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2020-09-22
|
Magnon-mediated spin currents in Tm3Fe5O12/Pt with perpendicular magnetic anisotropy
|
The control of pure spin currents carried by magnons in magnetic insulator
(MI) garnet films with a robust perpendicular magnetic anisotropy (PMA) is of
great interest to spintronic technology as they can be used to carry, transport
and process information. Garnet films with PMA present labyrinth domain
magnetic structures that enrich the magnetization dynamics, and could be
employed in more efficient wave-based logic and memory computing devices. In
MI/NM bilayers, where NM being a normal metal providing a strong spin-orbit
coupling, the PMA benefits the spin-orbit torque (SOT) driven magnetization's
switching by lowering the needed current and rendering the process faster,
crucial for developing magnetic random-access memories (SOT-MRAM). In this
work, we investigated the magnetic anisotropies in thulium iron garnet (TIG)
films with PMA via ferromagnetic resonance measurements, followed by the
excitation and detection of magnon-mediated pure spin currents in TIG/Pt driven
by microwaves and heat currents. TIG films presented a Gilbert damping constant
{\alpha}~0.01, with resonance fields above 3.5 kOe and half linewidths broader
than 60 Oe, at 300 K and 9.5 GHz. The spin-to-charge current conversion through
TIG/Pt was observed as a micro-voltage generated at the edges of the Pt film.
The obtained spin Seebeck coefficient was 0.54 {\mu}V/K, confirming also the
high interfacial spin transparency.
|
2009.10299v1
|
2020-10-17
|
Multiscale modelling of magnetostatic effects on magnetic nanoparticles with application to hyperthermia
|
We extend a renormalization group-based course-graining method for
micromagnetic simulations to include properly scaled magnetostatic
interactions. We apply the method in simulations of dynamic hysteresis loops at
clinically relevant sweep rates and at 310 K of iron oxide nanoparticles (NPs)
of the kind that have been used in preclinical studies of magnetic
hyperthermia. The coarse-graining method, along with a time scaling involving
sweep rate and Gilbert damping parameter, allow us to span length scales from
the unit cell to NPs approximately 50 nm in diameter with reasonable simulation
times. For both NPs and the nanorods composing them, we report effective
uniaxial anisotropy strengths and saturation magnetizations, which differ from
those of the bulk materials magnetite and maghemite of which they are made, on
account of the combined non-trivial effects of temperature, inter-rod exchange,
magnetostatic interactions and the degree of orientational order within the
nanorod composites. The effective parameters allow treating the NPs as single
macrospins, and we find for the test case of calculating loops for two aligned
NPs that using the dipole approximation is sufficient for distances beyond 1.5
times the NP diameter. We also present a study on relating integration time
step to micromagnetic cell size, finding that the optimal time step size scales
approximately linearly with cell volume.
|
2010.08848v1
|
2021-02-09
|
Unconventional quantum vortex matter state hosts quantum oscillations in the underdoped high-temperature cuprate superconductors
|
A central question in the underdoped cuprates pertains to the nature of the
pseudogap ground state. A conventional metallic ground state of the pseudogap
region has been argued to host quantum oscillations upon destruction of the
superconducting order parameter by modest magnetic fields. Here we use low
applied measurement currents and millikelvin temperatures on ultra-pure single
crystals of underdoped YBa$_2$Cu$_3$O$_{6+x}$ to unearth an unconventional
quantum vortex matter ground state characterized by vanishing electrical
resistivity, magnetic hysteresis, and non-ohmic electrical transport
characteristics beyond the highest laboratory accessible static fields. A new
model of the pseudogap ground state is now required to explain quantum
oscillations that are hosted by the bulk quantum vortex matter state without
experiencing sizeable additional damping in the presence of a large maximum
superconducting gap; possibilities include a pair density wave.
|
2102.04927v2
|
2021-03-08
|
Emerging magnetic nutation
|
Nutation has been recognized as of great significance for spintronics; but
justifying its presence has proven to be a hard problem. In this paper we show
that nutation can be understood as emerging from a systematic expansion of a
kernel that describes the history of the interaction of a magnetic moment with
a bath of colored noise. The parameter of the expansion is the ratio of the
colored noise timescale to the precession period. In the process we obtain the
Gilbert damping from the same expansion. We recover the known results, when the
coefficients of the two terms are proportional to one another, in the white
noise limit; and show how colored noise leads to situations where this simple
relation breaks down, but what replaces it can be understood by the appropriate
generalization of the fluctuation--dissipation theorem. Numerical simulations
of the stochastic equations support the analytic approach. In particular we
find that the equilibration time is about an order of magnitude longer than the
timescale set by the colored noise for a wide range of values of the latter and
we can identify the presence of nutation in the non-uniform way the
magnetization approaches equilibrium.
|
2103.04787v3
|
2021-03-11
|
Long-range spin transport on the surface of topological Dirac semimetal
|
We theoretically propose the long-range spin transport mediated by the
gapless surface states of topological Dirac semimetal (TDSM). Low-dissipation
spin current is a building block of next-generation spintronics devices. While
conduction electrons in metals and spin waves in ferromagnetic insulators
(FMIs) are the major carriers of spin current, their propagation length is
inevitably limited due to the Joule heating or the Gilbert damping. In order to
suppress dissipation and realize long-range spin transport, we here make use of
the spin-helical surface states of TDSMs, such as $\mathrm{Cd_3 As_2}$ and
$\mathrm{Na_3 Bi}$, which are robust against disorder. Based on a junction of
two FMIs connected by a TDSM, we demonstrate that the magnetization dynamics in
one FMI induces a spin current on the TDSM surface flowing to the other FMI. By
both the analytical transport theory on the surface and the numerical
simulation of real-time evolution in the bulk, we find that the induced spin
current takes a universal semi-quantized value that is insensitive to the
microscopic coupling structure between the FMI and the TDSM. We show that this
surface spin current is robust against disorder over a long range, which
indicates that the TDSM surface serves as a promising system for realizing
spintronics devices.
|
2103.06519v1
|
2021-04-09
|
Spin diffusion length associated to out-of-plane resistivity of Pt thin films in spin pumping experiments
|
We present a broadband ferromagnetic resonance study of the Gilbert damping
enhancement ($\Delta \alpha$) due to spin pumping in NiFe/Pt bilayers. The
bilayers, which have negligible interfacial spin memory loss, are studied as a
function of the Pt layer thickness ($t_{\text{Pt}}$) and temperature (100-293
K). Within the framework of diffusive spin pumping theory, we demonstrate that
Dyakonov-Perel (DP) or Elliot-Yaffet (EY) spin relaxation mechanisms acting
alone are incompatible with our observations. In contrast, if we consider that
the relation between spin relaxation characteristic time ($\tau_{\text{s}}$)
and momentum relaxation characteristic time ($\tau_{\text{p}}$) is determined
by a superposition of DP and EY mechanisms, the qualitative and quantitative
agreement with experimental results is excellent. Remarkably, we found that
$\tau_{\text{p}}$ must be determined by the out-of-plane electrical resistivity
($\rho$) of the Pt film and hence its spin diffusion length
($\lambda_{\text{Pt}}$) is independent of $t_{\text{Pt}}$. Our work settles the
controversy regarding the $t_{\text{Pt}}$ dependence of $\lambda_{\text{Pt}}$
by demonstrating its fundamental connection with $\rho$ considered along the
same direction of spin current flow. \end{abstract}
|
2104.04426v1
|
2021-05-05
|
Ni$_{80}$Fe$_{20}$ Nanotubes with Optimized Spintronic Functionalities Prepared by Atomic Layer Deposition
|
Permalloy Ni$_{80}$Fe$_{20}$ is one of the key magnetic materials in the
field of magnonics. Its potential would be further unveiled if it could be
deposited in three dimensional (3D) architectures of sizes down to the
nanometer. Atomic Layer Deposition, ALD, is the technique of choice for
covering arbitrary shapes with homogeneous thin films. Early successes with
ferromagnetic materials include nickel and cobalt. Still, challenges in
depositing ferromagnetic alloys reside in the synthesis via decomposing the
consituent elements at the same temperature and homogeneously. We report
plasma-enhanced ALD to prepare permalloy Ni$_{80}$Fe$_{20}$ thin films and
nanotubes using nickelocene and iron(III) tert-butoxide as metal precursors,
water as the oxidant agent and an in-cycle plasma enhanced reduction step with
hydrogen. We have optimized the ALD cycle in terms of Ni:Fe atomic ratio and
functional properties. We obtained a Gilbert damping of 0.013, a resistivity of
28 $\mu\Omega$cm and an anisotropic magnetoresistance effect of 5.6 $\%$ in the
planar thin film geometry. We demonstrate that the process also works for
covering GaAs nanowires, resulting in permalloy nanotubes with high aspect
ratios and diameters of about 150 nm. Individual nanotubes were investigated in
terms of crystal phase, composition and spin-dynamic response by microfocused
Brillouin Light Scattering. Our results enable NiFe-based 3D spintronics and
magnonic devices in curved and complex topology operated in the GHz frequency
regime.
|
2105.01969v1
|
2021-06-23
|
Spin dynamics of itinerant electrons: local magnetic moment formation and Berry phase
|
The state-of-the-art theoretical description of magnetic materials relies on
solving effective Heisenberg spin problems or their generalizations to
relativistic or multi-spin-interaction cases that explicitly assume the
presence of local magnetic moments in the system. We start with a general
interacting fermionic model that is often obtained in ab initio electronic
structure calculations and show that the corresponding spin problem can be
introduced even in the paramagnetic regime, which is characterized by a zero
average value of the magnetization. Further, we derive a physical criterion for
the formation of the local magnetic moment and confirm that the latter exists
already at high temperatures well above the transition to the ordered magnetic
state. The use of path-integral techniques allows us to disentangle spin and
electronic degrees of freedom and to carefully separate rotational dynamics of
the local magnetic moment from Higgs fluctuations of its absolute value. It
also allows us to accurately derive the topological Berry phase and relate it
to a physical bosonic variable that describes dynamics of the spin degrees of
freedom. As the result, we demonstrate that the equation of motion in the case
of a large magnetic moment takes a conventional Landau-Lifshitz form that
explicitly accounts for the Gilbert damping due to itinerant nature of the
original electronic model.
|
2106.12462v3
|
2021-11-20
|
Skyrmionics in correlated oxides
|
While chiral magnets, metal-based magnetic multilayers, or Heusler compounds
have been considered as the material workhorses in the field of skyrmionics,
oxides are now emerging as promising alternatives, as they host special
correlations between the spin-orbital-charge-lattice degrees of freedom and/or
coupled ferroic order parameters. These interactions open new possibilities for
practically exploiting skyrmionics. In this article, we review the recent
advances in the observation and control of topological spin textures in various
oxide systems. We start with the discovery of skyrmions and related
quasiparticles in bulk and heterostructure ferromagnetic oxides. Next, we
emphasize the shortcomings of implementing ferromagnetic textures, which have
led to the recent explorations of ferrimagnetic and antiferromagnetic oxide
counterparts, with higher Curie temperatures, stray-field immunity, low Gilbert
damping, ultrafast magnetic dynamics, and/or absence of skyrmion deflection.
Then, we highlight the development of novel pathways to control the stability,
motion, and detection of topological textures using electric fields and
currents. Finally, we present the outstanding challenges that need to be
overcome to achieve all-electrical, nonvolatile, low-power oxide skyrmionic
devices.
|
2111.10562v2
|
2021-12-01
|
Unconditional well-posedness and IMEX improvement of a family of predictor-corrector methods in micromagnetics
|
Recently, Kim & Wilkening (Convergence of a mass-lumped finite element method
for the Landau-Lifshitz equation, Quart. Appl. Math., 76, 383-405, 2018)
proposed two novel predictor-corrector methods for the Landau-Lifshitz-Gilbert
equation (LLG) in micromagnetics, which models the dynamics of the
magnetization in ferromagnetic materials. Both integrators are based on the
so-called Landau-Lifshitz form of LLG, use mass-lumped variational formulations
discretized by first-order finite elements, and only require the solution of
linear systems, despite the nonlinearity of LLG. The first(-order in time)
method combines a linear update with an explicit projection of an intermediate
approximation onto the unit sphere in order to fulfill the LLG-inherent
unit-length constraint at the discrete level. In the second(-order in time)
integrator, the projection step is replaced by a linear constraint-preserving
variational formulation. In this paper, we extend the analysis of the
integrators by proving unconditional well-posedness and by establishing a close
connection of the methods with other approaches available in the literature.
Moreover, the new analysis also provides a well-posed integrator for the
Schr\"odinger map equation (which is the limit case of LLG for vanishing
damping). Finally, we design an implicit-explicit strategy for the treatment of
the lower-order field contributions, which significantly reduces the
computational cost of the schemes, while preserving their theoretical
properties.
|
2112.00451v1
|
2021-12-21
|
Fast long-wavelength exchange spin waves in partially-compensated Ga:YIG
|
Spin waves in yttrium iron garnet (YIG) nano-structures attract increasing
attention from the perspective of novel magnon-based data processing
applications. For short wavelengths needed in small-scale devices, the group
velocity is directly proportional to the spin-wave exchange stiffness constant
$\lambda_\mathrm{ex}$. Using wave vector resolved Brillouin Light Scattering
(BLS) spectroscopy, we directly measure $\lambda_\mathrm{ex}$ in Ga-substituted
YIG thin films and show that it is about three times larger than for pure YIG.
Consequently, the spin-wave group velocity overcomes the one in pure YIG for
wavenumbers $k > 4$ rad/$\mu$m, and the ratio between the velocities reaches a
constant value of around 3.4 for all $k > 20$ rad/$\mu$m. As revealed by
vibrating-sample magnetometry (VSM) and ferromagnetic resonance (FMR)
spectroscopy, Ga:YIG films with thicknesses down to 59 nm have a low Gilbert
damping ($\alpha < 10^{-3}$), a decreased saturation magnetization $\mu_0
M_\mathrm{S}~\approx~20~$mT and a pronounced out-of-plane uniaxial anisotropy
of about $\mu_0 H_{\textrm{u1}} \approx 95 $ mT which leads to an out-of-plane
easy axis. Thus, Ga:YIG opens access to fast and isotropic spin-wave transport
for all wavelengths in nano-scale systems independently of dipolar effects.
|
2112.11348v1
|
2022-01-27
|
Magnon transport and thermoelectric effects in ultrathin Tm3Fe5O12/Pt nonlocal devices
|
The possibility of electrically exciting and detecting magnon currents in
magnetic insulators has opened exciting perspectives for transporting spin
information in electronic devices. However, the role of the magnetic field and
the nonlocal thermal gradients on the magnon transport remain unclear. Here, by
performing nonlocal harmonic voltage measurements, we investigate magnon
transport in perpendicularly magnetized ultrathin Tm3Fe5O12 (TmIG) films
coupled to Pt electrodes. We show that the first harmonic nonlocal voltage
captures spin-driven magnon transport in TmIG, as expected, and the second
harmonic is dominated by thermoelectric voltages driven by current-induced
thermal gradients at the detector. The magnon diffusion length in TmIG is found
to be on the order of 0.3 {\mu}m at 0.5 T and gradually decays to 0.2 {\mu}m at
0.8 T, which we attribute to the suppression of the magnon relaxation time due
to the increase of the Gilbert damping with field. By performing current,
magnetic field, and distance dependent nonlocal and local measurements we
demonstrate that the second harmonic nonlocal voltage exhibits five
thermoelectric contributions, which originate from the nonlocal spin Seebeck
effect and the ordinary, planar, spin, and anomalous Nernst effects. Our work
provides a guide on how to disentangle magnon signals from diverse
thermoelectric voltages of spin and magnetic origin in nonlocal magnon devices,
and establish the scaling laws of the thermoelectric voltages in
metal/insulator bilayers.
|
2201.11353v1
|
2022-01-31
|
Tuning spin-orbit torques across the phase transition in VO$_2$/NiFe heterostructure
|
The emergence of spin-orbit torques as a promising approach to
energy-efficient magnetic switching has generated large interest in material
systems with easily and fully tunable spin-orbit torques. Here, current-induced
spin-orbit torques in VO$_2$/NiFe heterostructures were investigated using
spin-torque ferromagnetic resonance, where the VO$_2$ layer undergoes a
prominent insulator-metal transition. A roughly two-fold increase in the
Gilbert damping parameter, $\alpha$, with temperature was attributed to the
change in the VO$_2$/NiFe interface spin absorption across the VO$_2$ phase
transition. More remarkably, a large modulation ($\pm$100%) and a sign change
of the current-induced spin-orbit torque across the VO$_2$ phase transition
suggest two competing spin-orbit torque generating mechanisms. The bulk spin
Hall effect in metallic VO$_2$, corroborated by our first-principles
calculation of spin Hall conductivity, $\sigma_{SH} \approx 10^4
\frac{\hbar}{e} \Omega^{-1} m^{-1}$, is verified as the main source of the
spin-orbit torque in the metallic phase. The self-induced/anomalous torque in
NiFe, of the opposite sign and a similar magnitude to the bulk spin Hall effect
in metallic VO$_2$, could be the other competing mechanism that dominates as
temperature decreases. For applications, the strong tunability of the torque
strength and direction opens a new route to tailor spin-orbit torques of
materials which undergo phase transitions for new device functionalities.
|
2201.12984v1
|
2022-02-03
|
Controlling spin pumping into superconducting Nb by proximity-induced spin-triplet Cooper pairs
|
Proximity-induced long-range spin-triplet supercurrents, important for the
field of superconducting spintronics, are generated in
superconducting/ferromagnetic heterostructures when interfacial magnetic
inhomogeneities responsible for spin mixing and spin flip scattering are
present. The multilayer stack Nb/Cr/Fe/Cr/Nb has been shown to support such
exotic currents when fabricated into Josephson junction devices. However,
creating pure spin currents controllably in superconductors outside of the
Josephson junction architecture is a bottleneck to progress. Recently,
ferromagnetic resonance was proposed as a possible direction, the signature of
pure supercurrent creation being an enhancement of the Gilbert damping below
the superconducting critical temperature, but the necessary conditions are
still poorly established. Consistent with theoretical prediction, we
demonstrate conclusively that pumping pure spin currents into a superconductor
is only possible when conditions supporting proximity-induced spin-triplet
effects are satisfied. Our study is an important step forward for
superconducting pure spin current creation and manipulation, considerably
advancing the field of superconducting spintronics.
|
2202.01520v1
|
2022-06-17
|
Multiscale Modelling of the Antiferromagnet Mn2Au: From ab-initio to Micromagnetics
|
Antiferromagnets (AFMs) are strong candidates for the future spintronic and
memory applications largely because of their inherently fast dynamics and lack
of stray fields, with Mn2Au being one of the most promising. For the numerical
modelling of magnetic material properties, it is common to use ab-initio
methods, atomistic models and micromagnetics. However, each method alone
describes the physics within certain limits. Multiscale methods bridging the
gap between these three approaches have been already proposed for ferromagnetic
materials. Here, we present a complete multiscale model of the AFM Mn2Au as an
exemplar material, starting with results from ab-initio methods going via
atomistic spin dynamics (ASD) to an AFM Landau-Lifshitz-Bloch (AFM-LLB) model.
Firstly, bulk is modelled using a classical spin Hamiltonian constructed based
on earlier first-principles calculations. Secondly, this spin model is used in
the stochastic Landau-Lifshitz-Gilbert (LLG) to calculate temperature-dependent
equilibrium properties, such as magnetization and magnetic susceptibilities.
Thirdly, the temperature dependent micromagnetic parameters are used in the
AFM-LLB. We validate our approach by comparing the ASD and AFM-LLB models for
three paradigmatic cases; (i) Damped magnetic oscillations, (ii) magnetization
dynamics following a heat pulse resembling pump-probe experiments, (iii)
magnetic domain wall motion under thermal gradients.
|
2206.08625v1
|
2022-10-29
|
Micromagnetic frequency-domain simulation methods for magnonic systems
|
We present efficient numerical methods for the simulation of small
magnetization oscillations in three-dimensional micromagnetic systems.
Magnetization dynamics is described by the Landau-Lifshitz-Gilbert (LLG)
equation, linearized in the frequency domain around a generic equilibrium
configuration, and formulated in a special operator form that allows leveraging
large-scale techniques commonly used to evaluate the effective field in
time-domain micromagnetic simulations. By using this formulation, we derive
numerical algorithms to compute the free magnetization oscillations (i.e., spin
wave eigenmodes) as well as magnetization oscillations driven by ac
radio-frequency fields for arbitrarily shaped nanomagnets. Moreover,
semi-analytical perturbation techniques based on the computation of a reduced
set of eigenmodes are provided for fast evaluation of magnetization frequency
response and absorption spectra as a function of damping and ac field. We
present both finite difference and finite element implementations and
demonstrate their effectiveness on a test case. These techniques open the
possibility to study generic magnonic systems discretized with several hundred
thousand (or even millions) of computational cells in a reasonably short time.
|
2210.16564v3
|
2023-03-07
|
Magnon currents excited by the spin Seebeck effect in ferromagnetic EuS thin films
|
A magnetic insulator is an ideal platform to propagate spin information by
exploiting magnon currents. However, until now, most studies have focused on
Y$_3$Fe$_5$O$_{12}$ (YIG) and a few other ferri- and antiferromagnetic
insulators, but not on pure ferromagnets. In this study, we demonstrate for the
first time that magnon currents can propagate in ferromagnetic insulating thin
films of EuS. By performing both local and non-local transport measurements in
18-nm-thick films of EuS using Pt electrodes, we detect magnon currents arising
from thermal generation by the spin Seebeck effect. By comparing the dependence
of the local and non-local signals with the temperature (< 30 K) and magnetic
field (< 9 T), we confirm the magnon transport origin of the non-local signal.
Finally, we extract the magnon diffusion length in the EuS film (~140 nm), a
short value in good correspondence with the large Gilbert damping measured in
the same film.
|
2303.03833v2
|
2023-04-01
|
A coupled magneto-structural continuum model for multiferroic $\mathrm{BiFeO}_3$
|
A continuum approach to study magnetoelectric multiferroic $\mathrm{BiFeO}_3$
(BFO) is proposed. Our modeling effort marries the ferroelectric (FE) phase
field method and micromagnetic simulations in order to describe the entire
multiferroic order parameter sector (polarization, oxygen antiphase tilts,
strain, and magnetism) self-consistently on the same time and length scale. In
this paper, we discuss our choice of ferroelectric and magnetic energy terms
and demonstrate benchmarks against known behavior. We parameterize the lowest
order couplings of the structural distortions against previous predictions from
density functional theory calculations giving access to simulations of the FE
domain wall (DW) topology. This allows us to estimate the energetic hierarchy
and thicknesses of the numerous structural DWs. We then extend the model to the
canted antiferromagnetic order and demonstrate how the ferroelectric domain
boundaries influence the resulting magnetic DWs. We also highlight some
capabilities of this model by providing two examples relevant for applications.
We demonstrate spin wave transmission through the multiferroic domain
boundaries which identify rectification in qualitative agreement with recent
experimental observations. As a second example of application, we model
fully-dynamical magnetoelectric switching, where we find a sensitivity on the
Gilbert damping with respect to switching pathways. We envision that this
modeling effort will set the basis for further work on properties of arbitrary
3D nanostructures of BFO (and related multiferroics) at the mesoscale.
|
2304.00270v1
|
2023-04-19
|
Thickness-dependent magnetic properties in Pt[CoNi]n multilayers with perpendicular magnetic anisotropy
|
We systematically investigated the Ni and Co thickness-dependent
perpendicular magnetic anisotropy (PMA) coefficient, magnetic domain
structures, and magnetization dynamics of Pt(5 nm)/[Co(t_Co nm)/Ni(t_Ni
nm)]5/Pt(1 nm) multilayers by combining the four standard magnetic
characterization techniques. The magnetic-related hysteresis loops obtained
from the field-dependent magnetization M and anomalous Hall resistivity (AHR)
\r{ho}_xy found that the two serial multilayers with t_Co = 0.2 and 0.3 nm have
the optimum PMA coefficient K_U well as the highest coercivity H_C at the Ni
thickness t_Ni = 0.6 nm. Additionally, the magnetic domain structures obtained
by Magneto-optic Kerr effect (MOKE) microscopy also significantly depend on the
thickness and K_U of the films. Furthermore, the thickness-dependent linewidth
of ferromagnetic resonance is inversely proportional to K_U and H_C, indicating
that inhomogeneous magnetic properties dominate the linewidth. However, the
intrinsic Gilbert damping constant determined by a linear fitting of
frequency-dependent linewidth does not depend on Ni thickness and K_U. Our
results could help promote the PMA [Co/Ni] multilayer applications in various
spintronic and spin-orbitronic devices.
|
2304.09366v1
|
2023-09-18
|
Coherent Tunneling and Strain Sensitivity of an All Heusler Alloy Magnetic Tunneling Junction: A First-Principles Study
|
Half-metallic Co-based full Heusler alloys have captured considerable
attention of the researchers in the realm of spintronic applications, owing to
their remarkable characteristics such as exceptionally high spin polarization
at Fermi level, ultra-low Gilbert damping, and high Curie temperature. In this
comprehensive study, employing density functional theory, we delve into the
stability and electron transport properties of a magnetic tunneling junction
(MTJ) comprising a Co$_2$MnSb/HfIrSb interface. Utilizing a standard model
given by Julliere, we estimate the tunnel magnetoresistance (TMR) ratio of this
heterojunction under external electric field, revealing a significantly high
TMR ratio (500%) that remains almost unaltered for electric field magnitudes up
to 0.5 V/A. In-depth investigation of K-dependent majority spin transmissions
uncovers the occurrence of coherent tunneling for the Mn-Mn/Ir interface,
particularly when a spacer layer beyond a certain thickness is employed.
Additionally, we explore the impact of bi-axial strain on the MTJ by varying
the in-plane lattice constants between -4% and +4%. Our spin-dependent
transmission calculations demonstrate that the Mn-Mn/Ir interface manifests
strain-sensitive transmission properties under both compressive and tensile
strain, and yields a remarkable three-fold increase in majority spin
transmission under tensile strain conditions. These compelling outcomes place
the Co2MnSb/HfIrSb junction among the highly promising candidates for nanoscale
spintronic devices, emphasizing the potential significance of the system in the
advancement of the field.
|
2309.09755v1
|
2023-10-27
|
Effect of interfacial Dzyaloshinskii-Moriya interaction in spin dynamics of an Antiferromagnet coupled Ferromagnetic double-barrier Magnetic Tunnel Junction
|
In this work, we have studied the spin dynamics of a synthethic
Antiferromagnet (SAFM)$|$Heavy Metal (HM)$|$Ferromagnet (FM) double barrier
magnetic tunnel junction (MTJ) in presence of Ruderman-Kittel-Kasuya-Yoside
interaction (RKKYI), interfacial Dzyaloshinskii-Moriya interaction (iDMI),
N\'eel field and Spin-Orbit Coupling (SOC) with different Spin Transfer Torque
(STT). We employ Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation to
investigate the AFM dynamics of the proposed system. We found that the system
exhibits a transition from regular to damped oscillations with the increase in
strength of STT for systems with weaker iDMI than RKKYI while display sustained
oscillatons for system having same order of iDMI and RKKYI. On the other hand
the iDMI dominating system exhibits self-similar but aperiodic patterns in
absence of N\'eel field. In the presence of N\'eel field, the RKKYI dominating
systems exhibit chaotic oscillations for low STT but display sustained
oscillation under moderate STT. Our results suggest that the decay time of
oscillations can be controlled via SOC. The system can works as an oscillator
for low SOC but display nonlinear characteristics with the rise in SOC for
systems having weaker iDMI than RKKYI while an opposite characteristic are
noticed for iDMI dominating systems. We found periodic oscillations under low
external magnetic field in RKKYI dominating systems while moderate field are
necessary for sustained oscillation in iDMI dominating systems. Moreover, the
system exhibits saddle-node bifurcation and chaos under moderate N\'eel field
and SOC with suitable iDMI and RKKYI. In addition, our results indicate that
the magnon lifetime can be enhanced by increasing the strength of iDMI for both
optical and acoustic modes.
|
2310.18175v1
|
2023-11-14
|
Berry curvature induced giant intrinsic spin-orbit torque in single layer magnetic Weyl semimetal thin films
|
Topological quantum materials can exhibit unconventional surface states and
anomalous transport properties, but their applications to spintronic devices
are restricted as they require the growth of high-quality thin films with
bulk-like properties. Here, we study 10--30 nm thick epitaxial ferromagnetic
Co$_{\rm 2}$MnGa films with high structural order. Very high values of the
anomalous Hall conductivity, $\sigma_{\rm xy}=1.35\times10^{5}$ $\Omega^{-1}
m^{-1}$, and the anomalous Hall angle, $\theta_{\rm H}=15.8\%$, both comparable
to bulk values. We observe a dramatic crystalline orientation dependence of the
Gilbert damping constant of a factor of two and a giant intrinsic spin Hall
conductivity, $\mathit{\sigma_{\rm SHC}}=(6.08\pm 0.02)\times 10^{5}$
($\hbar/2e$) $\Omega^{-1} m^{-1}$, which is an order of magnitude higher than
literature values of single-layer Ni$_{\rm 80}$Fe$_{\rm 20}$, Ni, Co, Fe, and
multilayer Co$_{\rm 2}$MnGa stacks. Theoretical calculations of the intrinsic
spin Hall conductivity, originating from a strong Berry curvature, corroborate
the results and yield values comparable to the experiment. Our results open up
for the design of spintronic devices based on single layers of topological
quantum materials.
|
2311.08145v2
|
2023-12-26
|
All solution grown epitaxial magnonic crystal of thulium iron garnet thin film
|
Magnonics has shown the immense potential of compatibility with CMOS devices
and the ability to be utilized in futuristic quantum computing. Therefore, the
magnonic crystals, both metallic and insulating, are under extensive
exploration. The presence of high spin-orbit interaction induced by the
presence of rare-earth elements in thulium iron garnet (TmIG) increases its
potential in magnonic applications. Previously, TmIG thin films were grown
using ultra-high vacuum-based techniques. Here, we present a cost-effective
solution-based approach that enables the excellent quality interface and
surface roughness of the epitaxial TmIG/GGG. The deposited TmIG (12.2 nm) thin
film's physical and spin dynamic properties are investigated in detail. The
confirmation of the epitaxy using X-ray diffraction in $\phi$-scan geometry
along with the X-ray reflectivity and atomic force for the thickness and
roughness analysis and topography, respectively. The epitaxial TmIG/GGG have
confirmed the perpendicular magnetic anisotropy utilizing the
polar-magneto-optic Kerr effect. Analyzing the ferromagnetic resonance study of
TmIG/GGG thin films provides the anisotropy constant K$_U$ = 20.6$\times$10$^3$
$\pm$ 0.2$\times$10$^3$ N/m$^2$ and the Gilbert damping parameter $\alpha$ =
0.0216 $\pm$ 0.0028. The experimental findings suggest that the
solution-processed TmIG/GGG thin films have the potential to be utilized in
device applications.
|
2312.15973v1
|
2024-03-01
|
Spin current control of magnetism
|
Exploring novel strategies to manipulate the order parameter of magnetic
materials by electrical means is of great importance, not only for advancing
our understanding of fundamental magnetism, but also for unlocking potential
practical applications. A well-established concept to date uses gate voltages
to control magnetic properties, such as saturation magnetization, magnetic
anisotropies, coercive field, Curie temperature and Gilbert damping, by
modulating the charge carrier population within a capacitor structure. Note
that the induced carriers are non-spin-polarized, so the control via the
electric-field is independent of the direction of the magnetization. Here, we
show that the magnetocrystalline anisotropy (MCA) of ultrathin Fe films can be
reversibly modified by a spin current generated in Pt by the spin Hall effect.
The effect decreases with increasing Fe thickness, indicating that the origin
of the modification can be traced back to the interface. Uniquely, the change
in MCA due to the spin current depends not only on the polarity of the charge
current but also on the direction of magnetization, i.e. the change in MCA has
opposite sign when the direction of magnetization is reversed. The control of
magnetism by the spin current results from the modified exchange splitting of
majority- and minority-spin bands, and differs significantly from the
manipulation by gate voltages via a capacitor structure, providing a
functionality that was previously unavailable and could be useful in advanced
spintronic devices.
|
2403.00709v1
|
2005-01-02
|
Effect of dipolar interactions on the magnetization of a cubic array of nanomagnets
|
We investigated the effect of intermolecular dipolar interactions on a cubic
3D ensemble of 5X5X4=100 nanomagnets, each with spin $S = 5$. We employed the
Landau-Lifshitz-Gilbert equation to solve for the magnetization $M(B)$ curves
for several values of the damping constant $\alpha$, the induction sweep rate,
the lattice constant $a$, the temperature $T$, and the magnetic anisotropy
field $H_A$. We find that the smaller the $\alpha$, the stronger the maximum
induction required to produce hysteresis. The shape of the hysteresis loops
also depends on the damping constant. We find further that the system
magnetizes and demagnetizes at decreasing magnetic field strengths with
decreasing sweep rates, resulting in smaller hysteresis loops. Variations of
$a$ within realistic values (1.5 nm - 2.5 nm) show that the dipolar interaction
plays an important role in the magnetic hysteresis by controlling the
relaxation process. The $T$ dependencies of $\alpha$ and of $M$ are presented
and discussed with regard to recent experimental data on nanomagnets. $H_A$
enhances the size of the hysteresis loops for external fields parallel to the
anisotropy axis, but decreases it for perpendicular external fields. Finally,
we reproduce and test an $M(B)$ curve for a 2D-system [M. Kayali and W. Saslow,
Phys. Rev. B {\bf 70}, 174404 (2004)]. We show that its hysteretic behavior is
only weakly dependent on the shape anisotropy field and the sweep rate, but
depends sensitively upon the dipolar interactions. Although in 3D systems,
dipole-dipole interactions generally diminish the hysteresis, in 2D systems,
they strongly enhance it. For both square 2D and rectangular 3D lattices with
${\bm B}||(\hat{\bm x}+\hat{\bm y})$, dipole-dipole interactions can cause
large jumps in the magnetization.
|
0501006v2
|
2019-03-07
|
Uniaxial anisotropy, intrinsic and extrinsic damping in Co$_{2}$FeSi Heusler alloy thin films
|
Ferromagnetic resonance (FMR) technique has been used to study the
magnetization relaxation processes and magnetic anisotropy in two different
series of the Co2FeSi (CFS) Heusler alloy thin films, deposited on the Si(111)
substrate by UHV sputtering. While the CFS films of fixed (50 nm) thickness,
deposited at different substrate temperatures (TS) ranging from room
temperature (RT) to 600^C, constitute the series-I, the CFS films with
thickness t varying from 12 nm to 100 nm and deposited at 550^C make up the
series-II. In series-I, the CFS films deposited at TS = RT and 200^C are
completely amorphous, the one at TS = 300^C is partially crystalline, and those
at TS equal 450^C, 550^C and 600^C are completely crystalline with B2 order. By
contrast, all the CFS films in series-II are in the fully-developed B2
crystalline state. Irrespective of the strength of disorder and film thickness,
angular variation of the resonance field in the film plane unambiguously
establishes the presence of global in-plane uniaxial anisotropy. Angular
variation of the linewidth in the film plane reveals that, in the CFS thin
films of varying thickness, a crossover from the in-plane local four-fold
symmetry (cubic anisotropy) to local two-fold symmetry (uniaxial anisotropy)
occurs as t exceeds 50 nm. Gilbert damping parameter {\alpha} decreases
monotonously from 0.047 to 0.0078 with decreasing disorder strength (increasing
TS) and jumps from 0.008 for the CFS film with t = 50 nm to 0.024 for the film
with t equal 75 nm. Such variations of {\alpha} with TS and t are understood in
terms of the changes in the total (spin-up and spin-down) density of states at
the Fermi level caused by the disorder and film thickness.
|
1903.02976v1
|
2023-09-19
|
Impact of strain on the SOT-driven dynamics of thin film Mn$_3$Sn
|
Mn$_3$Sn, a metallic antiferromagnet with an anti-chiral 120$^\circ$ spin
structure, generates intriguing magneto-transport signatures such as a large
anomalous Hall effect, spin-polarized current with novel symmetries, anomalous
Nernst effect, and magneto-optic Kerr effect. When grown epitaxially as
MgO(110)[001]$\parallel$ Mn$_3$Sn($0\bar{1}\bar{1}0$)[0001], Mn$_3$Sn
experiences a uniaxial tensile strain, which changes the bulk six-fold
anisotropy landscape to a perpendicular magnetic anisotropy with two stable
states. In this work, we investigate the field-assisted spin orbit-torque
(SOT)-driven response of the order parameter in single-domain Mn$_3$Sn with
uniaxial tensile strain. We find that for a non-zero external magnetic field,
the order parameter can be switched between the two stable states if the
magnitude of the input current is between two field-dependent critical
currents. Below the lower critical current, the order parameter exhibits a
stationary state in the vicinity of the initial stable state. On the other
hand, above the higher critical current, the order parameter shows oscillatory
dynamics which could be tuned from the 100's of megahertz to the gigahertz
range. We obtain approximate expressions of the two critical currents and find
them to agree very well with the numerical simulations for experimentally
relevant magnetic fields. We also obtain unified functional form of the
switching time versus the input current for different magnetic fields. Finally,
we show that for lower values of Gilbert damping ($\alpha \leq 2\times
10^{-3}$), the critical currents and the final steady states depend
significantly on the damping constant. The numerical and analytic results
presented in our work can be used by both theorists and experimentalists to
understand the SOT-driven order dynamics in PMA Mn$_3$Sn and design future
experiments and devices.
|
2309.10246v2
|
2008-12-31
|
Weak Solutions of the Stochastic Landau-Lifshitz-Gilbert Equation
|
The Landau-Lifshitz-Gilbert equation perturbed by a multiplicative
space-dependent noise is considered for a ferromagnet filling a bounded
three-dimensional domain. We show the existence of weak martingale solutions
taking values in a sphere $\mathbb S^2$. The regularity of weak solutions is
also discussed. Some of the regularity results are new even for the
deterministic Landau-Lifshitz-Gilbert equation.
|
0901.0039v1
|
2023-09-08
|
Branching points in the planar Gilbert--Steiner problem have degree 3
|
Gilbert--Steiner problem is a generalization of the Steiner tree problem on a
specific optimal mass transportation.
We show that every branching point in a solution of the planar
Gilbert--Steiner problem has degree 3.
|
2309.04202v2
|
2007-02-23
|
Organization of the Modulopt collection of optimization problems in the Libopt environment -- Version 1.0
|
This note describes how the optimization problems of the Modulopt collection
are organized within the Libopt environment. It is aimed at being a guide for
using and enriching this collection in this environment.
|
0702695v1
|
2005-10-17
|
Comment on "Operator Quantum Error Correction"
|
The attempt to equate operator quantum error correction (quant-ph/0504189v1)
with the quantum computer condition (quant-ph/0507141) in version two of
quant-ph/0504189 is shown to be invalid.
|
0510116v1
|
2007-09-17
|
H-Decompositions
|
We show that for all graphs H of size n, the complete graph $K_{2n+1}$ has an
$H$-decomposition.
|
0709.2525v5
|
2008-10-06
|
Unsolvability of the isomorphism problem for [free abelian]-by-free groups
|
The isomorphism problem for [free abelian]-by-free groups is unsolvable.
|
0810.0935v2
|
2011-11-27
|
Comment on "Capturing correlations in chaotic diffusion by approximation methods"
|
This is a comment on [G. Knight and R. Klages, Phys. Rev. E 84, 041135
(2011); also available at arXiv:1107.5293v2 [math-ph]].
|
1111.6271v1
|
2014-01-11
|
Hashimoto transform for stochastic Landau-Lifshitz-Gilbert equation
|
We show that Hashimoto transformation is applicable to the one dimensional
stochastic Landau-Lifshitz-Gilbert (LLG) equation and transforms it to the
stochastic generalized heat equation with nonlocal (in space) interaction.
|
1401.2520v1
|
2017-01-04
|
Non-linear Cyclic Codes that Attain the Gilbert-Varshamov Bound
|
We prove that there exist non-linear binary cyclic codes that attain the
Gilbert-Varshamov bound.
|
1701.01043v1
|
2019-01-28
|
Conformal deformations preserving the Finslerian $R$-Einstein criterion
|
Given a Finslerian metric $F$ on a $C^4$-manifold, conformal deformations of
$F$ preserving the $R$-Einstein criterion are presented. In particular, locally
conformal invariance between two Finslerian $R$-Einstein metrics is
characterized.
|
1902.00069v1
|
2022-04-07
|
How to design a network architecture using availability
|
The best way to design a network is to take into account Availability values
and Capacity Planning. You already saw Availability expressed with numbers such
as 99.99%. The purpose of this document is to introduce the way to compute
Availability values using Reliability Block Diagrams.
|
2204.03311v1
|
2010-02-22
|
Transport and magnetization dynamics in a superconductor/single-molecule magnet/superconductor junction
|
We study dc-transport and magnetization dynamics in a junction of arbitrary
transparency consisting of two spin-singlet superconducting leads connected via
a single classical spin precessing at the frequency $\Omega$. The presence of
the spin in the junction provides different transmission amplitudes for spin-up
and spin-down quasiparticles as well as a time-dependent spin-flip transmission
term. For a phase biased junction, we show that a steady-state superconducting
charge current flows through the junction and that an out-of-equilibrium
circularly polarized spin current, of frequency $\Omega$, is emitted in the
leads. Detailed understanding of the charge and spin currents is obtained in
the entire parameter range. In the adiabatic regime, $\hbar \Omega \ll 2\Delta$
where $\Delta$ is the superconducting gap, and for high transparencies of the
junction, a strong suppression of the current takes place around $\vp \approx
0$ due to an abrupt change in the occupation of the Andreev bound-states. At
higher values of the phase and/or precession frequency, extended
(quasi-particle like) states compete with the bound-states in order to carry
the current. Well below the superconducting transition, these results are shown
to be weakly affected by the back-action of the spin current on the dynamics of
the precessing spin. Indeed, we show that the Gilbert damping due to the
quasi-particle spin current is strongly suppressed at low-temperatures, which
goes along with a shift of the precession frequency due to the condensate. The
results obtained may be of interest for on-going experiments in the field of
molecular spintronics.
|
1002.3929v4
|
2013-06-18
|
Baryons do trace dark matter 380,000 years after the big bang: Search for compensated isocurvature perturbations with WMAP 9-year data
|
Primordial isocurvature fluctuations between photons and either neutrinos or
non-relativistic species such as baryons or dark matter are known to be
sub-dominant to adiabatic fluctuations. Perturbations in the relative densities
of baryons and dark matter (known as compensated isocurvature perturbations, or
CIPs), however, are surprisingly poorly constrained. CIPs leave no imprint in
the cosmic microwave background (CMB) on observable scales, at least at linear
order in their amplitude and zeroth order in the amplitude of adiabatic
perturbations. It is thus not yet empirically known if baryons trace dark
matter at the surface of last scattering. If CIPs exist, they would spatially
modulate the Silk damping scale and acoustic horizon, causing distinct
fluctuations in the CMB temperature/polarization power spectra across the sky:
this effect is first order in both the CIP and adiabatic mode amplitudes. Here,
temperature data from the Wilkinson Microwave Anisotropy Probe (WMAP) are used
to conduct the first CMB-based observational search for CIPs, using
off-diagonal correlations and the CMB trispectrum. Reconstruction noise from
weak lensing and point sources is shown to be negligible for this data set. No
evidence for CIPs is observed, and a 95%-confidence upper limit of $1.1\times
10^{-2}$ is imposed to the amplitude of a scale-invariant CIP power spectrum.
This limit agrees with CIP sensitivity forecasts for WMAP, and is competitive
with smaller scale constraints from measurements of the baryon fraction in
galaxy clusters. It is shown that the root-mean-squared CIP amplitude on 5-100
degree scales is smaller than 0.07-0.17 (depending on the scale) at the
95%-confidence level. Temperature data from the Planck satellite will provide
an even more sensitive probe for the existence of CIPs, as will the upcoming
ACTPol and SPTPol experiments on smaller angular scales.
|
1306.4319v1
|
2015-05-29
|
Microscopic Theory for Coupled Atomistic Magnetization and Lattice Dynamics
|
A coupled atomistic spin and lattice dynamics approach is developed which
merges the dynamics of these two degrees of freedom into a single set of
coupled equations of motion. The underlying microscopic model comprises local
exchange interactions between the electron spin and magnetic moment and the
local couplings between the electronic charge and lattice displacements. An
effective action for the spin and lattice variables is constructed in which the
interactions among the spin and lattice components are determined by the
underlying electronic structure. In this way, expressions are obtained for the
electronically mediated couplings between the spin and lattice degrees of
freedom, besides the well known inter-atomic force constants and spin-spin
interactions. These former susceptibilities provide an atomistic ab initio
description for the coupled spin and lattice dynamics. It is important to
notice that this theory is strictly bilinear in the spin and lattice variables
and provides a minimal model for the coupled dynamics of these subsystems and
that the two subsystems are treated on the same footing. Questions concerning
time-reversal and inversion symmetry are rigorously addressed and it is shown
how these aspects are absorbed in the tensor structure of the interaction
fields. By means of these results regarding the spin-lattice coupling, simple
explanations of ionic dimerization in double anti-ferromagnetic materials, as
well as, charge density waves induced by a non-uniform spin structure are
given. In the final parts, a set of coupled equations of motion for the
combined spin and lattice dynamics are constructed, which subsequently can be
reduced to a form which is analogous to the Landau-Lifshitz-Gilbert equations
for spin dynamics and damped driven mechanical oscillator for the ...
|
1505.08005v3
|
2016-04-28
|
Dynamics of skyrmionic states in confined helimagnetic nanostructures
|
In confined helimagnetic nanostructures, skyrmionic states in the form of
incomplete and isolated skyrmion states can emerge as the ground state in
absence of both external magnetic field and magnetocrystalline anisotropy. In
this work, we study the dynamic properties (resonance frequencies and
corresponding eigenmodes) of skyrmionic states in thin film FeGe disk samples.
We employ two different methods in finite-element based micromagnetic
simulation: eigenvalue and ringdown method. The eigenvalue method allows us to
identify all resonance frequencies and corresponding eigenmodes that can exist
in the simulated system. However, using a particular experimentally feasible
excitation can excite only a limited set of eigenmodes. Because of that, we
perform ringdown simulations that resemble the experimental setup using both
in-plane and out-of-plane excitations. In addition, we report the nonlinear
dependence of resonance frequencies on the external magnetic bias field and
disk sample diameter and discuss the possible reversal mode of skyrmionic
states. We compare the power spectral densities of incomplete skyrmion and
isolated skyrmion states and observe several key differences that can
contribute to the experimental identification of the state present in the
sample. We measure the FeGe Gilbert damping, and using its value we determine
what eigenmodes can be expected to be observed in experiments. Finally, we show
that neglecting the demagnetisation energy contribution or ignoring the
magnetisation variation in the out-of-film direction - although not changing
the eigenmode's magnetisation dynamics significantly - changes their resonance
frequencies substantially. Apart from contributing to the understanding of
skyrmionic states physics, this systematic work can be used as a guide for the
experimental identification of skyrmionic states in confined helimagnetic
nanostructures.
|
1604.08347v2
|
2017-04-13
|
Low energy magnon dynamics and magneto-optics of the skyrmionic Mott insulator Cu$_2$OSeO$_3$
|
In this work, we present a comprehensive study of the low energy optical
magnetic response of the skyrmionic Mott insulator Cu$_2$OSeO$_3$ via high
resolution time-domain THz spectroscopy. In zero field, a new magnetic
excitation not predicted by spin-wave theory with frequency $f$ = 2.03 THz is
observed and shown, with accompanying time-of-flight neutron scattering
experiments, to be a zone folded magnon from the $\mathrm{R}$ to
$\mathrm{\Gamma}$ points of the Brillouin zone. Highly sensitive polarimetry
experiments performed in weak magnetic fields, $\mu_0$H $<$ 200 mT, observe
Faraday and Kerr rotations which are proportional to the sample magnetization,
allowing for optical detection of the skyrmion phase and construction of a
magnetic phase diagram. From these measurements, we extract a critical exponent
of $\beta$ = 0.35 $\pm$ 0.04, in good agreement with the expected value for the
3D Heisenberg universality class of $\beta$ = 0.367. In large magnetic fields,
$\mu_0$H $>$ 5 T, we observe the magnetically active uniform mode of the
ferrimagnetic field polarized phase whose dynamics as a function of field and
temperature are studied. In addition to extracting a $g_\text{eff}$ = 2.08
$\pm$ 0.03, we observe the uniform mode to decay through a non-Gilbert damping
mechanism and to possesses a finite spontaneous decay rate, $\Gamma_0$
$\approx$ 25 GHz, in the zero temperature limit. Our observations are
attributed to Dzyaloshinkii-Moriya interactions, which have been proposed to be
exceptionally strong in Cu$_2$OSeO$_3$ and are expected to impact the low
energy magnetic response of such chiral magnets.
|
1704.04228v1
|
2017-08-25
|
Role of dimensional crossover on spin-orbit torque efficiency in magnetic insulator thin films
|
Magnetic insulators (MIs) attract tremendous interest for spintronic
applications due to low Gilbert damping and absence of Ohmic loss. Magnetic
order of MIs can be manipulated and even switched by spin-orbit torques (SOTs)
generated through spin Hall effect and Rashba-Edelstein effect in heavy
metal/MI bilayers. SOTs on MIs are more intriguing than magnetic metals since
SOTs cannot be transferred to MIs through direct injection of electron spins.
Understanding of SOTs on MIs remains elusive, especially how SOTs scale with
the film thickness. Here, we observe the critical role of dimensionality on the
SOT efficiency by systematically studying the MI layer thickness dependent SOT
efficiency in tungsten/thulium iron garnet (W/TmIG) bilayers. We first show
that the TmIG thin film evolves from two-dimensional to three-dimensional
magnetic phase transitions as the thickness increases, due to the suppression
of long-wavelength thermal fluctuation. Then, we report the significant
enhancement of the measured SOT efficiency as the thickness increases. We
attribute this effect to the increase of the magnetic moment density in concert
with the suppression of thermal fluctuations. At last, we demonstrate the
current-induced SOT switching in the W/TmIG bilayers with a TmIG thickness up
to 15 nm. The switching current density is comparable with those of heavy
metal/ferromagnetic metal cases. Our findings shed light on the understanding
of SOTs in MIs, which is important for the future development of ultrathin
MI-based low-power spintronics.
|
1708.07584v2
|
2018-07-04
|
Phase Boundary Exchange Coupling in the Mixed Magnetic Phase Regime of a Pd-doped FeRh Epilayer
|
Spin-wave resonance measurements were performed in the mixed magnetic phase
regime of a Pd-doped FeRh epilayer that appears as the first-order
ferromagnetic-antiferromagnetic phase transition takes place. It is seen that
the measured value of the exchange stiffness is suppressed throughout the
measurement range when compared to the expected value of the fully
ferromagnetic regime, extracted via the independent means of a measurement of
the Curie point, for only slight changes in the ferromagnetic volume fraction.
This behavior is attributed to the influence of the antiferromagnetic phase:
inspired by previous experiments that show ferromagnetism to be most persistent
at the surfaces and interfaces of FeRh thin films, we modelled the
antiferromagnetic phase as forming a thin layer in the middle of the epilayer
through which the two ferromagnetic layers are coupled up to a certain critical
thickness. The development of this exchange stiffness is then consistent with
that expected from the development of an exchange coupling across the magnetic
phase boundary, as a consequence of a thickness dependent phase transition
taking place in the antiferromagnetic regions and is supported by complimentary
computer simulations of atomistic spin-dynamics. The development of the Gilbert
damping parameter extracted from the ferromagnetic resonance investigations is
consistent with this picture.
|
1807.01615v6
|
2018-07-26
|
EPIC 246851721 b: A Tropical Jupiter Transiting a Rapidly Rotating Star in a Well-Aligned Orbit
|
We report the discovery of EPIC 246851721 b, a "tropical" Jupiter in a
6.18-day orbit around the bright ($V=11.439$) star EPIC 246851721 (TYC
1283-739-1). We present a detailed analysis of the system using $K2$ and
ground-based photometry, radial velocities, Doppler tomography and adaptive
optics imaging. From our global models, we infer that the host star is a
rapidly rotating ($v \sin i = 74.92 $ km s$^{-1}$) F dwarf with
$T_\mathrm{eff}$ = 6202 K, $R_\star = 1.586 \ R_\odot$ and $M_\star= 1.317 \
M_\odot$. EPIC 246851721 b has a radius of $1.051 \pm 0.044 R_J$, and a mass of
3.0$^{+1.1}_{-1.2} M_J$ . Doppler tomography reveals an aligned spin-orbit
geometry, with a projected obliquity of $-1.47^{\circ\ +0.87}_{\ -0.86}$,
making EPIC 246851721 the fourth hottest star to host a Jovian planet with $P >
5$ days and a known obliquity. Using quasi-periodic signatures in its light
curve that appear to be spot modulations, we estimate the star's rotation
period, and thereby infer the true obliquity of the system to be $3.7^{\circ\
+3.7}_{\ -1.8}$. We argue that this near-zero obliquity is likely to be
primordial rather than a result of tidal damping. The host star also has a
bound stellar companion, a $0.4 \ M_\odot$ M dwarf at a projected separation of
2100 AU, but the companion is likely incapable of emplacing EPIC 246851721 b in
its current orbit via high eccentricity Kozai-Lidov migration.
|
1807.10298v2
|
2018-09-10
|
Magnetic properties and field-driven dynamics of chiral domain walls in epitaxial Pt/Co/Au$_x$Pt$_{1-x}$ trilayers
|
Chiral domain walls in ultrathin perpendicularly magnetised layers have a
N\'{e}el structure stabilised by a Dzyaloshinskii-Moriya interaction (DMI) that
is generated at the interface between the ferromagnet and a heavy metal.
Different heavy metals are required above and below a ferromagnetic film in
order to generate the structural inversion asymmetry needed to ensure that the
DMI arising at the two interfaces does not cancel. Here we report on the
magnetic properties of epitaxial Pt/Co/Au$_x$Pt$_{1-x}$ trilayers grown by
sputtering onto sapphire substrates with 0.6 nm thick Co. As $x$ rises from 0
to 1 a structural inversion asymmetry is generated. We characterise the
epilayer structure with x-ray diffraction and cross-sectional transmission
electron microscopy, revealing (111) stacking. The saturation magnetization
falls as the proximity magnetisation in Pt is reduced, whilst the perpendicular
magnetic anisotropy $K_\mathrm{u}$ rises. The micromagnetic DMI strength $D$
was determined using the bubble expansion technique and also rises from a
negligible value when $x=0$ to $\sim 1$ mJ/m$^2$ for $x = 1$. The depinning
field at which field-driven domain wall motion crosses from the creep to the
depinning regime rises from $\sim 40$ to $\sim 70$ mT, attributed to greater
spatial fluctuations of the domain wall energy with increasing Au
concentration. Meanwhile, the increase in DMI causes the Walker field to rise
from $\sim 10$ to $\sim 280$ mT, meaning that only in the $x = 1$ sample is the
steady flow regime accessible. The full dependence of domain wall velocity on
driving field bears little resemblance to the prediction of a simple
one-dimensional model, but can be described very well using micromagnetic
simulations with a realistic model of disorder. These reveal a rise in Gilbert
damping as $x$ increases.
|
1809.03217v2
|
2019-09-06
|
Macrospin analysis of RF excitations within fully perpendicular magnetic tunnel junctions with second order easy-axis magnetic anisotropy contribution
|
The conditions of field and voltage for inducing steady state excitations in
fully perpendicular magnetic tunnel junctions (pMTJs), adapted for memory
applications, were numerically investigated by the resolution of the
Landau-Lifshitz-Gilbert equation in the macrospin approach. Both damping-like
and the field-like spin transfer torque terms were taken into account in the
simulations, as well as the contribution of the second order uniaxial
anisotropy term (K2), which has been recently revealed in MgO-based pMTJs. An
in-plane applied magnetic field balances the out of plane symmetry of the pMTJ
and allows the signal detection. Using this model, we assessed the states of
the free layer magnetization as a function of strength of K2 and polar theta_H
angle of the applied field (varied from 90 to 60 deg.). There are two stable
states, with the magnetization in-plane or out of plane of the layer, and two
dynamic states with self-sustained oscillations, called in-plane precession
state (IPP) or out of plane precession state (OPP). The IPP mode, with
oscillation frequencies up to 7 GHz, appears only for positive voltages if
theta_H = 90 deg. However, it shows a more complex distribution when the field
is slightly tilted out of plane. The OPP mode is excited only if K2 is
considered and reaches a maximum oscillation frequency of 15 GHz. Large areas
of dynamic states with high frequencies are obtained for strong values of the
field-like torque and K2, when applying a slightly tilted external field toward
the out of plane direction. The non-zero temperature does not modify the phase
diagrams, but reduces drastically the power spectral density peak amplitudes.
|
1909.02926v1
|
2021-04-21
|
Atomic Layer Deposition of Yttrium Iron Garnet Thin Films for 3D Magnetic Structures
|
A wide variety of new phenomena such as novel magnetization configurations
have been predicted to occur in three dimensional magnetic nanostructures.
However, the fabrication of such structures is often challenging due to the
specific shapes required, such as magnetic tubes and spirals. Furthermore, the
materials currently used to assemble these structures are predominantly
magnetic metals that do not allow to study the magnetic response of the system
separately from the electronic one. In the field of spintronics, the
prototypical material used for such experiments is the ferrimagnetic insulator
yttrium iron garnet (Y$_3$Fe$_5$O$_{12}$, YIG). YIG is one of the best
materials especially for magnonic studies due to its low Gilbert damping. Here,
we report the first successful fabrication of YIG thin films via atomic layer
deposition. To that end we utilize a supercycle approach based on the
combination of sub-nanometer thin layers of the binary systems Fe$_2$O$_3$ and
Y$_2$O$_3$ in the correct atomic ratio on Y$_3$Al$_5$O$_{12}$ substrates with a
subsequent annealing step. Our process is robust against typical growth-related
deviations, ensuring a good reproducibility. The ALD-YIG thin films exhibit a
good crystalline quality as well as magnetic properties comparable to other
deposition techniques. One of the outstanding characteristics of atomic layer
deposition is its ability to conformally coat arbitrarily-shaped substrates.
ALD hence is the ideal deposition technique to grant an extensive freedom in
choosing the shape of the magnetic system. The atomic layer deposition of YIG
enables the fabrication of novel three dimensional magnetic nanostructures,
which in turn can be utilized for experimentally investigating the phenomena
predicted in those structures.
|
2104.10293v2
|
2022-11-03
|
Skyrmion Jellyfish in Driven Chiral Magnets
|
Chiral magnets can host topological particles known as skyrmions, which carry
an exactly quantised topological charge $Q=-1$. In the presence of an
oscillating magnetic field ${\bf B}_1(t)$, a single skyrmion embedded in a
ferromagnetic background will start to move with constant velocity ${\bf
v}_{\text{trans}}$. The mechanism behind this motion is similar to the one used
by a jellyfish when it swims through water. We show that the skyrmion's motion
is a universal phenomenon, arising in any magnetic system with translational
modes. By projecting the equation of motion onto the skyrmion's translational
modes and going to quadratic order in ${\bf B}_1(t)$, we obtain an analytical
expression for ${\bf v}_{\text{trans}}$ as a function of the system's linear
response. The linear response and consequently ${\bf v}_{\text{trans}}$ are
influenced by the skyrmion's internal modes and scattering states, as well as
by the ferromagnetic background's Kittel mode. The direction and speed of ${\bf
v}_{\text{trans}}$ can be controlled by changing the polarisation, frequency
and phase of the driving field ${\bf B}_1(t)$. For systems with small Gilbert
damping parameter $\alpha$, we identify two distinct physical mechanisms used
by the skyrmion to move. At low driving frequencies, the skyrmion's motion is
driven by friction, and $v_{\text{trans}}\sim\alpha$, whereas at higher
frequencies above the ferromagnetic gap, the skyrmion moves by magnon emission,
and $v_{\text{trans}}$ becomes independent of $\alpha$.
|
2211.01714v5
|
2023-04-05
|
Threshold current of field-free perpendicular magnetization switching using anomalous spin-orbit torque
|
Spin-orbit torque (SOT) is a candidate technique in next generation magnetic
random-access memory (MRAM). Recently, experiments show that some material with
low-symmetric crystalline or magnetic structures can generate anomalous SOT
that has an out-of-plane component, which is crucial in switching perpendicular
magnetization of adjacent ferromagnetic (FM) layer in the field-free condition.
In this work, we analytically derive the threshold current of field-free
perpendicular magnetization switching using the anomalous SOT. And we
numerically calculate the track of the magnetic moment in a FM free layer when
an applied current is smaller and greater than the threshold current. After
that, we study the applied current dependence of the switching time and the
switching energy consumption, which shows the minimum energy consumption
decreases as out-of-plane torque proportion increases. Then we study the
dependences of the threshold current on anisotropy strength, out-of-plane
torque proportion, FM free layer thickness and Gilbert damping constant, and
the threshold current shows negative correlation with the out-of-plane torque
proportion and positive correlation with the other three parameters. Finally,
we demonstrate that when the applied current is smaller than the threshold
current, although it cannot switch the magnetization of FM free layer, it can
still equivalently add an effective exchange bias field H_{bias} on the FM free
layer. The H_{bias} is proportional to the applied current J_{SOT}, which
facilitates the determination of the anomalous SOT efficiency. This work helps
us to design new spintronic devices that favor field-free switching
perpendicular magnetization using the anomalous SOT, and provides a way to
adjust the exchange bias field, which is helpful in controlling FM layer
magnetization depinning.
|
2304.02248v2
|
2023-08-07
|
$\textit{In situ}$ electric-field control of ferromagnetic resonance in the low-loss organic-based ferrimagnet V[TCNE]$_{x\sim 2}$
|
We demonstrate indirect electric-field control of ferromagnetic resonance
(FMR) in devices that integrate the low-loss, molecule-based, room-temperature
ferrimagnet vanadium tetracyanoethylene (V[TCNE]$_{x \sim 2}$) mechanically
coupled to PMN-PT piezoelectric transducers. Upon straining the V[TCNE]$_x$
films, the FMR frequency is tuned by more than 6 times the resonant linewidth
with no change in Gilbert damping for samples with $\alpha = 6.5 \times
10^{-5}$. We show this tuning effect is due to a strain-dependent magnetic
anisotropy in the films and find the magnetoelastic coefficient $|\lambda_S|
\sim (1 - 4.4)$ ppm, backed by theoretical predictions from DFT calculations
and magnetoelastic theory. Noting the rapidly expanding application space for
strain-tuned FMR, we define a new metric for magnetostrictive materials,
$\textit{magnetostrictive agility}$, given by the ratio of the magnetoelastic
coefficient to the FMR linewidth. This agility allows for a direct comparison
between magnetostrictive materials in terms of their comparative efficacy for
magnetoelectric applications requiring ultra-low loss magnetic resonance
modulated by strain. With this metric, we show V[TCNE]$_x$ is competitive with
other magnetostrictive materials including YIG and Terfenol-D. This combination
of ultra-narrow linewidth and magnetostriction in a system that can be directly
integrated into functional devices without requiring heterogeneous integration
in a thin-film geometry promises unprecedented functionality for electric-field
tuned microwave devices ranging from low-power, compact filters and circulators
to emerging applications in quantum information science and technology.
|
2308.03353v1
|
2023-08-18
|
Large thermo-spin effects in Heusler alloy based spin-gapless semiconductor thin films
|
Recently, Heusler alloys-based spin gapless semiconductors (SGSs) with high
Curie temperature (TC) and sizeable spin polarization have emerged as potential
candidates for tunable spintronic applications. We report comprehensive
investigation of the temperature dependent ANE and intrinsic longitudinal spin
Seebeck effect (LSSE) in CoFeCrGa thin films grown on MgO substrates. Our
findings show the anomalous Nernst coefficient for the MgO/CoFeCrGa (95 nm)
film is $\cong 1.86$ micro V/K at room temperature which is nearly two orders
of magnitude higher than that of the bulk polycrystalline sample of CoFeCrGa (=
0.018 micro V/K) but comparable to that of the magnetic Weyl semimetal Co2MnGa
thin film (2-3 micro V/K). Furthermore, the LSSE coefficient for our
MgO/CoFeCrGa(95nm)/Pt(5nm) heterostructure is $\cong 20.5$ $\mu$V/K/$\Omega$ at
room temperature which is twice larger than that of the half-metallic
ferromagnetic La$_{0.7}$Sr$_{0.3}$MnO$_3$ thin films ($\cong$ 20.5
$\mu$V/K/$\Omega$). We show that both ANE and LSSE coefficients follow
identical temperature dependences and exhibit a maximum at $\cong$ 225 K which
is understood as the combined effects of inelastic magnon scatterings and
reduced magnon population at low temperatures. Our analyses not only indicate
that the extrinsic skew scattering is the dominating mechanism for ANE in these
films but also provide critical insights into the functional form of the
observed temperature dependent LSSE at low temperatures. Furthermore, by
employing radio frequency transverse susceptibility and broadband ferromagnetic
resonance in combination with the LSSE measurements, we establish a correlation
among the observed LSSE signal, magnetic anisotropy and Gilbert damping of the
CoFeCrGa thin films, which will be beneficial for fabricating tunable and
highly efficient Heusler alloys based spincaloritronic nanodevices.
|
2308.09843v1
|
2017-07-28
|
Measurements of the Temperature and E-Mode Polarization of the CMB from 500 Square Degrees of SPTpol Data
|
We present measurements of the $E$-mode polarization angular auto-power
spectrum ($EE$) and temperature-$E$-mode cross-power spectrum ($TE$) of the
cosmic microwave background (CMB) using 150 GHz data from three seasons of
SPTpol observations. We report the power spectra over the spherical harmonic
multipole range $50 < \ell \leq 8000$, and detect nine acoustic peaks in the
$EE$ spectrum with high signal-to-noise ratio. These measurements are the most
sensitive to date of the $EE$ and $TE$ power spectra at $\ell > 1050$ and $\ell
> 1475$, respectively. The observations cover 500 deg$^2$, a fivefold increase
in area compared to previous SPTpol analyses, which increases our sensitivity
to the photon diffusion damping tail of the CMB power spectra enabling tighter
constraints on \LCDM model extensions. After masking all sources with
unpolarized flux $>50$ mJy we place a 95% confidence upper limit on residual
polarized point-source power of $D_\ell = \ell(\ell+1)C_\ell/2\pi
<0.107\,\mu{\rm K}^2$ at $\ell=3000$, suggesting that the $EE$ damping tail
dominates foregrounds to at least $\ell = 4050$ with modest source masking. We
find that the SPTpol dataset is in mild tension with the $\Lambda CDM$ model
($2.1\,\sigma$), and different data splits prefer parameter values that differ
at the $\sim 1\,\sigma$ level. When fitting SPTpol data at $\ell < 1000$ we
find cosmological parameter constraints consistent with those for $Planck$
temperature. Including SPTpol data at $\ell > 1000$ results in a preference for
a higher value of the expansion rate ($H_0 = 71.3 \pm
2.1\,\mbox{km}\,s^{-1}\mbox{Mpc}^{-1}$ ) and a lower value for present-day
density fluctuations ($\sigma_8 = 0.77 \pm 0.02$).
|
1707.09353v3
|
2021-02-09
|
Binet's factorial series and extensions to Laplace transforms
|
We investigate a generalization of Binet's factorial series in the parameter
$\alpha$ \[ \mu\left( z\right) =\sum_{m=1}^{\infty}\frac{b_{m}\left(
\alpha\right) }{\prod_{k=0}^{m-1}(z+\alpha+k)}% \] due to Gilbert, for the
Binet function \[ \mu\left( z\right) =\log\Gamma\left( z\right) -\left(
z-\frac{1} {2}\right) \log z+z-\frac{1}{2}\log\left( 2\pi\right) \] After a
review of the Binet function $\mu\left( z\right) $ and Gilbert's investigations
of $\mu\left( z\right) $, several properties of the Binet polynomials
$b_{m}\left( \alpha\right) $ are presented. We compare Gilbert's generalized
factorial series with Stirling's asymptotic expansion and demonstrate by a
numerical example that, with a same number of terms evaluated, the Gilbert
generalized factorial series with an optimized value of $\alpha$ can beat the
best possible accuracy of Stirling's expansion. Finally, we extend Binet's
method to factorial series of Laplace transforms.
|
2102.04891v7
|
2000-05-29
|
Entropy Production in a Persistent Random Walk
|
We consider a one-dimensional persisent random walk viewed as a deterministic
process with a form of time reversal symmetry. Particle reservoirs placed at
both ends of the system induce a density current which drives the system out of
equilibrium. The phase space distribution is singular in the stationary state
and has a cumulative form expressed in terms of generalized Takagi functions.
The entropy production rate is computed using the coarse-graining formalism of
Gaspard, Gilbert and Dorfman. In the continuum limit, we show that the value of
the entropy production rate is independent of the coarse-graining and agrees
with the phenomenological entropy production rate of irreversible
thermodynamics.
|
0005063v1
|
2012-12-13
|
A convergent finite element approximation for the quasi-static Maxwell--Landau--Lifshitz--Gilbert equations
|
We propose a $\theta$-linear scheme for the numerical solution of the
quasi-static Maxwell-Landau-Lifshitz-Gilbert (MLLG) equations. Despite the
strong nonlinearity of the Landau-Lifshitz-Gilbert equation, the proposed
method results in a linear system at each time step. We prove that as the time
and space steps tend to zero (with no further conditions when
$\theta\in(1/2,1]$), the finite element solutions converge weakly to a weak
solution of the MLLG equations. Numerical results are presented to show the
applicability of the method.
|
1212.3369v1
|
2013-09-28
|
Global Well-Posedness of the Landau-Lifshitz-Gilbert equation for initial data in Morrey space
|
We establish the global well-posedness of the Landau-Lifshitz-Gilbert
equation in $\mathbb R^n$ for any initial data ${\bf m}_0\in H^1_*(\mathbb
R^n,\mathbb S^2)$ whose gradient belongs to the Morrey space $M^{2,2}(\mathbb
R^n)$ with small norm $\displaystyle\|\nabla {\bf m}_0\|_{M^{2,2}(\mathbb
R^n)}$. The method is based on priori estimates of a dissipative Schr\"odinger
equation of Ginzburg-Landau types obtained from the Landau-Lifshitz-Gilbert
equation by the moving frame technique.
|
1309.7426v1
|
2016-10-26
|
Iterated Gilbert Mosaics and Poisson Tropical Plane Curves
|
We propose an iterated version of the Gilbert model, which results in a
sequence of random mosaics of the plane. We prove that under appropriate
scaling, this sequence of mosaics converges to that obtained by a classical
Poisson line process with explicit cylindrical measure. Our model arises from
considerations on tropical plane curves, which are zeros of random tropical
polynomials in two variables. In particular, the iterated Gilbert model
convergence allows one to derive a scaling limit for Poisson tropical plane
curves. Our work raises a number of open questions at the intersection of
stochastic and tropical geometry.
|
1610.08533v1
|
2017-05-29
|
Strong solvability of regularized stochastic Landau-Lifshitz-Gilbert equation
|
We examine a stochastic Landau-Lifshitz-Gilbert equation based on an exchange
energy functional containing second-order derivatives of the unknown field.
Such regularizations are featured in advanced micromagnetic models recently
introduced in connection with nanoscale topological solitons. We show that, in
contrast to the classical stochastic Landau-Lifshitz-Gilbert equation based on
the Dirichlet energy alone, the regularized equation is solvable in the
stochastically strong sense. As a consequence it preserves the topology of the
initial data, almost surely.
|
1705.10184v1
|
2021-04-03
|
Improving the Gilbert-Varshamov Bound by Graph Spectral Method
|
We improve Gilbert-Varshamov bound by graph spectral method. Gilbert graph
$G_{q,n,d}$ is a graph with all vectors in $\mathbb{F}_q^n$ as vertices where
two vertices are adjacent if their Hamming distance is less than $d$. In this
paper, we calculate the eigenvalues and eigenvectors of $G_{q,n,d}$ using the
properties of Cayley graph. The improved bound is associated with the minimum
eigenvalue of the graph. Finally we give an algorithm to calculate the bound
and linear codes which satisfy the bound.
|
2104.01403v3
|
2006-02-20
|
Fluctuation theorem applied to the Nosé-Hoover thermostated Lorentz gas
|
We present numerical evidence supporting the validity of the Gallavotti-Cohen
Fluctuation Theorem applied to the driven Lorentz gas with Nos\'e-Hoover
thermostating. It is moreover argued that the asymptotic form of the
fluctuation formula is independent of the amplitude of the driving force, in
the limit where it is small.
|
0602458v1
|
1997-12-28
|
Model-independent $\tan β$ bounds in the MSSM
|
We demonstrate, through the study of the one-loop effective potential in the
MSSM, the existence of fully model-independent lower and upper theoretical
bounds on $\tan \beta$. We give their general analytic form and illustrate some
of their implications.
|
9712529v1
|
2006-12-26
|
Very Light Gravitino Dark Matter
|
We address the question of dark matter in the context of gauge mediated
supersymmetry breaking models. In contrast with mSUGRA scenarios, the messenger
of the susy breaking to the visible sector can play an important role allowing
a relic gravitino in the $\sim {keV}$ to $10 {MeV}$ mass range to account for
the cold dark matter in the Universe.
|
0612331v1
|
2004-09-15
|
Characterizing rigid simplicial actions on trees
|
We extend Forester's rigidity theorem so as to give a complete
characterization of rigid group actions on trees (an action is rigid if it is
the only reduced action in its deformation space, in particular it is invariant
under automorphisms preserving the set of elliptic subgroups).
|
0409245v1
|
2005-12-14
|
Efficient Construction of Photonic Quantum Computational Clusters
|
We demonstrate a method of creating photonic two-dimensional cluster states
that is considerably more efficient than previously proposed approaches. Our
method uses only local unitaries and type-I fusion operations. The increased
efficiency of our method compared to previously proposed constructions is
obtained by identifying and exploiting local equivalence properties inherent in
cluster states.
|
0512110v1
|
2007-10-26
|
The dark matter as a light gravitino
|
We address the question of gravitino dark matter in the context of gauge
mediated supersymmetry breaking models. A special emphasis is put on the role
played by the MSSM singlet messenger in the case of SO(10) grand unification.
|
0710.5121v1
|
2008-03-05
|
Innovative Weak Formulation for The Landau-Lifshitz-Gilbert Equations
|
A non-conventional finite element formalism is proposed to solve the dynamic
Landau-Lifshitz-Gilbert micromagnetic equations. Two bidimensional test
problems are treated to estimate the validity and the accuracy of this finite
element approach
|
0803.0599v1
|
2008-10-27
|
The profile of bubbling solutions of a class of fourth order geometric equations on 4-manifolds
|
We study a class of fourth order geometric equations defined on a
4-dimensional compact Riemannian manifold which includes the Q-curvature
equation. We obtain sharp estimates on the difference near the blow-up points
between a bubbling sequence of solutions and the standard bubble.
|
0810.4879v1
|
2009-03-02
|
Asymptotic Improvement of the Binary Gilbert-Varshamov Bound on the Code Rate
|
We compute the code parameters for binary linear codes obtained by greedy
constructing the parity check matrix. Then we show that these codes improve the
Gilbert-Varshamov (GV) bound on the code size and rate. This result counter
proves the conjecture on the asymptotical exactness of the binary GV bound.
|
0903.0302v2
|
2009-11-16
|
The Independent Chip Model and Risk Aversion
|
We consider the Independent Chip Model (ICM) for expected value in poker
tournaments. Our first result is that participating in a fair bet with one
other player will always lower one's expected value under this model. Our
second result is that the expected value for players not participating in a
fair bet between two players always increases. We show that neither result
necessarily holds for a fair bet among three or more players.
|
0911.3100v1
|
2011-03-29
|
Statistical properties of $r$-adic processes and their connections to families of popular fractal curves
|
Results concerning the statists of $r$-adic processes and their fractal
properties are reviewed. The connection between singular eigenstates of the
statistical evolution of such processes and popular fractal curves is
emphasized.
|
1103.5683v1
|
2011-05-09
|
Global Solvability of the Cauchy Problem for the Landau-Lifshitz-Gilbert Equation in Higher Dimensions
|
We prove existence, uniqueness and asymptotics of global smooth solutions for
the Landau-Lifshitz-Gilbert equation in dimension $n \ge 3$, valid under a
smallness condition of initial gradients in the $L^n$ norm. The argument is
based on the method of moving frames that produces a covariant complex
Ginzburg-Landau equation, and a priori estimates that we obtain by the method
of weighted-in-time norms as introduced by Fujita and Kato.
|
1105.1597v1
|
2012-03-28
|
Fibonacci numbers in phyllotaxis : a simple model
|
A simple model is presented which explains the occurrence of high order
Fibonacci number parastichies in asteracae flowers by two distinct steps. First
low order parastichies result from the fact that a new floret, at its
appearance is repelled by two former ones, then, in order to accommodate for
the increase of the radius, parastichies numbers have to evolve and can do it
only by applying the Fibonacci recurrence formula.
|
1203.6257v1
|
2014-02-19
|
Ordered groupoids and the holomorph of an inverse semigroup
|
We present a construction for the holomorph of an inverse semigroup, derived
from the cartesian closed structure of the category of ordered groupoids. We
compare the holomorph with the monoid of mappings that preserve the ternary
heap operation on an inverse semigroup: for groups these two constructions
coincide. We present detailed calculations for semilattices of groups and for
the polycyclic monoids.
|
1402.4592v1
|
2014-11-01
|
Functorial Zeta Integrals
|
The functional equation for nonarchimedean Rankin-Selberg local Euler factors
was proved by Jacquet, Piatetski-Shapiro, and Shalika in 1983. In this
expository note we translate the original proof into the purely functorial
language of parabolic induction-restriction of Bernstein-Zelevinsky. This new
language gives a clearer presentation of the ideas, and works over arbitrary
fields with characteristic not equal to the residue characteristic.
|
1411.0148v1
|
2016-02-17
|
Dispersion and Scaling Law of Dynamic Hysteresis Based on the Landau-Lifshitz-Gilbert Model
|
Hysteresis dispersion under a varying external field Hex is investigated
through numerical simulations based on the Landau-Lifshitz-Gilbert (LLG)
equation, indicating the energy dissipation can be determined by W({\eta}) = A
(f, H0). A linear relation between area of hysteresis and magnitude of external
field is discovered. Evolution of hysteresis is also investigated under
oscillating external field.
|
1602.05375v1
|
2016-06-06
|
Proof of tightness of Varshamov - Gilbert bound for binary codes
|
We prove tightness of right logarithmic asymptotic of Varshamov- Gilbert
bound for linear binary codes We find general asymptotic coding bound for
linear codes
|
1606.01592v5
|
2017-02-21
|
Cohomology and extensions of ordered groupoids
|
We adapt and generalise results of Loganathan on the cohomology of inverse
semigroups to the cohomology of ordered groupoids. We then derive a five-term
exact sequence in cohomology from an extension of ordered groupoids, and show
that this sequence leads to a classification of extensions by a second
cohomology group. Our methods use structural ideas in cohomology as far as
possible, rather than computation with cocycles.
|
1702.06333v1
|
2017-05-11
|
Two Gilbert-Varshamov Type Existential Bounds for Asymmetric Quantum Error-Correcting Codes
|
In this note we report two versions of Gilbert-Varshamov type existential
bounds for asymmetric quantum error-correcting codes.
|
1705.04087v2
|
2017-09-21
|
Self-Dual Codes better than the Gilbert--Varshamov bound
|
We show that every self-orthogonal code over $\mathbb F_q$ of length $n$ can
be extended to a self-dual code, if there exists self-dual codes of length $n$.
Using a family of Galois towers of algebraic function fields we show that over
any nonprime field $\mathbb F_q$, with $q\geq 64$, except possibly $q=125$,
there are self-dual codes better than the asymptotic Gilbert--Varshamov bound.
|
1709.07221v1
|
2018-10-12
|
A convex approach to the Gilbert-Steiner problem
|
We describe a convex relaxation for the Gilbert-Steiner problem both in $R^d$
and on manifolds, extending the framework proposed in [9], and we discuss its
sharpness by means of calibration type arguments. The minimization of the
resulting problem is then tackled numerically and we present results for an
extensive set of examples. In particular we are able to address the Steiner
tree problem on surfaces.
|
1810.05417v1
|
2018-11-09
|
Finslerian metrics locally conformally $R$-Einstein
|
Let $R$ be the $hh$-curvature associated with the Chern connection or the
Cartan connection. Adopting the pulled-back tangent bundle approach to the
Finslerian Geometry, an intrinsic characterization of $R$-Einstein metrics is
given. Finslerian metrics which are locally conformally $R$-Einstein are
classified.
|
1811.04077v3
|
2019-02-05
|
Harmonic maps with prescribed singularities and applications in general relativity
|
This paper presents a general existence and uniqueness result for harmonic
maps with prescribed singularities into non-positively curved targets, and
surveys a number of applications to general relativity. It is based on a talk
delivered by the author at The 11th Mathematical Society of Japan Seasonal
Institute, The Role of Metrics in the Theory of Partial Differential Equations.
|
1902.01576v2
|
2020-03-13
|
3D Stochastic Landau-Lifshitz-Gilbert Equations coupled with Maxwell's Equations with full energy
|
We consider 3D stochastic Landau-Lifshitz-Gilbert equations coupled with the
Maxwell equations with the full energy. We have proved the existence and some
further regularities of the weak solution.
|
2003.06091v4
|
2021-12-09
|
Induced Semi-Riemannian structures on null submanifolds
|
In this paper, we induce a semi-Riemannian metric on the $r$-null
submanifold. We establish the links between the null geometry and basics
invariants of the associated semi-Riemannian geometry on $r$-null submanifold
and semi-Riemannian constructed from a semi-Riemannian ambient.
|
2112.07348v1
|
2022-04-12
|
How to design a network architecture using capacity planning
|
Building a network architecture must answer to organization needs, but also
to two major elements which are the need for dependability and performance. By
performance, we must understand the ability to meet an immediate need and the
ability to scale without reducing the performance of the whole as new elements
are added to the network infrastructure. This last point is covered by Capacity
Planning domain.
|
2204.05916v2
|
2022-07-31
|
Moduli of Representations of Skewed-Gentle Algebras
|
We prove irreducible components of moduli spaces of semistable
representations of skewed-gentle algebras, and more generally, clannish
algebras, are isomorphic to products of projective spaces. This is achieved by
showing irreducible components of varieties of representations of clannish
algebras can be viewed as irreducible components of skewed-gentle algebras,
which we show are always normal. The main theorem generalizes an analogous
result for moduli of representations of special biserial algebras proven by
Carroll-Chindris-Kinser-Weyman.
|
2208.00336v1
|
2022-08-01
|
iOCR: Informed Optical Character Recognition for Election Ballot Tallies
|
The purpose of this study is to explore the performance of Informed OCR or
iOCR. iOCR was developed with a spell correction algorithm to fix errors
introduced by conventional OCR for vote tabulation. The results found that the
iOCR system outperforms conventional OCR techniques.
|
2208.00865v1
|
2023-03-13
|
Adaptive mesh refinement for the Landau-Lifshitz-Gilbert equation
|
We propose a new adaptive algorithm for the approximation of the
Landau-Lifshitz-Gilbert equation via a higher-order tangent plane scheme. We
show that the adaptive approximation satisfies an energy inequality and
demonstrate numerically, that the adaptive algorithm outperforms uniform
approaches.
|
2303.07463v1
|
2023-05-08
|
Evaluation of the Gilbert-Varshamov Bound using Multivariate Analytic Combinatorics
|
Analytic combinatorics in several variables refers to a suite of tools that
provide sharp asymptotic estimates for certain combinatorial quantities. In
this paper, we apply these tools to determine the Gilbert-Varshamov (GV) bound
for the sticky insertion and the constrained-synthesis channel.
|
2305.04439v1
|
2023-12-11
|
Matrix Formulae and Skein Relations for Quasi-cluster Algebras
|
In this paper, we give matrix formulae for non-orientable surfaces that
provide the Laurent expansion for quasi-cluster variables, generalizing the
orientable surface matrix formulae by Musiker-Williams. We additionally use our
matrix formulas to prove the skein relations for the elements in the
quasi-cluster algebra associated to curves on the non-orientable surface.
|
2312.06148v1
|
2009-08-12
|
Linear Fractionally Damped Oscillator
|
In this paper the linearly damped oscillator equation is considered with the
damping term generalized to a Caputo fractional derivative. The order of the
derivative being considered is 0 less than or equal to nu which is less than or
equal to 1 . At the lower end, nu = 0, the equation represents an un-damped
oscillator and at the upper end, nu = 1, the ordinary linearly damped
oscillator equation is recovered. A solution is found analytically and a
comparison with the ordinary linearly damped oscillator is made. It is found
that there are nine distinct cases as opposed to the usual three for the
ordinary equation (damped, over-damped, and critically damped). For three of
these cases it is shown that the frequency of oscillation actually increases
with increasing damping order before eventually falling to the limiting value
given by the ordinary damped oscillator equation. For the other six cases the
behavior is as expected, the frequency of oscillation decreases with increasing
order of the derivative (damping term).
|
0908.1683v1
|
2004-05-06
|
On a theorem of Kac and Gilbert
|
We prove a general operator theoretic result that asserts that many
multiplicity two selfadjoint operators have simple singular spectrum.
|
0405110v1
|
2011-01-05
|
Beating the Gilbert-Varshamov Bound for Online Channels
|
In the online channel coding model, a sender wishes to communicate a message
to a receiver by transmitting a codeword x =(x_1,...,x_n) in {0,1}^n bit by bit
via a channel limited to at most pn corruptions. The channel is online in the
sense that at the ith step the channel decides whether to flip the ith bit or
not and its decision is based only on the bits transmitted so far, i.e.,
(x_1,...,x_i). This is in contrast to the classical adversarial channel in
which the corruption is chosen by a channel that has full knowledge on the sent
codeword x. The best known lower bound on the capacity of both the online
channel and the classical adversarial channel is the well-known
Gilbert-Varshamov bound. In this paper we prove a lower bound on the capacity
of the online channel which beats the Gilbert-Varshamov bound for any positive
p such that H(2p) < 0.5 (where H is the binary entropy function). To do so, we
prove that for any such p, a code chosen at random combined with the nearest
neighbor decoder achieves with high probability a rate strictly higher than the
Gilbert-Varshamov bound (for the online channel).
|
1101.1045v1
|
2014-11-25
|
From heavy-tailed Boolean models to scale-free Gilbert graphs
|
Define the scale-free Gilbert graph based on a Boolean model with
heavy-tailed radius distribution on the $d$-dimensional torus by connecting two
centers of balls by an edge if at least one of the balls contains the center of
the other. We investigate two asymptotic properties of this graph as the size
of the torus tends to infinity. First, we determine the tail index associated
with the asymptotic distribution of the sum of all power-weighted incoming and
outgoing edge lengths at a randomly chosen vertex. Second, we study the
behavior of chemical distances on scale-free Gilbert graphs and show the
existence of different regimes depending on the tail index of the radius
distribution. Despite some similarities to long-range percolation and
ultra-small scale-free geometric networks, scale-free Gilbert graphs are
actually more closely related to fractal percolation and this connection gives
rise to different scaling limits. We also propose a modification of the graph,
where the total number of edges can be reduced substantially at the cost of
introducing a logarithmic factor in the chemical distances.
|
1411.6824v1
|
2016-03-16
|
Recent Results from SPLASH: Chemical Abundances and Kinematics of Andromeda's Stellar Halo
|
Large scale surveys of Andromeda's resolved stellar populations have
revolutionized our view of this galaxy over the past decade. The combination of
large-scale, contiguous photometric surveys and pointed spectroscopic surveys
has been particularly powerful for discovering substructure and disentangling
the structural components of Andromeda. The SPLASH (Spectroscopic and
Photometric Landscape of Andromeda's Stellar Halo) survey consists of broad-
and narrow-band imaging and spectroscopy of red giant branch stars in lines of
sight ranging in distance from 2 kpc to more than 200 kpc from Andromeda's
center. The SPLASH data reveal a power-law surface brightness profile extending
to at least two-thirds of Andromeda's virial radius (Gilbert et al. 2012), a
metallicity gradient extending to at least 100 kpc from Andromeda's center
(Gilbert et al. 2014), and evidence of a significant population of heated disk
stars in Andromeda's inner halo (Dorman et al. 2013). We are also using the
velocity distribution of halo stars to measure the tangential motion of
Andromeda (Beaton et al., in prep).
|
1603.05160v1
|
2017-05-09
|
Gilbert's disc model with geostatistical marking
|
We study a variant of Gilbert's disc model, in which discs are positioned at
the points of a Poisson process in $\mathbb{R}^2$ with radii determined by an
underlying stationary and ergodic random field
$\varphi:\mathbb{R}^2\to[0,\infty)$, independent of the Poisson process. When
the random field is independent of the point process one often talks about
'geostatistical marking'. We examine how typical properties of interest in
stochastic geometry and percolation theory, such as coverage probabilities and
the existence of long-range connections, differ between Gilbert's model with
radii given by some random field and Gilbert's model with radii assigned
independently, but with the same marginal distribution. Among our main
observations we find that complete coverage of $\mathbb{R}^2$ does not
necessarily happen simultaneously, and that the spatial dependence induced by
the random field may both increase as well as decrease the critical threshold
for percolation.
|
1705.03337v2
|
2019-01-30
|
Is the mailing Gilbert-Steiner problem convex?
|
A convexification of the mailing version of the finite Gilbert problem for
optimal networks is introduced. It is ia convex functional on the set of
probability measures subject to the Wasserstein $p-$ metric. The minimizer of
this convex functional is a measure supported in a graph. If this graph is a
tree (i.e contains no cycles) then this tree is also a minimum of the
corresponding mailing Gilbert problem. A numerical algorithm for the
implementation of the convexified Gilbert-mailing problem is also suggested,
based on entropic regularization.
|
1901.10924v4
|
2019-11-06
|
Phase transitions for chase-escape models on Gilbert graphs
|
We present results on phase transitions of local and global survival in a
two-species model on Gilbert graphs. At initial time there is an infection at
the origin that propagates on the Gilbert graph according to a continuous-time
nearest-neighbor interacting particle system. The Gilbert graph consists of
susceptible nodes and nodes of a second type, which we call white knights. The
infection can spread on susceptible nodes without restriction. If the infection
reaches a white knight, this white knight starts to spread on the set of
infected nodes according to the same mechanism, with a potentially different
rate, giving rise to a competition of chase and escape. We show
well-definedness of the model, isolate regimes of global survival and
extinction of the infection and present estimates on local survival. The proofs
rest on comparisons to the process on trees, percolation arguments and
finite-degree approximations of the underlying random graphs.
|
1911.02622v2
|
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