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2019-09-10
|
Spin Pumping from Permalloy into Uncompensated Antiferromagnetic Co doped Zinc Oxide
|
Heterostructures of Co-doped ZnO and Permalloy were investigated for their
static and dynamic magnetic interaction. The highly Co-doped ZnO is
paramagentic at room temperature and becomes an uncompensated antiferromagnet
at low temperatures, showing a narrowly opened hysteresis and a vertical
exchange bias shift even in the absence of any ferromagnetic layer. At low
temperatures in combination with Permalloy an exchange bias is found causing a
horizontal as well as vertical shift of the hysteresis of the heterostructure
together with an increase in coercive field. Furthermore, an increase in the
Gilbert damping parameter at room temperature was found by multifrequency FMR
evidencing spin pumping. Temperature dependent FMR shows a maximum in magnetic
damping close to the magnetic phase transition. These measurements also
evidence the exchange bias interaction of Permalloy and long-range ordered
Co-O-Co structures in ZnO, that are barely detectable by SQUID due to the
shorter probing times in FMR.
|
1909.04362v3
|
2020-02-27
|
Ultrafast magnetization dynamics in half-metallic Co$_2$FeAl Heusler alloy
|
We report on optically induced, ultrafast magnetization dynamics in the
Heusler alloy $\mathrm{Co_{2}FeAl}$, probed by time-resolved magneto-optical
Kerr effect. Experimental results are compared to results from electronic
structure theory and atomistic spin-dynamics simulations. Experimentally, we
find that the demagnetization time ($\tau_{M}$) in films of
$\mathrm{Co_{2}FeAl}$ is almost independent of varying structural order, and
that it is similar to that in elemental 3d ferromagnets. In contrast, the
slower process of magnetization recovery, specified by $\tau_{R}$, is found to
occur on picosecond time scales, and is demonstrated to correlate strongly with
the Gilbert damping parameter ($\alpha$). Our results show that
$\mathrm{Co_{2}FeAl}$ is unique, in that it is the first material that clearly
demonstrates the importance of the damping parameter in the remagnetization
process. Based on these results we argue that for $\mathrm{Co_{2}FeAl}$ the
remagnetization process is dominated by magnon dynamics, something which might
have general applicability.
|
2002.12255v1
|
2020-06-05
|
Controlling the nonlinear relaxation of quantized propagating magnons in nanodevices
|
Relaxation of linear magnetization dynamics is well described by the viscous
Gilbert damping processes. However, for strong excitations, nonlinear damping
processes such as the decay via magnon-magnon interactions emerge and trigger
additional relaxation channels. Here, we use space- and time-resolved
microfocused Brillouin light scattering spectroscopy and micromagnetic
simulations to investigate the nonlinear relaxation of strongly driven
propagating spin waves in yttrium iron garnet nanoconduits. We show that the
nonlinear magnon relaxation in this highly quantized system possesses
intermodal features, i.e., magnons scatter to higher-order quantized modes
through a cascade of scattering events. We further show how to control such
intermodal dissipation processes by quantization of the magnon band in
single-mode devices, where this phenomenon approaches its fundamental limit.
Our study extends the knowledge about nonlinear propagating spin waves in
nanostructures which is essential for the construction of advanced spin-wave
elements as well as the realization of Bose-Einstein condensates in scaled
systems.
|
2006.03400v2
|
2021-05-16
|
Anatomy of inertial magnons in ferromagnets
|
We analyze dispersion relations of magnons in ferromagnetic nanostructures
with uniaxial anisotropy taking into account inertial terms, i.e. magnetic
nutation. Inertial effects are parametrized by damping-independent parameter
$\beta$, which allows for an unambiguous discrimination of inertial effects
from Gilbert damping parameter $\alpha$. The analysis of magnon dispersion
relation shows its two branches are modified by the inertial effect, albeit in
different ways. The upper nutation branch starts at $\omega=1/ \beta$, the
lower branch coincides with FMR in the long-wavelength limit and deviates from
the zero-inertia parabolic dependence $\simeq\omega_{FMR}+Dk^2$ of the exchange
magnon. Taking a realistic experimental geometry of magnetic thin films,
nanowires and nanodiscs, magnon eigenfrequencies, eigenvectors and $Q$-factors
are found to depend on the shape anisotropy. The possibility of phase-matched
magneto-elastic excitation of nutation magnons is discussed and the condition
was found to depend on $\beta$, exchange stiffness $D$ and the acoustic
velocity.
|
2105.07376v1
|
2021-11-16
|
Ultrathin ferrimagnetic GdFeCo films with very low damping
|
Ferromagnetic materials dominate as the magnetically active element in
spintronic devices, but come with drawbacks such as large stray fields, and low
operational frequencies. Compensated ferrimagnets provide an alternative as
they combine the ultrafast magnetization dynamics of antiferromagnets with a
ferromagnet-like spin-orbit-torque (SOT) behavior. However to use ferrimagnets
in spintronic devices their advantageous properties must be retained also in
ultrathin films (t < 10 nm). In this study, ferrimagnetic Gdx(Fe87.5Co12.5)1-x
thin films in the thickness range t = 2-20 nm were grown on high resistance
Si(100) substrates and studied using broadband ferromagnetic resonance
measurements at room temperature. By tuning their stoichiometry, a nearly
compensated behavior is observed in 2 nm Gdx(Fe87.5Co12.5)1-x ultrathin films
for the first time, with an effective magnetization of Meff = 0.02 T and a low
effective Gilbert damping constant of {\alpha} = 0.0078, comparable to the
lowest values reported so far in 30 nm films. These results show great promise
for the development of ultrafast and energy efficient ferrimagnetic spintronic
devices.
|
2111.08768v1
|
2021-11-30
|
First and second order magnetic anisotropy and damping of europium iron garnet under high strain
|
Understanding and tailoring static and dynamic properties of magnetic
insulator thin films is important for spintronic device applications. Here, we
grow atomically flat epitaxial europium iron garnet (EuIG) thin films by pulsed
laser deposition on (111)-oriented garnet substrates with a range of lattice
parameters. By controlling the lattice mismatch between EuIG and the
substrates, we tune the strain in EuIG films from compressive to tensile
regime, which is characterized by X-ray diffraction. Using ferromagnetic
resonance, we find that in addition to the first-order perpendicular magnetic
anisotropy which depends linearly on the strain, there is a significant
second-order one that has a quadratic strain dependence. Inhomogeneous
linewidth of the ferromagnetic resonance increases notably with increasing
strain, while the Gilbert damping parameter remains nearly constant (~
2x10^-2). These results provide valuable insight into the spin dynamics in
ferrimagnetic insulators and useful guidance for material synthesis and
engineering of next-generation spintronics applications.
|
2111.15142v1
|
2022-10-01
|
Nonlinear features of the superconductor--ferromagnet--superconductor $\varphi_0$ Josephson junction in ferromagnetic resonance region
|
We demonstrate the manifestations of the nonlinear features in magnetic
dynamics and IV-characteristics of the $\varphi_0$ Josephson junction in the
ferromagnetic resonance region. We show that at small values of system
parameters, namely, damping, spin-orbit interaction, and Josephson to magnetic
energy ratio, the magnetic dynamics is reduced to the dynamics of the scalar
Duffing oscillator, driven by the Josephson oscillations. The role of
increasing superconducting current in the resonance region is clarified.
Shifting of the ferromagnetic resonant frequency and the reversal of its
damping dependence due to nonlinearity are demonstrated by the full
Landau-Lifshitz-Gilbert-Josephson system of equations, and in its different
approximations. Finally, we demonstrate the negative differential resistance in
the IV--characteristics, and its correlation with the foldover effect.
|
2210.00366v1
|
2023-12-16
|
Spin-torque nano-oscillator based on two in-plane magnetized synthetic ferrimagnets
|
We report the dynamic characterization of the spin-torque-driven in-plane
precession modes of a spin-torque nano-oscillator based on two different
synthetic ferrimagnets: a pinned one characterized by a strong RKKY interaction
which is exchange coupled to an antiferromagnetic layer; and a second one,
non-pinned characterized by weak RKKY coupling. The microwave properties
associated with the steady-state precession of both SyFs are characterized by
high spectral purity and power spectral density. However, frequency dispersion
diagrams of the damped and spin transfer torque modes reveal drastically
different dynamical behavior and microwave emission properties in both SyFs. In
particular, the weak coupling between the magnetic layers of the non-pinned SyF
raises discontinuous dispersion diagrams suggesting a strong influence of mode
crossing. An interpretation of the different dynamical features observed in the
damped and spin torque modes of both SyF systems was obtained by solving
simultaneously, in a macrospin approach, a linearized version of the
Landau-Lifshitz-Gilbert equation including the spin transfer torque term.
|
2312.10451v2
|
2013-08-17
|
Thickness and power dependence of the spin-pumping effect in Y3Fe5O12/Pt heterostructures measured by the inverse spin Hall effect
|
The dependence of the spin-pumping effect on the yttrium iron garnet
(Y3Fe5O12, YIG) thickness detected by the inverse spin Hall effect (ISHE) has
been investigated quantitatively. Due to the spin-pumping effect driven by the
magnetization precession in the ferrimagnetic insulator YIG film a
spin-polarized electron current is injected into the Pt layer. This spin
current is transformed into electrical charge current by means of the ISHE. An
increase of the ISHE-voltage with increasing film thickness is observed and
compared to the theoretically expected behavior. The effective damping
parameter of the YIG/Pt samples is found to be enhanced with decreasing YIG
film thickness. The investigated samples exhibit a spin mixing conductance of
g=(7.43 \pm 0.36) \times 10^{18} m^{-2} and a spin Hall angle of theta_{ISHE} =
0.009 \pm 0.0008. Furthermore, the influence of nonlinear effects on the
generated voltage and on the Gilbert damping parameter at high excitation
powers are revealed. It is shown that for small YIG film thicknesses a
broadening of the linewidth due to nonlinear effects at high excitation powers
is suppressed because of a lack of nonlinear multi-magnon scattering channels.
We have found that the variation of the spin-pumping efficiency for thick YIG
samples exhibiting pronounced nonlinear effects is much smaller than the
nonlinear enhancement of the damping.
|
1308.3787v1
|
2020-05-28
|
Spintronics meets nonadiabatic molecular dynamics: Geometric spin torque and damping on noncollinear classical magnetism due to electronic open quantum system
|
We analyze a quantum-classical hybrid system of steadily precessing slow
classical localized magnetic moments, forming a head-to-head domain wall,
embedded into an open quantum system of fast nonequilibrium electrons. The
electrons reside within a metallic wire connected to macroscopic reservoirs.
The model captures the essence of dynamical noncollinear and noncoplanar
magnetic textures in spintronics, while making it possible to obtain the exact
time-dependent nonequilibrium density matrix of electronic system and split it
into four contributions. The Fermi surface contribution generates dissipative
(or damping-like in spintronics terminology) spin torque on the moments, and
one of the two Fermi sea contributions generates geometric torque dominating in
the adiabatic regime. When the coupling to the reservoirs is reduced, the
geometric torque is the only nonzero contribution. Locally it has both
nondissipative (or field-like in spintronics terminology) and damping-like
components, but with the sum of latter being zero, which act as the
counterparts of geometric magnetism force and electronic friction in
nonadiabatic molecular dynamics. Such current-independent geometric torque is
absent from widely used micromagnetics or atomistic spin dynamics modeling of
magnetization dynamics based on the Landau-Lifshitz-Gilbert equation, where
previous analysis of Fermi surface-type torque has severely underestimated its
magnitude.
|
2005.14153v2
|
2020-09-29
|
Structural Phase Dependent Giant Interfacial Spin Transparency in W/CoFeB Thin Film Heterostructure
|
Pure spin current has transfigured the energy-efficient spintronic devices
and it has the salient characteristic of transport of the spin angular
momentum. Spin pumping is a potent method to generate pure spin current and for
its increased efficiency high effective spin-mixing conductance (Geff) and
interfacial spin transparency (T) are essential. Here, a giant T is reported in
Sub/W(t)/Co20Fe60B20(d)/SiO2(2 nm) heterostructures in \beta-tungsten (\beta-W)
phase by employing all-optical time-resolved magneto-optical Kerr effect
technique. From the variation of Gilbert damping with W and CoFeB thicknesses,
the spin diffusion length of W and spin-mixing conductances are extracted.
Subsequently, T is derived as 0.81 \pm 0.03 for the \beta-W/CoFeB interface. A
sharp variation of Geff and T with W thickness is observed in consonance with
the thickness-dependent structural phase transition and resistivity of W. The
spin memory loss and two-magnon scattering effects are found to have negligible
contributions to damping modulation as opposed to spin pumping effect which is
reconfirmed from the invariance of damping with Cu spacer layer thickness
inserted between W and CoFeB. The observation of giant interfacial spin
transparency and its strong dependence on crystal structures of W will be
important for pure spin current based spin-orbitronic devices.
|
2009.14143v1
|
2023-12-31
|
Molecular Hybridization Induced Antidamping and Sizable Enhanced Spin-to-Charge Conversion in Co20Fe60B20/$β$-W/C60 Heterostructures
|
Development of power efficient spintronics devices has been the compelling
need in the post-CMOS technology era. The effective tunability of
spin-orbit-coupling (SOC) in bulk and at the interfaces of hybrid materials
stacking is a prerequisite for scaling down the dimension and power consumption
of these devices. In this work, we demonstrate the strong chemisorption of C60
molecules when grown on the high SOC $\beta$-W layer. The parent CFB/$\beta$-W
bilayer exhibits large spin-to-charge interconversion efficiency, which can be
ascribed to the interfacial SOC observed at the Ferromagnet/Heavy metal
interface. Further, the adsorption of C60 molecules on $\beta$-W reduces the
effective Gilbert damping by $\sim$15% in the CFB/$\beta$-W/C60
heterostructures. The anti-damping is accompanied by a gigantic $\sim$115%
enhancement in the spin-pumping induced output voltage owing to the molecular
hybridization. The non-collinear Density Functional Theory calculations confirm
the long-range enhancement of SOC of $\beta$-W upon the chemisorption of C60
molecules, which in turn can also enhance the SOC at the CFB/$\beta$-W
interface in CFB/$\beta$-W/C60 heterostructures. The combined amplification of
bulk as well interfacial SOC upon molecular hybridization stabilizes the
anti-damping and enhanced spin-to-charge conversion, which can pave the way for
the fabrication of power efficient spintronics devices.
|
2401.00486v1
|
2002-04-25
|
Statics and Fast Dynamics of Nanomagnets with Vortex Structure
|
Within the framework of the Landau-Lifshitz-Gilbert equation, using permalloy
parameters, we study the statics and dynamics of flat circular magnetic
nano-structures with an in-plane magnetic vortex configuration, putting
particular emphasis on the (planar) vorticity of the magnetic state and on the
(perpendicular) polarisation of the vortex center (which may be shifted with
respect to the center of the circle). These binary degrees of freedom can in
principle be used to manipulate two independent bits of information.
Studying switching processes induced by in-plane and out-of plane field
pulses we find that it is possible to switch the vorticity of the magnetic dot
on a time scale of 40 ps in strong enough and short enough perpendicular
external field pulses (B_z^ext \approx 0.5 T, duration \approx 40 ps). But for
realistically small values of the Gilbert damping, only the vorticity can be
switched this fast, and it turns out that it is better to dismiss the center of
the circle totally, concentrating on flat 'nano-rings' with an inner radius R_1
and an outer radius R_2. On these 'nano-rings' the vortex state is more stable,
and with respect to the switching of the vorticity these structures have
similar properties as circular dots.
|
0204541v3
|
2007-03-15
|
Functional Keldysh Theory of Spin Torques
|
We present a microscopic treatment of current-induced torques and thermal
fluctuations in itinerant ferromagnets based on a functional formulation of the
Keldysh formalism. We find that the nonequilibrium magnetization dynamics is
governed by a stochastic Landau-Lifschitz-Gilbert equation with spin transfer
torques. We calculate the Gilbert damping parameter $\alpha$ and the
non-adiabatic spin transfer torque parameter $\beta$ for a model ferromagnet.
We find that $\beta \neq \alpha$, in agreement with the results obtained using
imaginary-time methods of Kohno, Tatara and Shibata [J. Phys. Soc. Japan 75,
113706 (2006)]. We comment on the relationship between $s-d$ and
isotropic-Stoner toy models of ferromagnetism and more realistic
density-functional-theory models, and on the implications of these
relationships for predictions of the $\beta/\alpha$ ratio which plays a central
role in domain wall motion. Only for a single-parabolic-band isotropic-Stoner
model with an exchange splitting that is small compared to the Fermi energy
does $\beta/\alpha$ approach one. In addition, our microscopic formalism
incorporates naturally the fluctuations needed in a nonzero-temperature
description of the magnetization. We find that to first order in the applied
electric field, the usual form of thermal fluctuations via a phenomenological
stochastic magnetic field holds.
|
0703414v2
|
2010-10-04
|
Thermal fluctuation field for current-induced domain wall motion
|
Current-induced domain wall motion in magnetic nanowires is affected by
thermal fluctuation. In order to account for this effect, the
Landau-Lifshitz-Gilbert equation includes a thermal fluctuation field and
literature often utilizes the fluctuation-dissipation theorem to characterize
statistical properties of the thermal fluctuation field. However, the theorem
is not applicable to the system under finite current since it is not in
equilibrium. To examine the effect of finite current on the thermal
fluctuation, we adopt the influence functional formalism developed by Feynman
and Vernon, which is known to be a useful tool to analyze effects of
dissipation and thermal fluctuation. For this purpose, we construct a quantum
mechanical effective Hamiltonian describing current-induced domain wall motion
by generalizing the Caldeira-Leggett description of quantum dissipation. We
find that even for the current-induced domain wall motion, the statistical
properties of the thermal noise is still described by the
fluctuation-dissipation theorem if the current density is sufficiently lower
than the intrinsic critical current density and thus the domain wall tilting
angle is sufficiently lower than pi/4. The relation between our result and a
recent result, which also addresses the thermal fluctuation, is discussed. We
also find interesting physical meanings of the Gilbert damping alpha and the
nonadiabaticy parameter beta; while alpha characterizes the coupling strength
between the magnetization dynamics (the domain wall motion in this paper) and
the thermal reservoir (or environment), beta characterizes the coupling
strength between the spin current and the thermal reservoir.
|
1010.0478v2
|
2015-06-03
|
Antidamping spin-orbit torque driven by spin-flip reflection mechanism on the surface of a topological insulator: A time-dependent nonequilibrium Green function approach
|
Motivated by recent experiments observing spin-orbit torque (SOT) acting on
the magnetization $\vec{m}$ of a ferromagnetic (F) overlayer on the surface of
a three-dimensional topological insulator (TI), we investigate the origin of
the SOT and the magnetization dynamics in such systems. We predict that lateral
F/TI bilayers of finite length, sandwiched between two normal metal leads, will
generate a large antidamping-like SOT per very low charge current injected
parallel to the interface. The large values of antidamping-like SOT are {\it
spatially localized} around the transverse edges of the F overlayer. Our
analysis is based on adiabatic expansion (to first order in $\partial
\vec{m}/\partial t$) of time-dependent nonequilibrium Green functions (NEGFs),
describing electrons pushed out of equilibrium both by the applied bias voltage
and by the slow variation of a classical degree of freedom [such as
$\vec{m}(t)$]. From it we extract formulas for spin torque and charge pumping,
which show that they are reciprocal effects to each other, as well as Gilbert
damping in the presence of SO coupling. The NEGF-based formula for SOT
naturally splits into four components, determined by their behavior (even or
odd) under the time and bias voltage reversal. Their complex angular dependence
is delineated and employed within Landau-Lifshitz-Gilbert simulations of
magnetization dynamics in order to demonstrate capability of the predicted SOT
to efficiently switch $\vec{m}$ of a perpendicularly magnetized F overlayer.
|
1506.01303v3
|
2015-07-11
|
Realization of the thermal equilibrium in inhomogeneous magnetic systems by the Landau-Lifshitz-Gilbert equation with stochastic noise, and its dynamical aspects
|
It is crucially important to investigate effects of temperature on magnetic
properties such as critical phenomena, nucleation, pinning, domain wall motion,
coercivity, etc. The Landau-Lifshitz-Gilbert (LLG) equation has been applied
extensively to study dynamics of magnetic properties. Approaches of Langevin
noises have been developed to introduce the temperature effect into the LLG
equation. To have the thermal equilibrium state (canonical distribution) as the
steady state, the system parameters must satisfy some condition known as the
fluctuation-dissipation relation. In inhomogeneous magnetic systems in which
spin magnitudes are different at sites, the condition requires that the ratio
between the amplitude of the random noise and the damping parameter depends on
the magnitude of the magnetic moment at each site. Focused on inhomogeneous
magnetic systems, we systematically showed agreement between the stationary
state of the stochastic LLG equation and the corresponding equilibrium state
obtained by Monte Carlo simulations in various magnetic systems including
dipole-dipole interactions. We demonstrated how violations of the condition
result in deviations from the true equilibrium state. We also studied the
characteristic features of the dynamics depending on the choice of the
parameter set. All the parameter sets satisfying the condition realize the same
stationary state (equilibrium state). In contrast, different choices of
parameter set cause seriously different relaxation processes. We show two
relaxation types, i.e., magnetization reversals with uniform rotation and with
nucleation.
|
1507.03075v1
|
2017-01-12
|
Dynamic coupling of ferromagnets via spin Hall magnetoresistance
|
The synchronized magnetization dynamics in ferromagnets on a nonmagnetic
heavy metal caused by the spin Hall effect is investigated theoretically. The
direct and inverse spin Hall effects near the ferromagnetic/nonmagnetic
interface generate longitudinal and transverse electric currents. The
phenomenon is known as the spin Hall magnetoresistance effect, whose magnitude
depends on the magnetization direction in the ferromagnet due to the spin
transfer effect. When another ferromagnet is placed onto the same nonmagnet,
these currents are again converted to the spin current by the spin Hall effect
and excite the spin torque to this additional ferromagnet, resulting in the
excitation of the coupled motions of the magnetizations. The in-phase or
antiphase synchronization of the magnetization oscillations, depending on the
value of the Gilbert damping constant and the field-like torque strength, is
found in the transverse geometry by solving the Landau-Lifshitz-Gilbert
equation numerically. On the other hand, in addition to these synchronizations,
the synchronization having a phase difference of a quarter of a period is also
found in the longitudinal geometry. The analytical theory clarifying the
relation among the current, frequency, and phase difference is also developed,
where it is shown that the phase differences observed in the numerical
simulations correspond to that giving the fixed points of the energy supplied
by the coupling torque.
|
1701.03201v2
|
2018-10-16
|
Superfluid spin transport in ferro- and antiferromagnets
|
This paper focuses on spin superfluid transport, observation of which was
recently reported in antiferromagnet Cr$_2$O$_3$ [Yuan et al., Sci. Adv. 4,
eaat1098 (2018)]. This paper analyzes the role of dissipation in transformation
of spin current injected with incoherent magnons to a superfluid spin current
near the interface where spin is injected. The Gilbert damping parameter in the
Landau-Lifshitz-Gilbert theory does not describe dissipation properly, and the
dissipation parameters are calculated from the Boltzmann equation for magnons
scattered by defects. The two-fluid theory is developed similar to the
two-fluid theory for superfluids. This theory shows that the influence of
temperature variation in bulk on the superfluid spin transport (bulk Seebeck
effect) is weak at low temperatures. The scenario that the results of Yuan et
al. are connected with the Seebeck effect at the interface between the spin
detector and the sample is also discussed.
The Landau criterion for an antiferromagnet put in a magnetic field is
derived from the spectrum of collective spin modes. The Landau instability
starts in the gapped mode earlier than in the Goldstone gapless mode, in
contrast to easy-plane ferromagnets where the Goldstone mode becomes unstable.
The structure of the magnetic vortex in the geometry of the experiment is
determined. The vortex core has the skyrmion structure with finite
magnetization component normal to the magnetic field. This magnetization
creates stray magnetic fields around the exit point of the vortex line from the
sample, which can be used for experimental detection of vortices.
|
1810.07020v4
|
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
|
2023-08-14
|
Temperature Evolution of Magnon Propagation Length in Tm$_3$Fe$_5$O$_{12}$ Thin Films: Roles of Magnetic Anisotropy and Gilbert Damping
|
The magnon propagation length ($\langle\xi\rangle$) of a ferro/ferrimagnet
(FM) is one of the key factors that controls the generation and propagation of
thermally-driven spin current in FM/heavy metal (HM) bilayer based
spincaloritronic devices. Theory predicts that for the FM layer,
$\langle\xi\rangle$ is inversely proportional to the Gilbert damping ($\alpha$)
and the square root of the effective magnetic anisotropy constant ($K_{\rm
eff}$). However, direct experimental evidence of this relationship is lacking.
To experimentally confirm this prediction, we employ a combination of
longitudinal spin Seebeck effect (LSSE), transverse susceptibility, and
ferromagnetic resonance experiments to investigate the temperature evolution of
$\langle\xi\rangle$ and establish its correlation with the effective magnetic
anisotropy field, $H_K^{\rm eff}$ ($\propto K_{\rm eff}$) and $\alpha$ in
Tm$_3$Fe$_5$O$_{12}$ (TmIG)/Pt bilayers. We observe concurrent drops in the
LSSE voltage and $\langle\xi\rangle$ below 200$^\circ$K in TmIG/Pt bilayers
regardless of TmIG film thickness and substrate choice and attribute it to the
noticeable increases in $H_K^{\rm eff}$ and $\alpha$ that occur within the same
temperature range. From the TmIG thickness dependence of the LSSE voltage, we
determined the temperature dependence of $\langle\xi\rangle$ and highlighted
its correlation with the temperature-dependent $H_K^{\rm eff}$ and $\alpha$ in
TmIG/Pt bilayers, which will be beneficial for the development of rare-earth
iron garnet-based efficient spincaloritronic nanodevices.
|
2308.07236v3
|
2014-01-08
|
Dynamic exchange via spin currents in acoustic and optical modes of ferromagnetic resonance in spin-valve structures
|
Two ferromagnetic layers magnetically decoupled by a thick normal metal
spacer layer can be, nevertheless, dynamically coupled via spin currents
emitted by the spin-pump and absorbed through the spin-torque effects at the
neighboring interfaces. A decrease of damping in both layers due to a partial
compensation of the angular momentum leakage in each layer was previously
observed at the coincidence of the two ferromagnetic resonances. In case of
non-zero magnetic coupling, such a dynamic exchange will depend on the mutual
precession of the magnetic moments in the layers. A difference in the linewidth
of the resonance peaks is expected for the acoustic and optical regimes of
precession. However, the interlayer coupling hybridizes the resonance responses
of the layers and therefore can also change their linewidths. The interplay
between the two mechanisms has never been considered before. In the present
work, the joint influence of the hybridization and non-local damping on the
linewidth has been studied in weakly coupled NiFe/CoFe/Cu/CoFe/MnIr spin-valve
multilayers. It has been found that the dynamic exchange by spin currents is
different in the optical and acoustic modes, and this difference is dependent
on the interlayer coupling strength. In contrast to the acoustic precession
mode, the dynamic exchange in the optical mode works as an additional damping
source. A simulation in the framework of the Landau-Lifshitz-Gilbert formalism
for two ferromagnetic layers coupled magnetically and by spin currents has been
done to separate the effects of the non-local damping from the resonance modes
hybridization. In our samples both mechanisms bring about linewidth changes of
the same order of magnitude, but lead to a distinctly different angular
behavior. The obtained results are relevant for a broad class of coupled
magnetic multilayers with ballistic regime of the spin transport.
|
1401.1672v1
|
2020-08-29
|
Exploring a quantum-information-relevant magnonic material: Ultralow damping at low temperature in the organic ferrimagnet V[TCNE]x
|
Quantum information science and engineering requires novel low-loss magnetic
materials for magnon-based quantum-coherent operations. The search for low-loss
magnetic materials, traditionally driven by applications in microwave
electronics near room-temperature, has gained additional constraints from the
need to operate at cryogenic temperatures for many applications in quantum
information science and technology. Whereas yttrium iron garnet (YIG) has been
the material of choice for decades, the emergence of molecule-based materials
with robust magnetism and ultra-low damping has opened new avenues for
exploration. Specifically, thin-films of vanadium tetracyanoethylene (V[TCNE]x)
can be patterned into the multiple, connected structures needed for hybrid
quantum elements and have shown room-temperature Gilbert damping ({\alpha} = 4
\times 10^-5) that rivals the intrinsic (bulk) damping otherwise seen only in
highly-polished YIG spheres (far more challenging to integrate into arrays).
Here, we present a comprehensive and systematic study of the low-temperature
magnetization dynamics for V[TCNE]x thin films, with implications for their
application in quantum systems. These studies reveal a temperature-driven,
strain-dependent magnetic anisotropy that compensates the thin-film shape
anisotropy, and the recovery of a magnetic resonance linewidth at 5 K that is
comparable to room-temperature values (roughly 2 G at 9.4 GHz). We can account
for these variations of the V[TCNE]x linewidth within the context of scattering
from very dilute paramagnetic impurities, and anticipate additional linewidth
narrowing as the temperature is further reduced.
|
2008.13061v3
|
2003-04-18
|
Elementary Excitations of Ferromagnetic Metal Nanoparticles
|
We present a theory of the elementary spin excitations in transition metal
ferromagnet nanoparticles which achieves a unified and consistent quantum
description of both collective and quasiparticle physics. The theory starts by
recognizing the essential role played by spin-orbit interactions in determining
the energies of ferromagnetic resonances in the collective excitation spectrum
and the strength of their coupling to low-energy particle-hole excitations. We
argue that a crossover between Landau-damped ferromagnetic resonance and
pure-state collective magnetic excitations occurs as the number of atoms in
typical transition metal ferromagnet nanoparticles drops below approximately
$10^4$, approximately where the single-particle level spacing, $\delta$,
becomes larger than, $\sqrt{\alpha} E_{\rm res}$, where $E_{\rm res}$ is the
ferromagnetic resonance frequency and $\alpha$ is the Gilbert damping
parameter. We illustrate our ideas by studying the properties of semi-realistic
model Hamiltonians, which we solve numerically for nanoparticles containing
several hundred atoms. For small nanoparticles, we find one isolated
ferromagnetic resonance collective mode below the lowest particle-hole
excitation energy, at $E_{\rm res} \approx 0.1$ meV. The spectral weight of
this pure excitation nearly exhausts the transverse dynamical susceptibility
spectral weight. As $\delta$ approaches $\sqrt{\alpha} E_{\rm res}$, the
ferromagnetic collective excitation is more likely to couple strongly with
discrete particle-hole excitations. In this regime the distinction between the
two types of excitations blurs. We discuss the significance of this picture for
the interpretation of recent single-electron tunneling experiments.
|
0304427v1
|
2014-08-02
|
Tunnel magnetoresistance and spin-transfer-torque switching in polycrystalline Co2FeAl full-Heusler alloy magnetic tunnel junctions on Si/SiO2 amorphous substrates
|
We studied polycrystalline B2-type Co2FeAl (CFA) full-Heusler alloy based
magnetic tunnel junctions (MTJs) fabricated on a Si/SiO2 amorphous substrate.
Polycrystalline CFA films with a (001) orientation, a high B2 ordering, and a
flat surface were achieved using a MgO buffer layer. A tunnel magnetoresistance
(TMR) ratio up to 175% was obtained for an MTJ with a CFA/MgO/CoFe structure on
a 7.5-nm-thick MgO buffer. Spin-transfer torque induced magnetization switching
was achieved in the MTJs with a 2-nm-thick polycrystalline CFA film as a
switching layer. Using a thermal activation model, the intrinsic critical
current density (Jc0) was determined to be 8.2 x 10^6 A/cm^2, which is lower
than 2.9 x 10^7 A/cm^2, the value for epitaxial CFA-MTJs [Appl. Phys. Lett.
100, 182403 (2012)]. We found that the Gilbert damping constant evaluated using
ferromagnetic resonance measurements for the polycrystalline CFA film was
~0.015 and was almost independent of the CFA thickness (2~18 nm). The low Jc0
for the polycrystalline MTJ was mainly attributed to the low damping of the CFA
layer compared with the value in the epitaxial one (~0.04).
|
1408.0341v1
|
2018-02-20
|
Ultrafast magnetization dynamics in pure and doped Heusler and inverse Heusler alloys
|
By using a multiscale approach based on first-principles density functional
theory combined with atomistic spin dynamics, we investigate the electronic
structure and magnetization dynamics of an inverse Heusler and a Heusler
compound and their alloys, i. e. Mn$_{2-x}Z_x$CoAl and Mn$_{2-x}Z_x$VAl, where
$Z$ = Mo, W, Os and Ru, respectively. A signature of the ferrimagnetic ordering
of Mn$_{2}$CoAl and Mn$_{2}$VAl Heusler alloys is reflected in the calculated
Heisenberg exchange constants. They decay very rapidly with the interatomic
distance and have short range, which is a consequence of the existence of the
finite gap in the minority spin band. The calculated Gilbert damping parameter
of both Mn$_2$CoAl and Mn$_2$VAl is high compared to other half-metals, but
interestingly in the particular case of the inverse Mn$_{2}$CoAl alloys and due
to the spin-gapless semiconducting property, the damping parameters decrease
with the doping concentration in clear contradiction to the general trend.
Atomistic spin dynamics simulations predict ultrafast magnetisation switching
in Mn$_{2}$CoAl and Mn$_{2}$VAl under the influence of an external magnetic
field, starting from a threshold field of $2\text{T}$. Our overall finding
extends with Heusler and inverse Heusler alloys, the class of materials that
exhibits laser induced magnetic switching.
|
1802.07195v1
|
2018-04-10
|
GONG Catalog of Solar Filament Oscillations Near Solar Maximum
|
We have catalogued 196 filament oscillations from the GONG $H{\alpha}$
network data during several months near the maximum of solar cycle 24 (January
- June 2014). Selected examples from the catalog are described in detail, along
with our statistical analyses of all events. Oscillations were classified
according to their velocity amplitude: 106 small-amplitude oscillations (SAOs),
with velocities $<10\mathrm{\, km \; s^{-1}}$, and 90 large-amplitude
oscillations (LAOs), with velocities $>10\mathrm{\, km \; s^{-1}}$. Both SAOs
and LAOs are common, with one event of each class every two days on the visible
side of the Sun. For nearly half of the events we identified their apparent
trigger. The period distribution has a mean value of 58$\pm$15 min for both
types of oscillations. The distribution of the damping time per period peaks at
$\tau/P=1.75$ and $1.25$ for SAOs and LAOs respectively. We confirmed that LAO
damping rates depend nonlinearly on the oscillation velocity. The angle between
the direction of motion and the filament spine has a distribution centered at
$27^\circ$ for all filament types. This angle agrees with the observed
direction of filament-channel magnetic fields, indicating that most of the
catalogued events are longitudinal (i.e., undergo field-aligned motions). We
applied seismology to determine the average radius of curvature in the magnetic
dips, $R\approx89$ Mm, and the average minimum magnetic-field strength,
$B\approx16$ G. The catalog is available to the community online, and is
intended to be expanded to cover at least 1 solar cycle.
|
1804.03743v1
|
2018-10-16
|
Spin-wave-induced lateral temperature gradient in a YIG thin film/GGG system excited in an ESR cavity
|
Lateral thermal gradient of an yttrium iron garnet (YIG) film under the
microwave application in the cavity of the electron spin resonance system (ESR)
was measured at room temperature by fabricating a Cu/Sb thermocouple onto it.
To date, thermal transport in YIG films caused by the Damon-Eshbach mode (DEM)
- the unidirectional spin-wave heat conveyer effect - was demonstrated only by
the excitation using coplanar waveguides. Here we show that effect exists even
under YIG excitation using the ESR cavity - tool often employed to realize spin
pumping. The temperature difference observed around the ferromagnetic resonance
(FMR) field under the 4 mW microwave power peaked at 13 mK. The observed
thermoelectric signal indicates the imbalance of the population between the
DEMs that propagate near the top and bottom surfaces of the YIG film. We
attribute the DEM population imbalance to the different magnetic damping near
the top and bottom YIG surfaces. Additionally, the spin wave dynamics of the
system were investigated using the micromagnetic simulations. The micromagnetic
simulations confirmed the existence of the DEM imbalance in the system with the
increased Gilbert damping at one of the YIG interfaces. The reported results
are indispensable for the quantitative estimation of the electromotive force in
the spin-charge conversion experiments using ESR cavities.
|
1810.06875v1
|
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
|
2023-05-16
|
Non-Hermitian Casimir Effect of Magnons
|
There has been a growing interest in non-Hermitian quantum mechanics. The key
concepts of quantum mechanics are quantum fluctuations. Quantum fluctuations of
quantum fields confined in a finite-size system induce the zero-point energy
shift. This quantum phenomenon, the Casimir effect, is one of the most striking
phenomena of quantum mechanics in the sense that there are no classical analogs
and has been attracting much attention beyond the hierarchy of energy scales,
ranging from elementary particle physics to condensed matter physics, together
with photonics. However, the non-Hermitian extension of the Casimir effect and
the application to spintronics have not yet been investigated enough, although
exploring energy sources and developing energy-efficient nanodevices are its
central issues. Here we fill this gap. By developing a magnonic analog of the
Casimir effect into non-Hermitian systems, we show that this non-Hermitian
Casimir effect of magnons is enhanced as the Gilbert damping constant (i.e.,
the energy dissipation rate) increases. When the damping constant exceeds a
critical value, the non-Hermitian Casimir effect of magnons exhibits an
oscillating behavior, including a beating one, as a function of the film
thickness and is characterized by the exceptional point. Our result suggests
that energy dissipation serves as a key ingredient of Casimir engineering.
|
2305.09231v1
|
2002-09-07
|
Neural network analysis of the magnetization reversal in magnetic dot arrays
|
We simulated the remagnetization dynamics of the ultra-dense and ultra-thin
magnetic dot array system with dipole-dipole and exchange coupling
interactions. Within the proposed 2D XY superlattice model, the square dots are
modeled by the spatially modulated exchange-couplings. The dipole-dipole
interactions were approximated by the hierarchical sums and dynamics was
reduced to damping term of the Landau-Lifshitz-Gilbert equation. The simulation
of 40 000 spin system leads to nonequilibrium nonuniform configurations with
soliton-antisoliton pairs detected at intra-dot and inter-dot scales. The
classification of intra-dot magnetic configurations was performed using the
self-adaptive neural networks with varying number of neurons.
|
0209186v1
|
2005-04-06
|
Macrospin Models of Spin Transfer Dynamics
|
The current-induced magnetization dynamics of a spin valve are studied using
a macrospin (single domain) approximation and numerical solutions of a
generalized Landau-Lifshitz-Gilbert equation. For the purpose of quantitative
comparison with experiment [Kiselev {\it et al.} Nature {\bf 425}, 380 (2003)],
we calculate the resistance and microwave power as a function of current and
external field including the effects of anisotropies, damping, spin-transfer
torque, thermal fluctuations, spin-pumping, and incomplete absorption of
transverse spin current. While many features of experiment appear in the
simulations, there are two significant discrepancies: the current dependence of
the precession frequency and the presence/absence of a microwave quiet magnetic
phase with a distinct magnetoresistance signature. Comparison is made with
micromagnetic simulations designed to model the same experiment.
|
0504142v1
|
2006-02-01
|
Mapping Monte Carlo to Langevin dynamics: A Fokker-Planck approach
|
We propose a general method of using the Fokker-Planck equation (FPE) to link
the Monte-Carlo (MC) and the Langevin micromagnetic schemes. We derive the
drift and disusion FPE terms corresponding to the MC method and show that it is
analytically equivalent to the stochastic Landau-Lifshitz-Gilbert (LLG)
equation of Langevin-based micromagnetics. Subsequent results such as the time
quantification factor for the Metropolis MC method can be rigorously derived
from this mapping equivalence. The validity of the mapping is shown by the
close numerical convergence between the MC method and the LLG equation for the
case of a single magnetic particle as well as interacting arrays of particles.
We also found that our Metropolis MC is accurate for a large range of damping
factors $\alpha$, unlike previous time-quantified MC methods which break down
at low $\alpha$, where precessional motion dominates.
|
0602011v2
|
2006-04-21
|
Dynamic approach for micromagnetics close to the Curie temperature
|
In conventional micromagnetism magnetic domain configurations are calculated
based on a continuum theory for the magnetization which is assumed to be of
constant length in time and space. Dynamics is usually described with the
Landau-Lifshitz-Gilbert (LLG) equation the stochastic variant of which includes
finite temperatures. Using simulation techniques with atomistic resolution we
show that this conventional micromagnetic approach fails for higher
temperatures since we find two effects which cannot be described in terms of
the LLG equation: i) an enhanced damping when approaching the Curie temperature
and, ii) a magnetization magnitude that is not constant in time. We show,
however, that both of these effects are naturally described by the
Landau-Lifshitz-Bloch equation which links the LLG equation with the theory of
critical phenomena and turns out to be a more realistic equation for
magnetization dynamics at elevated temperatures.
|
0604508v1
|
2007-02-20
|
Spin dynamics in a superconductor / ferromagnet proximity system
|
The ferromagnetic resonance of thin sputtered Ni80Fe20 films grown on Nb is
measured. By varying the temperature and thickness of the Nb the role of the
superconductivity on the whole ferromagnetic layer in these heterostructures is
explored. The change in the spin transport properties below the superconducting
transition of the Nb is found to manifest itself in the Ni80Fe20 layer by a
sharpening in the resonance of the ferromagnet, or a decrease in the effective
Gilbert damping co-efficient. This dynamic proximity effect is in contrast to
low frequency studies in these systems, where the effect of the superconductor
is confined to a small region in the ferromagnet. We interpret this in terms of
the spin pumping model.
|
0702461v1
|
2007-02-21
|
Domain wall mobility, stability and Walker breakdown in magnetic nanowires
|
We present an analytical calculation of the velocity of a single 180 degree
domain wall in a magnetic structure with reduced thickness and/or lateral
dimension under the combined action of an external applied magnetic field and
an electrical current. As for the case of field-induced domain wall propagation
in thick films, two motion regimes with different mobilities are obtained,
below and far above the so-called Walker field. Additionally, for the case of
current induced motion, a Walker-like current density threshold can be defined.
When the dimensions of the system become comparable to the domain wall width,
the threshold field and current density, stating the wall's internal structure
stability, are reduced by the same geometrical demagnetising factor which
accounts for the confinement. This points out the fact that the velocity
dependence over an extended field/current range and the knowledge of the Walker
breakdown are mandatory to draw conclusions about the phenomenological Gilbert
damping parameter tuning the magnetisation dynamics.
|
0702492v1
|
2001-01-09
|
Hysteresis in layered spring magnets
|
This article addresses a problem of micromagnetics: the reversal of magnetic
moments in layered spring magnets. A one-dimensional model is used of a film
consisting of several atomic layers of a soft material on top of several atomic
layers of a hard material. Each atomic layer is taken to be uniformly
magnetized, and spatial inhomogeneities within an atomic layer are neglected.
The state of such a system is described by a chain of magnetic spin vectors.
Each spin vector behaves like a spinning top driven locally by the effective
magnetic field and subject to damping (Landau-Lifshitz-Gilbert equation). A
numerical integration scheme for the LLG equation is presented that is
unconditionally stable and preserves the magnitude of the magnetization vector
at all times. The results of numerical investigations for a bilayer in a
rotating in-plane magnetic field show hysteresis with a basic period of $2\pi$
at moderate fields and hysteresis with a basic period of $\pi$ at strong
fields.
|
0101077v1
|
2005-01-01
|
Equatorial and related non-equilibrium states in magnetization dynamics of ferromagnets: Generalization of Suhl's spin-wave instabilities
|
We investigate the nonlinear dynamics underlying the evolution of a 2-D
nanoscale ferromagnetic film with uniaxial anisotropy in the presence of
perpendicular pumping. Considering the associated Landau-Lifshitz spin
evolution equation with Gilbert damping together with Maxwell equation for the
demagnetization field, we study the dynamics in terms of the stereographic
variable. We identify several new fixed points for suitable choice of external
field in a rotating frame of reference. In particular, we identify explicit
equatorial and related fixed points of the spin vector in the plane transverse
to the anisotropy axis when the pumping frequency coincides with the amplitude
of the static parallel field. We then study the linear stability of these novel
fixed points under homogeneous and spin wave perturbations and obtain a
generalized Suhl's instability criterion, giving the condition for exponential
growth of P-modes under spin wave perturbations. Two parameter phase diagrams
(in terms of amplitudes of static parallel and oscillatory perpendicular
magnetic fields) for stability are obtained, which differ qualitatively from
those for the conventional ferromagnetic resonance near thermal equilibrium and
are amenable to experimental tests.
|
0501002v2
|
2002-12-30
|
Stochastic resonance in periodic potentials: realization in a dissipative optical lattice
|
We have observed the phenomenon of stochastic resonance on the Brillouin
propagation modes of a dissipative optical lattice. Such a mode has been
excited by applying a moving potential modulation with phase velocity equal to
the velocity of the mode. Its amplitude has been characterized by the
center-of-mass (CM) velocity of the atomic cloud. At Brillouin resonance, we
studied the CM-velocity as a function of the optical pumping rate at a given
depth of the potential wells. We have observed a resonant dependence of the CM
velocity on the optical pumping rate, corresponding to the noise strength. This
corresponds to the experimental observation of stochastic resonance in a
periodic potential in the low-damping regime.
|
0212156v1
|
2007-05-03
|
Planar spin-transfer device with a dynamic polarizer
|
In planar nano-magnetic devices magnetization direction is kept close to a
given plane by the large easy-plane magnetic anisotropy, for example by the
shape anisotropy in a thin film. In this case magnetization shows effectively
in-plane dynamics with only one angle required for its description. Moreover,
the motion can become overdamped even for small values of Gilbert damping. We
derive the equations of effective in-plane dynamics in the presence of
spin-transfer torques. The simplifications achieved in the overdamped regime
allow to study systems with several dynamic magnetic pieces (``free layers'').
A transition from a spin-transfer device with a static polarizer to a device
with two equivalent magnets is observed. When the size difference between the
magnets is less than critical, the device does not exhibit switching, but goes
directly into the ``windmill'' precession state.
|
0705.0406v1
|
2007-05-03
|
Effective attraction induced by repulsive interaction in a spin-transfer system
|
In magnetic systems with dominating easy-plane anisotropy the magnetization
can be described by an effective one dimensional equation for the in-plane
angle. Re-deriving this equation in the presence of spin-transfer torques, we
obtain a description that allows for a more intuitive understanding of
spintronic devices' operation and can serve as a tool for finding new dynamic
regimes. A surprising prediction is obtained for a planar ``spin-flip
transistor'': an unstable equilibrium point can be stabilized by a current
induced torque that further repels the system from that point. Stabilization by
repulsion happens due to the presence of dissipative environment and requires a
Gilbert damping constant that is large enough to ensure overdamped dynamics at
zero current.
|
0705.0508v1
|
2007-06-21
|
Spin pumping by a field-driven domain wall
|
We calculate the charge current in a metallic ferromagnet to first order in
the time derivative of the magnetization direction. Irrespective of the
microscopic details, the result can be expressed in terms of the conductivities
of the majority and minority electrons and the non-adiabatic spin transfer
torque parameter $\beta$. The general expression is evaluated for the specific
case of a field-driven domain wall and for that case depends strongly on the
ratio of $\beta$ and the Gilbert damping constant. These results may provide an
experimental method to determine this ratio, which plays a crucial role for
current-driven domain-wall motion.
|
0706.3160v3
|
2007-09-18
|
Theory of current-driven magnetization dynamics in inhomogeneous ferromagnets
|
We give a brief account of recent developments in the theoretical
understanding of the interaction between electric currents and inhomogeneous
ferromagnetic order parameters. We start by discussing the physical origin of
the spin torques responsible for this interaction and construct a
phenomenological description. We then consider the electric current-induced
ferromagnetic instability and domain-wall motion. Finally, we present a
microscopic justification of the phenomenological description of current-driven
magnetization dynamics, with particular emphasis on the dissipative terms, the
so-called Gilbert damping $\alpha$ and the $\beta$ component of the adiabatic
current-driven torque.
|
0709.2937v2
|
2008-02-12
|
Temperature dependent magnetization dynamics of magnetic nanoparticles
|
Recent experimental and theoretical studies show that the switching behavior
of magnetic nanoparticles can be well controlled by external time-dependent
magnetic fields. In this work, we inspect theoretically the influence of the
temperature and the magnetic anisotropy on the spin-dynamics and the switching
properties of single domain magnetic nanoparticles (Stoner-particles). Our
theoretical tools are the Landau-Lifshitz-Gilbert equation extended as to deal
with finite temperatures within a Langevine framework. Physical quantities of
interest are the minimum field amplitudes required for switching and the
corresponding reversal times of the nanoparticle's magnetic moment. In
particular, we contrast the cases of static and time-dependent external fields
and analyze the influence of damping for a uniaxial and a cubic anisotropy.
|
0802.1740v1
|
2008-05-21
|
Non-equilibrium thermodynamic study of magnetization dynamics in the presence of spin-transfer torque
|
The dynamics of magnetization in the presence of spin-transfer torque was
studied. We derived the equation for the motion of magnetization in the
presence of a spin current by using the local equilibrium assumption in
non-equilibrium thermodynamics. We show that, in the resultant equation, the
ratio of the Gilbert damping constant, $\alpha$, and the coefficient, $\beta$,
of the current-induced torque, called non-adiabatic torque, depends on the
relaxation time of the fluctuating field $\tau_{c}$. The equality
$\alpha=\beta$ holds when $\tau_c$ is very short compared to the time scale of
magnetization dynamics. We apply our theory to current-induced magnetization
reversal in magnetic multilayers and show that the switching time is a
decreasing function of $\tau_{c}$.
|
0805.3306v1
|
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
|
2008-11-13
|
Intrinsic Coupling between Current and Domain Wall Motion in (Ga,Mn)As
|
We consider current-induced domain wall motion and, the reciprocal process,
moving domain wall-induced current. The associated Onsager coefficients are
expressed in terms of scattering matrices. Uncommonly, in (Ga,Mn)As, the
effective Gilbert damping coefficient $\alpha_w$ and the effective out-of-plane
spin transfer torque parameter $\beta_w$ are dominated by spin-orbit
interaction in combination with scattering off the domain wall, and not
scattering off extrinsic impurities. Numerical calculations give $\alpha_w \sim
0.01$ and $\beta_w \sim 1$ in dirty (Ga,Mn)As. The extraordinary large
$\beta_w$ parameter allows experimental detection of current or voltage induced
by domain wall motion in (Ga,Mn)As.
|
0811.2235v2
|
2008-11-21
|
Spin Transfer Torque as a Non-Conservative Pseudo-Field
|
In this paper we show that the spin transfer torque can be described by a
pseudo magnetic field, proportional to the magnetic moment of the itinerant
electrons that enters the Landau-Lifshitz-Gilbert equation in the same way as
other external or internal magnetic fields. However, unlike an ordinary
magnetic field, which is always conservative in nature, the spin torque induced
pseudo field may have both conservative and non-conservative components. We
further show that the magnetic moment of itinerant electrons develops an
out-of-plane component only at non-equilibrium and this component is
responsible for the Slonczewski type switching that acts against the damping
and is always non-conservative. On the other hand, the in-plane components of
the pseudo field exist both at equilibrium and out-of-equilibrium, and are
responsible for the field like term. For tunnel based devices, this term
results in lower switching current for anti-parallel (AP) to parallel (P)
switching compared to P to AP, even when the torque magnitudes are completely
symmetric with voltage.
|
0811.3472v1
|
2008-12-13
|
Non-Adiabatic Spin Transfer Torque in Real Materials
|
The motion of simple domain walls and of more complex magnetic textures in
the presence of a transport current is described by the
Landau-Lifshitz-Slonczewski (LLS) equations. Predictions of the LLS equations
depend sensitively on the ratio between the dimensionless material parameter
$\beta$ which characterizes non-adiabatic spin-transfer torques and the Gilbert
damping parameter $\alpha$. This ratio has been variously estimated to be close
to 0, close to 1, and large compared to 1. By identifying $\beta$ as the
influence of a transport current on $\alpha$, we derive a concise, explicit and
relatively simple expression which relates $\beta$ to the band structure and
Bloch state lifetimes of a magnetic metal. Using this expression we demonstrate
that intrinsic spin-orbit interactions lead to intra-band contributions to
$\beta$ which are often dominant and can be (i) estimated with some confidence
and (ii) interpreted using the "breathing Fermi surface" model.
|
0812.2570v1
|
2009-05-01
|
Spin excitations in a monolayer scanned by a magnetic tip
|
Energy dissipation via spin excitations is investigated for a hard
ferromagnetic tip scanning a soft magnetic monolayer. We use the classical
Heisenberg model with Landau-Lifshitz-Gilbert (LLG)-dynamics including a
stochastic field representing finite temperatures. The friction force depends
linearly on the velocity (provided it is small enough) for all temperatures.
For low temperatures, the corresponding friction coefficient is proportional to
the phenomenological damping constant of the LLG equation. This dependence is
lost at high temperatures, where the friction coefficient decreases
exponentially. These findings can be explained by properties of the spin
polarization cloud dragged along with the tip.
|
0905.0112v2
|
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
|
2011-03-30
|
Spin motive forces due to magnetic vortices and domain walls
|
We study spin motive forces, i.e, spin-dependent forces, and voltages induced
by time-dependent magnetization textures, for moving magnetic vortices and
domain walls. First, we consider the voltage generated by a one-dimensional
field-driven domain wall. Next, we perform detailed calculations on
field-driven vortex domain walls. We find that the results for the voltage as a
function of magnetic field differ between the one-dimensional and vortex domain
wall. For the experimentally relevant case of a vortex domain wall, the
dependence of voltage on field around Walker breakdown depends qualitatively on
the ratio of the so-called $\beta$-parameter to the Gilbert damping constant,
and thus provides a way to determine this ratio experimentally. We also
consider vortices on a magnetic disk in the presence of an AC magnetic field.
In this case, the phase difference between field and voltage on the edge is
determined by the $\beta$ parameter, providing another experimental method to
determine this quantity.
|
1103.5858v3
|
2011-07-04
|
Influence of randomness and retardation on the FMR-linewidth
|
The theory predicts that the spin-wave lifetime $\tau_L$ and the linewidth of
ferromagnetic resonance $\Delta B$ can be governed by random fields and spatial
memory. To that aim the effective field around which the magnetic moments
perform a precession is superimposed by a stochastic time dependent magnetic
field with finite correlation time. The magnetization dynamics is altered by
inclusion of a spatial memory effect monitoring a non-local interaction of size
$\xi$. The underlying Landau-Lifshitz-Gilbert equation (LLG) is modified
accordingly. The stochastic LLG is equivalent to a Fokker-Planck equation which
enables to calculate the mean values of the magnetization vector. Within the
spin-wave approximation we present an analytical solution for the excitation
energy and its damping. The lifetime and the linewidth are analyzed depending
on the strength of the random field $D$ and its correlation time $\tau_c$ as
well as the retardation strength $\Gamma_0$ and the size $\xi$. Whereas
$\tau_L$ decreases with increasing $D$, retardation strength $\Gamma_0$ and
$\tau_c$, the lifetime is enhanced for growing width $\xi$ of the spatial
retardation kernel. In the same manner we calculate the experimentally
measurable linewidth $\Delta B$ is increased strongly when the correlation time
$\tau_c$ ranges in the nanosecond interval.
|
1107.0638v1
|
2011-11-18
|
Charge and Spin Transport in Magnetic Tunnel Junctions: Microscopic Theory
|
We study the charge and spin currents passing through a magnetic tunnel
junction (MTJ) on the basis of a tight-binding model. The currents are
evaluated perturbatively with respect to the tunnel Hamiltonian. The charge
current has the form $A[\bm M_1(t)\times\dot{\bm M}_1(t)]\cdot\bm M_2+B\dot{\bm
M}_1(t)\cdot\bm M_2$, where $\bm M_1(t)$ and $\bm M_2$ denote the directions of
the magnetization in the free layer and fixed layer, respectively. The constant
$A$ vanishes when one or both layers are insulators, {while the constant $B$
disappears when both layers are insulators or the same ferromagnets.} The first
term in the expression for charge current represents dissipation driven by the
effective electric field induced by the dynamic magnetization. In addition,
from an investigation of the spin current, we obtain the microscopic expression
for the enhanced Gilbert damping constant $\varDelta \alpha$. We show that
$\varDelta\alpha$ is proportional to the tunnel conductance and depends on the
bias voltage.
|
1111.4295v2
|
2012-01-17
|
Magnetic vortex echoes: application to the study of arrays of magnetic nanostructures
|
We propose the use of the gyrotropic motion of vortex cores in nanomagnets to
produce a magnetic echo, analogous to the spin echo in NMR. This echo occurs
when an array of nanomagnets, e.g., nanodisks, is magnetized with an in-plane
(xy) field, and after a time \tau a field pulse inverts the core magnetization;
the echo is a peak in M_{xy} at t=2\tau. Its relaxation times depend on the
inhomogeneity, on the interaction between the nanodots and on the Gilbert
damping constant \alpha. Its feasibility is demonstrated using micromagnetic
simulation. To illustrate an application of the echoes, we have determined the
inhomogeneity and measured the magnetic interaction in an array of nanodisks
separated by a distance d, finding a d^{-n} dependence, with n\approx 4.
|
1201.3553v1
|
2012-02-15
|
Current-induced motion of a transverse magnetic domain wall in the presence of spin Hall effect
|
We theoretically study the current-induced dynamics of a transverse magnetic
domain wall in bi-layer nanowires consisting of a ferromagnet on top of a
nonmagnet having strong spin-orbit coupling. Domain wall dynamics is
characterized by two threshold current densities, $J_{th}^{WB}$ and
$J_{th}^{REV}$, where $J_{th}^{WB}$ is a threshold for the chirality switching
of the domain wall and $J_{th}^{REV}$ is another threshold for the reversed
domain wall motion caused by spin Hall effect. Domain walls with a certain
chirality may move opposite to the electron-flow direction with high speed in
the current range $J_{th}^{REV} < J < J_{th}^{WB}$ for the system designed to
satisfy the conditions $J_{th}^{WB} > J_{th}^{REV}$ and \alpha > \beta, where
\alpha is the Gilbert damping constant and \beta is the nonadiabaticity of spin
torque. Micromagnetic simulations confirm the validity of analytical results.
|
1202.3450v1
|
2012-04-23
|
Rotating skyrmion lattices by spin torques and field or temperature gradients
|
Chiral magnets like MnSi form lattices of skyrmions, i.e. magnetic whirls,
which react sensitively to small electric currents j above a critical current
density jc. The interplay of these currents with tiny gradients of either the
magnetic field or the temperature can induce a rotation of the magnetic pattern
for j>jc. Either a rotation by a finite angle of up to 15 degree or -- for
larger gradients -- a continuous rotation with a finite angular velocity is
induced. We use Landau-Lifshitz-Gilbert equations extended by extra damping
terms in combination with a phenomenological treatment of pinning forces to
develop a theory of the relevant rotational torques. Experimental neutron
scattering data on the angular distribution of skyrmion lattices suggests that
continuously rotating domains are easy to obtain in the presence of remarkably
small currents and temperature gradients.
|
1204.5051v1
|
2012-07-09
|
Thermal vortex dynamics in thin circular ferromagnetic nanodisks
|
The dynamics of gyrotropic vortex motion in a thin circular nanodisk of soft
ferromagnetic material is considered. The demagnetization field is calculated
using two-dimensional Green's functions for the thin film problem and fast
Fourier transforms. At zero temperature, the dynamics of the
Landau-Lifshitz-Gilbert equation is simulated using fourth order Runge-Kutta
integration. Pure vortex initial conditions at a desired position are obtained
with a Lagrange multipliers constraint. These methods give accurate estimates
of the vortex restoring force constant $k_F$ and gyrotropic frequency, showing
that the vortex core motion is described by the Thiele equation to very high
precision. At finite temperature, the second order Heun algorithm is applied to
the Langevin dynamical equation with thermal noise and damping. A spontaneous
gyrotropic motion takes place without the application of an external magnetic
field, driven only by thermal fluctuations. The statistics of the vortex radial
position and rotational velocity are described with Boltzmann distributions
determined by $k_F$ and by a vortex gyrotropic mass $m_G=G^2/k_F$,
respectively, where $G$ is the vortex gyrovector.
|
1207.2192v2
|
2013-02-19
|
Chirality Sensitive Domain Wall Motion in Spin-Orbit Coupled Ferromagnets
|
Using the Lagrangian formalism, we solve analytically the equations of motion
for current-induced domain-wall dynamics in a ferromagnet with Rashba
spin-orbit coupling. An exact solution for the domain wall velocity is
provided, including the effect of non-equilibrium conduction electron
spin-density, Gilbert damping, and the Rashba interaction parameter. We
demonstrate explicitly that the influence of spin-orbit interaction can be
qualitatively different from the role of non-adiabatic spin-torque in the sense
that the former is sensitive to the chirality of the domain wall whereas the
latter is not: the domain wall velocity shows a reentrant behavior upon
changing the chirality of the domain wall. This could be used to experimentally
distinguish between the spin-orbit and non-adiabatic contribution to the wall
speed. A quantitative estimate for the attainable domain wall velocity is
given, based on an experimentally relevant set of parameters for the system.
|
1302.4744v1
|
2013-12-17
|
Control of the in-plane anisotropy in off-stoichiometric NiMnSb
|
NiMnSb is a ferromagnetic half-metal which, because of its rich anisotropy
and very low Gilbert damping, is a promising candidate for applications in
information technologies. We have investigated the in-plane anisotropy
properties of thin, MBE-grown NiMnSb films as a function of their Mn
concentration. Using ferromagnetic resonance (FMR) to determine the uniaxial
and four-fold anisotropy fields, 2KU/Ms and 2K1/Ms, we find that a small
variation in composition is sufficient to change the film from primarily
four-fold to primarily uniaxial behavior, allowing for continuous tuning of the
anisotropy. This provides valuable flexibility in designing new device
geometries.
|
1312.4781v2
|
2014-05-09
|
Current-induced magnetization dynamics in two magnetic insulators separated by a normal metal
|
We study the dynamics of spin valves consisting of two layers of magnetic
insulators separated by a normal metal in the macrospin model. A current
through the spacer generates a spin Hall current that can actuate the
magnetization via the spin-transfer torque. We derive expressions for the
effective Gilbert damping and the critical currents for the onset of
magnetization dynamics including the effects of spin pumping that can be tested
by ferromagnetic resonance experiments. The current generates an amplitude
asymmetry between the in-phase and out-of-phase modes. We briefly discuss
superlattices of metals and magnetic insulators.
|
1405.2267v1
|
2014-05-25
|
Spin Hall phenomenology of magnetic dynamics
|
We study the role of spin-orbit interactions in the coupled magnetoelectric
dynamics of a ferromagnetic film coated with an electrical conductor. While the
main thrust of this work is phenomenological, several popular simple models are
considered microscopically in some detail, including Rashba and Dirac
two-dimensional electron gases coupled to a magnetic insulator, as well as a
diffusive spin Hall system. We focus on the long-wavelength magnetic dynamics
that experiences current-induced torques and produces fictitious electromotive
forces. Our phenomenology provides a suitable framework for analyzing
experiments on current-induced magnetic dynamics and reciprocal charge pumping,
including the effects of magnetoresistance and Gilbert-damping anisotropies,
without a need to resort to any microscopic considerations or modeling.
Finally, some remarks are made regarding the interplay of spin-orbit
interactions and magnetic textures.
|
1405.6354v2
|
2014-08-21
|
Brownian motion of massive skyrmions forced by spin polarized currents
|
We report on the thermal effects on the motion of current-driven massive
magnetic skyrmions. The reduced equation for the motion of skyrmion has the
form of a stochastic generalized Thiele's equation. We propose an ansatz for
the magnetization texture of a non-rigid single skyrmion that depends linearly
with the velocity. By utilizing this ansatz it is is found that the mass of
skyrmion is closely related to intrinsic skyrmion parameters, such as Gilbert
damping, skyrmion-charge and dissipative force. We have found an exact
expression for the average drift velocity as well as the mean-square velocity
of the skyrmion. The longitudinal and transverse mobility of skyrmions for
small spin-velocity of electrons is also determined and found to be independent
of the skyrmion mass.
|
1408.4861v2
|
2014-11-11
|
Capturing of a Magnetic Skyrmion with a Hole
|
Magnetic whirls in chiral magnets, so-called skyrmions, can be manipulated by
ultrasmall current densities. Here we study both analytically and numerically
the interactions of a single skyrmion in two dimensions with a small hole in
the magnetic layer. Results from micromagnetic simulations are in good
agreement with effective equations of motion obtained from a generalization of
the Thiele approach. Skyrmion-defect interactions are described by an effective
potential with both repulsive and attractive components. For small current
densities a previously pinned skyrmion stays pinned whereas an unpinned
skyrmion moves around the impurities and never gets captured. For higher
current densities, j_c1 < j < j_c2, however, single holes are able to capture
moving skyrmions. The maximal cross section is proportional to the skyrmion
radius and to Sqrt(alpha), where alpha is the Gilbert damping. For j > j_c2 all
skyrmions are depinned. Small changes of the magnetic field strongly change the
pinning properties, one can even reach a regime without pinning, j_c2=0. We
also show that a small density of holes can effectively accelerate the motion
of the skyrmion and introduce a Hall effect for the skyrmion.
|
1411.2857v1
|
2014-12-01
|
Dissipation due to pure spin-current generated by spin pumping
|
Based on spin-dependent transport theory and thermodynamics, we develop a
generalized theory of the Joule heating in the presence of a spin current.
Along with the conventional Joule heating consisting of an electric current and
electrochemical potential, it is found that the spin current and spin
accumulation give an additional dissipation because the spin-dependent
scatterings inside bulk and ferromagnetic/nonmagnetic interface lead to a
change of entropy. The theory is applied to investigate the dissipation due to
pure spin-current generated by spin pumping across a
ferromagnetic/nonmagnetic/ferromagnetic multilayer. The dissipation arises from
an interface because the spin pumping is a transfer of both the spin angular
momentum and the energy from the ferromagnet to conduction electrons near the
interface. It is found that the dissipation is proportional to the enhancement
of the Gilbert damping constant by spin pumping.
|
1412.0688v1
|
2015-01-30
|
Head-to-Head Domain Wall Structures in Wide Permalloy Strips
|
We analyze the equilibrium micromagnetic domain wall structures encountered
in Permalloy strips of a wide range of thicknesses and widths, with strip
widths up to several micrometers. By performing an extensive set of
micromagnetic simulations, we show that the equilibrium phase diagram of the
domain wall structures exhibits in addition to the previously found structures
(symmetric and asymmetric transverse walls, vortex wall) also double vortex and
triple vortex domain walls for large enough strip widths and thicknesses. Also
several metastable domain wall structures are found for wide and/or thick
strips. We discuss the details of the relaxation process from random
magnetization initial states towards the stable domain wall structure, and show
that our results are robust with respect to changes of e.g. the magnitude of
the Gilbert damping constant and details of the initial conditions.
|
1501.07731v1
|
2015-02-19
|
Characterization of spin relaxation anisotropy in Co using spin pumping
|
Ferromagnets are believed to exhibit strongly anisotropic spin relaxation,
with relaxation lengths for spin longitudinal to magnetization significantly
longer than those for spin transverse to magnetization. Here we characterize
the anisotropy of spin relaxation in Co using the spin pumping contribution to
Gilbert damping in noncollinearly magnetized Py$_{1-x}$Cu$_{x}$/Cu/Co trilayer
structures. The static magnetization angle between Py$_{1-x}$Cu$_{x}$ and Co,
adjusted under field bias perpendicular to film planes, controls the
projections of longitudinal and transverse spin current pumped from
Py$_{1-x}$Cu$_{x}$ into Co. We find nearly isotropic absorption of pure spin
current in Co using this technique; fits to a diffusive transport model yield
the longitudinal spin relaxation length $< 2$ nm in Co. The longitudinal spin
relaxation lengths found are an order of magnitude smaller than those
determined by current-perpendicular-to-planes giant magnetoresistance
measurements, but comparable with transverse spin relaxation lengths in Co
determined by spin pumping.
|
1502.05687v3
|
2015-03-26
|
Thermophoresis of an Antiferromagnetic Soliton
|
We study dynamics of an antiferromagnetic soliton under a temperature
gradient. To this end, we start by phenomenologically constructing the
stochastic Landau-Lifshitz-Gilbert equation for an antiferromagnet with the aid
of the fluctuation-dissipation theorem. We then derive the Langevin equation
for the soliton's center of mass by the collective coordinate approach. An
antiferromagentic soliton behaves as a classical massive particle immersed in a
viscous medium. By considering a thermodynamic ensemble of solitons, we obtain
the Fokker-Planck equation, from which we extract the average drift velocity of
a soliton. The diffusion coefficient is inversely proportional to a small
damping constant $\alpha$, which can yield a drift velocity of tens of m/s
under a temperature gradient of $1$ K/mm for a domain wall in an easy-axis
antiferromagnetic wire with $\alpha \sim 10^{-4}$.
|
1503.07854v2
|
2015-04-01
|
Multiscale modeling of ultrafast element-specific magnetization dynamics of ferromagnetic alloys
|
A hierarchical multiscale approach to model the magnetization dynamics of
ferromagnetic ran- dom alloys is presented. First-principles calculations of
the Heisenberg exchange integrals are linked to atomistic spin models based
upon the stochastic Landau-Lifshitz-Gilbert (LLG) equation to calculate
temperature-dependent parameters (e.g., effective exchange interactions,
damping param- eters). These parameters are subsequently used in the
Landau-Lifshitz-Bloch (LLB) model for multi-sublattice magnets to calculate
numerically and analytically the ultrafast demagnetization times. The developed
multiscale method is applied here to FeNi (permalloy) as well as to copper-
doped FeNi alloys. We find that after an ultrafast heat pulse the Ni sublattice
demagnetizes faster than the Fe sublattice for the here-studied FeNi-based
alloys.
|
1504.00199v1
|
2015-05-04
|
High-topological-number magnetic skyrmions and topologically protected dissipative structure
|
The magnetic skyrmion with the topological number of unity ($Q=1$) is a
well-known nanometric swirling spin structure in the nonlinear $\sigma$ model
with the Dzyaloshinskii-Moriya interaction. Here, we show that magnetic
skyrmion with the topological number of two ($Q=2$) can be created and
stabilized by applying vertical spin-polarized current though it cannot exist
as a static stable excitation. Magnetic skyrmion with $Q=2$ is a nonequilibrium
dynamic object, subsisting on a balance between the energy injection from the
current and the energy dissipation by the Gilbert damping. Once it is created,
it becomes a topologically protected object against fluctuations of various
variables including the injected current itself. Hence, we may call it a
topologically protected dissipative structure. We also elucidate the nucleation
and destruction mechanisms of the magnetic skyrmion with $Q=2$ by studying the
evolutions of the magnetization distribution, the topological charge density as
well as the energy density. Our results will be useful for the study of the
nontrivial topology of magnetic skyrmions with higher topological numbers.
|
1505.00522v2
|
2015-08-06
|
Large spin-wave bullet in a ferrimagnetic insulator driven by spin Hall effect
|
Due to its transverse nature, spin Hall effects (SHE) provide the possibility
to excite and detect spin currents and magnetization dynamics even in magnetic
insulators. Magnetic insulators are outstanding materials for the investigation
of nonlinear phenomena and for novel low power spintronics applications because
of their extremely low Gilbert damping. Here, we report on the direct imaging
of electrically driven spin-torque ferromagnetic resonance (ST-FMR) in the
ferrimagnetic insulator Y$_3$Fe$_5$O$_{12}$ based on the excitation and
detection by SHEs. The driven spin dynamics in Y$_3$Fe$_5$O$_{12}$ is directly
imaged by spatially-resolved microfocused Brillouin light scattering (BLS)
spectroscopy. Previously, ST-FMR experiments assumed a uniform precession
across the sample, which is not valid in our measurements. A strong spin-wave
localization in the center of the sample is observed indicating the formation
of a nonlinear, self-localized spin-wave `bullet'.
|
1508.01427v1
|
2015-12-02
|
Bose-Einstein Condensation of Magnons Pumped by the Bulk Spin Seebeck Effect
|
We propose inducing Bose-Einstein condensation of magnons in a magnetic
insulator by a heat flow oriented toward its boundary. At a critical heat flux,
the oversaturated thermal gas of magnons accumulated at the boundary
precipitates the condensate, which then grows gradually as the thermal bias is
dialed up further. The thermal magnons thus pumped by the magnonic bulk (spin)
Seebeck effect must generally overcome both the local Gilbert damping
associated with the coherent magnetic dynamics as well as the radiative
spin-wave losses toward the magnetic bulk, in order to achieve the threshold of
condensation. We quantitatively estimate the requisite bias in the case of the
ferrimagnetic yttrium iron garnet, discuss different physical regimes of
condensation, and contrast it with the competing (so-called Doppler-shift) bulk
instability.
|
1512.00557v1
|
2016-01-10
|
Interfacial Dzyaloshinskii-Moriya interaction, surface anisotropy energy,and spin pumping at spin orbit coupled Ir/Co interface
|
The interfacial Dzyaloshinskii-Moriya interaction (iDMI), surface anisotropy
energy, and spin pumping at the Ir/Co interface are experimentally investigated
by performing Brillouin light scattering. Contrary to previous reports, we
suggest that the sign of the iDMI at the Ir/Co interface is the same as in the
case of the Pt/Co interface. We also find that the magnitude of the iDMI energy
density is relatively smaller than in the case of the Pt/Co interface, despite
the large strong spin-orbit coupling (SOC) of Ir. The saturation magnetization
and the perpendicular magnetic anisotropy (PMA) energy are significantly
improved due to a strong SOC. Our findings suggest that an SOC in an Ir/Co
system behaves in different ways for iDMI and PMA. Finally, we determine the
spin pumping effect at the Ir/Co interface, and it increases the Gilbert
damping constant from 0.012 to 0.024 for 1.5 nmthick Co.
|
1601.02210v3
|
2016-02-23
|
Relaxation of a classical spin coupled to a strongly correlated electron system
|
A classical spin which is antiferromagnetically coupled to a system of
strongly correlated conduction electrons is shown to exhibit unconventional
real-time dynamics which cannot be described by Gilbert damping. Depending on
the strength of the local Coulomb interaction, the two main electronic
dissipation channels, transport of excitations via correlated hopping and via
excitations of correlation-induced magnetic moments, become active on largely
different time scales. We demonstrate that this can lead to a prethermalization
scenario which so far has been observed in purely electronic systems only and
which is governed here by proximity to the divergent magnetic time scale in the
infinite-U limit.
|
1602.07317v2
|
2016-04-24
|
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
|
2016-06-07
|
The temperature dependence of FeRh's transport properties
|
The finite-temperature transport properties of FeRh compounds are
investigated by first-principles Density Functional Theory-based calculations.
The focus is on the behavior of the longitudinal resistivity with rising
temperature, which exhibits an abrupt decrease at the metamagnetic transition
point, $T = T_m$ between ferro- and antiferromagnetic phases. A detailed
electronic structure investigation for $T \geq 0$ K explains this feature and
demonstrates the important role of (i) the difference of the electronic
structure at the Fermi level between the two magnetically ordered states and
(ii) the different degree of thermally induced magnetic disorder in the
vicinity of $T_m$, giving different contributions to the resistivity. To
support these conclusions, we also describe the temperature dependence of the
spin-orbit induced anomalous Hall resistivity and Gilbert damping parameter.
For the various response quantities considered the impact of thermal lattice
vibrations and spin fluctuations on their temperature dependence is
investigated in detail. Comparison with corresponding experimental data finds
in general a very good agreement.
|
1606.02072v1
|
2016-09-05
|
Coarsening dynamics of topological defects in thin Permalloy films
|
We study the dynamics of topological defects in the magnetic texture of
rectangular Permalloy thin film elements during relaxation from random
magnetization initial states. Our full micromagnetic simulations reveal complex
defect dynamics during relaxation towards the stable Landau closure domain
pattern, manifested as temporal power-law decay, with a system-size dependent
cut-off time, of various quantities. These include the energy density of the
system, and the number densities of the different kinds of topological defects
present in the system. The related power-law exponents assume non-trivial
values, and are found to be different for the different defect types. The
exponents are robust against a moderate increase in the Gilbert damping
constant and introduction of quenched structural disorder. We discuss details
of the processes allowed by conservation of the winding number of the defects,
underlying their complex coarsening dynamics.
|
1609.01094v1
|
2016-09-27
|
Anomalous Feedback and Negative Domain Wall Resistance
|
Magnetic induction can be regarded as a negative feedback effect, where the
motive-force opposes the change of magnetic flux that generates the
motive-force. In artificial electromagnetics emerging from spintronics,
however, this is not necessarily the case. By studying the current-induced
domain wall dynamics in a cylindrical nanowire, we show that the spin
motive-force exerting on electrons can either oppose or support the applied
current that drives the domain wall. The switching into the anomalous feedback
regime occurs when the strength of the dissipative torque {\beta} is about
twice the value of the Gilbert damping constant {\alpha}. The anomalous
feedback manifests as a negative domain wall resistance, which has an analogy
with the water turbine.
|
1609.08250v1
|
2016-10-04
|
Magnetomechanical coupling and ferromagnetic resonance in magnetic nanoparticles
|
We address the theory of the coupled lattice and magnetization dynamics of
freely suspended single-domain nanoparticles. Magnetic anisotropy generates
low-frequency satellite peaks in the microwave absorption spectrum and a
blueshift of the ferromagnetic resonance (FMR) frequency. The low-frequency
resonances are very sharp with maxima exceeding that of the FMR, because their
magnetic and mechanical precessions are locked, thereby suppressing Gilbert
damping. Magnetic nanoparticles can operate as nearly ideal motors that convert
electromagnetic into mechanical energy. The Barnett/Einstein-de Haas effect is
significant even in the absence of a net rotation.
|
1610.01072v2
|
2016-10-05
|
Finite-dimensional colored fluctuation-dissipation theorem for spin systems
|
When nano-magnets are coupled to random external sources, their magnetization
becomes a random variable, whose properties are defined by an induced
probability density, that can be reconstructed from its moments, using the
Langevin equation, for mapping the noise to the dynamical degrees of freedom.
When the spin dynamics is discretized in time, a general
fluctuation-dissipation theorem, valid for non-Markovian noise, can be
established, even when zero modes are present. We discuss the subtleties that
arise, when Gilbert damping is present and the mapping between noise and spin
degrees of freedom is non--linear.
|
1610.01622v1
|
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