389 research outputs found
Nonlinear dispersion relation in anharmonic periodic mass-spring and mass-in-mass systems
The study of wave propagation in chains of anharmonic periodic systems is of
fundamental importance to understand the response of dynamical absorbers of
vibrations and acoustic metamaterials working in nonlinear regime. Here, we
derive an analytical nonlinear dispersion relation for periodic chains of
anharmonic mass-spring and mass-in-mass systems resulting from considering the
hypothesis of weak anharmonic energy and a periodic distribution function as
ansatz of a general solution of the nonlinear equations of motion. Numerical
simulations show that this expression is valid for anharmonic potential energy
up to 50% of the harmonic one. This work provides a simple tool to design and
study nonlinear dynamics for a class of seismic metamaterials.Comment: 18 pages, 5 figure
Micromagnetic simulations of persistent oscillatory modes excited by spin-polarized current in nanoscale exchange-biased spin valves
We perform 3D micromagnetic simulations of current-driven magnetization
dynamics in nanoscale exchange biased spin-valves that take account of (i) back
action of spin-transfer torque on the pinned layer, (ii) non-linear damping and
(iii) random thermal torques. Our simulations demonstrate that all these
factors significantly impact the current-driven dynamics and lead to a better
agreement between theoretical predictions and experimental results. In
particular, we observe that, at a non-zero temperature and a sub-critical
current, the magnetization dynamics exhibits nonstationary behaviour in which
two independent persistent oscillatory modes are excited which compete for the
angular momentum supplied by spin-polarized current. Our results show that this
multi-mode behaviour can be induced by combined action of thermal and spin
transfer torques.Comment: 7pages, 2 figures, submitted JAP via MMM 200
A strategy for the design of skyrmion racetrack memories
Magnetic storage based on racetrack memory is very promising for the design
of ultra-dense, low-cost and low-power storage technology. Information can be
coded in a magnetic region between two domain walls or, as predicted recently,
in topological magnetic objects known as skyrmions. Here, we show the
technological advantages and limitations of using Bloch and Neel skyrmions
manipulated by spin current generated within the ferromagnet or via the
spin-Hall effect arising from a non-magnetic heavy metal underlayer. We found
that the Neel skyrmion moved by the spin-Hall effect is a very promising
strategy for technological implementation of the next generation of skyrmion
racetrack memories (zero field, high thermal stability, and ultra-dense
storage). We employed micromagnetics reinforced with an analytical formulation
of skyrmion dynamics that we developed from the Thiele equation. We identified
that the excitation, at high currents, of a breathing mode of the skyrmion
limits the maximal velocity of the memory
Spin-torque driven magnetic vortex self-oscillations in perpendicular magnetic fields
We have employed complete micromagnetic simulations to analyze dc current
driven self-oscillations of a vortex core in a spin-valve nanopillar in a
perpendicular field by including the coupled effect of the spin-torque and the
magnetostatic field computed self-consistently for the entire spin-valve. The
vortex in the thicker nanomagnet moves along a quasi-elliptical trajectory that
expands with applied current, resulting in blue-shifting of the frequency,
while the magnetization of the thinner nanomagnet is non-uniform due to the
bias current. The simulations explain the experimental magnetoresistance-field
hysteresis loop and yield good agreement with the measured frequency vs.
current behavior of this spin-torque vortex oscillator.Comment: 10 pages, 3 figures, to be appear on AP
Magnetization switching driven by spin-transfer-torque in high-TMR magnetic tunnel junctions
This paper describes a numerical experiment of magnetization switching driven
by spin-polarized current in high-TMR magnetic tunnel junctions (TMR>100%).
Differently from other works, the current density distribution throughout the
cross-sectional area of the free-layer is here computed dinamically, by
modeling the ferromagnet/insulator/ferromagnet trilayer as a series of parallel
resistances. The validity of the main postulated hypothesis, which states that
the current density vector is perpendicular to the sample plane, has been
verified by numerically solving the Poisson equation. Our results show that the
nonuniform current density distribution is a source of asymmetry for the
switching. Furthermore, we found out that the switching processes are
characterized by well defined localized pre-switching oscillation modes.Comment: 19 pages and 8 figures, submmitted to JMM
Robust-to-outliers square-root LASSO, simultaneous inference with a MOM approach
We consider the least-squares regression problem with unknown noise variance,
where the observed data points are allowed to be corrupted by outliers.
Building on the median-of-means (MOM) method introduced by Lecue and Lerasle
Ann.Statist.48(2):906-931(April 2020) in the case of known noise variance, we
propose a general MOM approach for simultaneous inference of both the
regression function and the noise variance, requiring only an upper bound on
the noise level. Interestingly, this generalization requires care due to
regularity issues that are intrinsic to the underlying convex-concave
optimization problem. In the general case where the regression function belongs
to a convex class, we show that our simultaneous estimator achieves with high
probability the same convergence rates and a similar risk bound as if the noise
level was unknown, as well as convergence rates for the estimated noise
standard deviation.
In the high-dimensional sparse linear setting, our estimator yields a robust
analog of the square-root LASSO. Under weak moment conditions, it jointly
achieves with high probability the minimax rates of estimation for the -norm of the coefficient vector, and
the rate for the estimation of the noise standard
deviation. Here denotes the sample size, the dimension and the
sparsity level. We finally propose an extension to the case of unknown sparsity
level , providing a jointly adaptive estimator . It simultaneously estimates the coefficient
vector, the noise level and the sparsity level, with proven bounds on each of
these three components that hold with high probability.Comment: 70 page
Combined frequency-amplitude nonlinear modulation: theory and applications
In this work we formulate a generalized theoretical model to describe the
nonlinear dynamics observed in combined frequency-amplitude modulators whose
characteristic parameters exhibit a nonlinear dependence on the input
modulating signal. The derived analytical solution may give a satisfactory
explanation of recent laboratory observations on magnetic spin-transfer
oscillators and fully agrees with results of micromagnetic calculations. Since
the theory has been developed independently of the mechanism causing the
nonlinearities, it may encompass the description of modulation processes of any
physical nature, a promising feature for potential applications in the field of
communication systems.Comment: 8 pages, 4 figures, to be published on IEEE Transactions on Magnetic
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