometry and perfect film
lattices, which show neither significant compositional strain nor geometric
mosaicity, but sharp interfaces. Their magneto-static and dynamic behavior is
similar to that of single crystalline bulk YIG. We found, that the Gilbert
damping coefficient alpha is independent of the film thickness and close to 1 x
10-4, and that together with an inhomogeneous peak-to-peak linewidth broadening
of delta H0|| = 0.4 G, these values are among the lowest ever reported for YIG
films with a thickness smaller than 40 nm. These results suggest, that
nanometer-thin LPE films can be used to fabricate nano- and micro-scaled
circuits with the required quality for magnonic devices. The LPE technique is
easily scalable to YIG sample diameters of several inches. | 1911.09400v1 |
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 |
2018-04-20 | A Weakly Nonlinear Model for the Damping of Resonantly Forced Density Waves in Dense Planetary Rings | In this paper we address the stability of resonantly forced density waves in
dense planetary rings.
Already by Goldreich & Tremaine (1978) it has been argued that density waves
might be unstable, depending on the relationship between the ring's viscosity
and the surface mass density.
In the recent paper Schmidt et al. (2016) we have pointed out that when -
within a fluid description of the ring dynamics - the criterion for viscous
overstability is satisfied, forced spiral density waves become unstable as
well.
In this case, linear theory fails to describe the damping, but nonlinearity
of the underlying equations guarantees a finite amplitude and eventually a
damping of the wave.
We apply the multiple scale formalism to derive a weakly nonlinear damping
relation from a hydrodynamical model.
This relation describes the resonant excitation and nonlinear viscous damping
of spiral density waves in a vertically integrated fluid disk with density
dependent transport coefficients.
The model consistently predicts density waves to be (linearly) unstable in a
ring region where the conditions for viscous overstability are met.
Sufficiently far away from the Lindblad resonance, the surface mass density
perturbation is predicted to saturate to a constant value due to nonlinear
viscous damping.
The wave's damping lengths of the model depend on certain input parameters,
such as the distance to the threshold for viscous overstability in parameter
space and the ground state surface mass density. | 1804.07674v1 |
2019-03-02 | Complex Stiffness Model of Physical Human-Robot Interaction: Implications for Control of Performance Augmentation Exoskeletons | Human joint dynamic stiffness plays an important role in the stability of
performance augmentation exoskeletons. In this paper, we consider a new
frequency domain model of the human joint dynamics which features a complex
value stiffness. This complex stiffness consists of a real stiffness and a
hysteretic damping. We use it to explain the dynamic behaviors of the human
connected to the exoskeleton, in particular the observed non-zero low frequency
phase shift and the near constant damping ratio of the resonant as stiffness
and inertia vary. We validate this concept by experimenting with an elbow-joint
exoskeleton testbed on a subject while modifying joint stiffness behavior,
exoskeleton inertia, and strength augmentation gains. We compare three
different models of elbow-joint dynamic stiffness: a model with real stiffness,
viscous damping and inertia, a model with complex stiffness and inertia, and a
model combining the previous two models. Our results show that the hysteretic
damping term improves modeling accuracy, using a statistical F-test. Moreover
this improvement is statistically more significant than using classical viscous
damping term. In addition, we experimentally observe a linear relationship
between the hysteretic damping and the real part of the stiffness which allows
us to simplify the complex stiffness model as a 1-parameter system. Ultimately,
we design a fractional order controller to demonstrate how human hysteretic
damping behavior can be exploited to improve strength amplification performance
while maintaining stability. | 1903.00704v4 |
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 |
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 |
2007-09-18 | 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 |
2008-06-28 | Theory of spin magnetohydrodynamics | We develop a phenomenological hydrodynamic theory of coherent magnetic
precession coupled to electric currents. Exchange interaction between electron
spin and collective magnetic texture produces two reciprocal effects:
spin-transfer torque on the magnetic order parameter and the Berry-phase gauge
field experienced by the itinerant electrons. The dissipative processes are
governed by three coefficients: the ohmic resistance, Gilbert damping of the
magnetization, and the "beta coefficient" describing viscous coupling between
magnetic dynamics and electric current, which stems from spin mistracking of
the magnetic order. We develop general magnetohydrodynamic equations and
discuss the net dissipation produced by the coupled dynamics. The latter in
particular allows us to determine a lower bound on the magnetic-texture
resistivity. | 0806.4656v2 |
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 |
2008-12-13 | 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 |
2010-08-03 | Determination of the spin-flip time in ferromagnetic SrRuO3 from time-resolved Kerr measurements | We report time-resolved Kerr effect measurements of magnetization dynamics in
ferromagnetic SrRuO3. We observe that the demagnetization time slows
substantially at temperatures within 15K of the Curie temperature, which is ~
150K. We analyze the data with a phenomenological model that relates the
demagnetization time to the spin flip time. In agreement with our observations
the model yields a demagnetization time that is inversely proportional to T-Tc.
We also make a direct comparison of the spin flip rate and the Gilbert damping
coefficient showing that their ratio very close to kBTc, indicating a common
origin for these phenomena. | 1008.0674v1 |
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 |
2011-07-04 | 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 |
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 |
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 |
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 |
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 |
2016-04-24 | Coupled Spin-Light dynamics in Cavity Optomagnonics | Experiments during the past two years have shown strong resonant
photon-magnon coupling in microwave cavities, while coupling in the optical
regime was demonstrated very recently for the first time. Unlike with
microwaves, the coupling in optical cavities is parametric, akin to
optomechanical systems. This line of research promises to evolve into a new
field of optomagnonics, aimed at the coherent manipulation of elementary
magnetic excitations by optical means. In this work we derive the microscopic
optomagnonic Hamiltonian. In the linear regime the system reduces to the
well-known optomechanical case, with remarkably large coupling. Going beyond
that, we study the optically induced nonlinear classical dynamics of a
macrospin. In the fast cavity regime we obtain an effective equation of motion
for the spin and show that the light field induces a dissipative term
reminiscent of Gilbert damping. The induced dissipation coefficient however can
change sign on the Bloch sphere, giving rise to self-sustained oscillations.
When the full dynamics of the system is considered, the system can enter a
chaotic regime by successive period doubling of the oscillations. | 1604.07053v3 |
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 |
2016-05-21 | Landau-Lifshitz theory of the magnon-drag thermopower | Metallic ferromagnets subjected to a temperature gradient exhibit a magnonic
drag of the electric current. We address this problem by solving a stochastic
Landau-Lifshitz equation to calculate the magnon-drag thermopower. The
long-wavelength magnetic dynamics result in two contributions to the
electromotive force acting on electrons: (1) An adiabatic Berry-phase force
related to the solid angle subtended by the magnetic precession and (2) a
dissipative correction thereof, which is rooted microscopically in the
spin-dephasing scattering. The first contribution results in a net force
pushing the electrons towards the hot side, while the second contribution drags
electrons towards the cold side, i.e., in the direction of the magnonic drift.
The ratio between the two forces is proportional to the ratio between the
Gilbert damping coefficient $\alpha$ and the coefficient $\beta$ parametrizing
the dissipative contribution to the electromotive force. | 1605.06578v1 |
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 |
2017-10-30 | Probe of Spin Dynamics in Superconducting NbN Thin Films via Spin Pumping | The emerging field of superconductor (SC) spintronics has attracted intensive
attentions recently. Many fantastic spin dependent properties in SC have been
discovered, including the observation of large magnetoresistance, long spin
lifetimes and the giant spin Hall effect in SC, as well as spin supercurrent in
Josephson junctions, etc. Regarding the spin dynamic in SC films, few studies
has been reported yet. Here, we report the investigation of the spin dynamics
in an s-wave superconducting NbN film via spin pumping from an adjacent
insulating ferromagnet GdN layer. A profound coherence peak of the Gilbert
damping is observed slightly below the superconducting critical temperature of
the NbN layer, which is consistent with recent theoretical studies. Our results
further indicate that spin pumping could be a powerful tool for investigating
the spin dynamics in 2D crystalline superconductors. | 1710.10833v2 |
2017-11-17 | Shot noise of charge and spin transport in a junction with a precessing molecular spin | Magnetic molecules and nanomagnets can be used to influence the electronic
transport in mesoscopic junction. In a magnetic field the precessional motion
leads to resonances in the dc- and ac-transport properties of a nanocontact, in
which the electrons are coupled to the precession. Quantities like the
dc-conductance or the ac-response provide valuable information like the level
structure and the coupling parameters. Here, we address the current noise
properties of such contacts. This encompasses the charge current and
spin-torque shot noise, which both show a step-like behavior as functions of
bias voltage and magnetic field. The charge current noise shows pronounced dips
around the steps, which we trace back to interference effects of electron in
quasienergy levels coupled by the molecular spin precession. We show that some
components of the noise of the spin-torque currents are directly related to the
Gilbert damping and, hence, are experimentally accessible. Our results show
that the noise characteristics allow to investigate in more detail the
coherence of spin transport in contacts containing magnetic molecules. | 1711.06759v2 |
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