16,237 research outputs found
Magnetization Switching in Small Ferromagnetic Particles: Nucleation and Coherent Rotation
The mechanisms of thermally activated magnetization switching in small
ferromagnetic particles driven by an external magnetic field are investigated.
For low uniaxial anisotropy the spins rotate coherently while for sufficiently
large uniaxial anisotropy they behave Ising-like, i.e. the switching then is
due to nucleation. The crossover from coherent rotation to nucleation is
studied for the classical three-dimensional Heisenberg model with uniaxial
anisotropy by Monte Carlo simulations. From the temperature dependence of the
metastable lifetime the energy barrier of a switching process can be
determined. For the case of infinite anisotropy we compare numerical results
from simulations of the Ising model with theoretical results for energy
barriers for both, single-droplet and multi-droplet nucleation. The simulated
barriers are in agreement with the theoretical predictions.Comment: 3 pages, Revtex, 4 Figures include
Exchange Bias driven by Dzyaloshinskii-Moriya interactions
The exchange bias effect in compensated IrMn3/Co(111) system is studied using
multiscale modeling from "ab initio" to atomistic calculations. We evaluate
numerically the out-of-plane hysteresis loops of the bi-layer for different
thickness of the ferromagnetic layer. The results show the existence of a
perpendicular exchange bias field and an enhancement of the coercivity of the
system. In order to elucidate the possible origin of the exchange bias, we
analyze the hysteresis loops of a selected bi-layer by tuning the different
contributions to the exchange interactions across the interface. Our results
indicate that the exchange bias is primarily induced by the
Dzyaloshinskii-Moriya interactions, while the coercivity is increased mainly
due to a spin-flop mechanism
Heat and work distributions for mixed Gauss-Cauchy process
We analyze energetics of a non-Gaussian process described by a stochastic
differential equation of the Langevin type. The process represents a
paradigmatic model of a nonequilibrium system subject to thermal fluctuations
and additional external noise, with both sources of perturbations considered as
additive and statistically independent forcings. We define thermodynamic
quantities for trajectories of the process and analyze contributions to
mechanical work and heat. As a working example we consider a particle subjected
to a drag force and two independent Levy white noises with stability indices
and . The fluctuations of dissipated energy (heat) and
distribution of work performed by the force acting on the system are addressed
by examining contributions of Cauchy fluctuations to either bath or external
force acting on the system
Role of temperature-dependent spin model parameters in ultra-fast magnetization dynamics
In the spirit of multi-scale modelling magnetization dynamics at elevated
temperature is often simulated in terms of a spin model where the model
parameters are derived from first principles. While these parameters are mostly
assumed temperature-independent and thermal properties arise from spin
fluctuations only, other scenarios are also possible. Choosing bcc Fe as an
example, we investigate the influence of different kinds of model assumptions
on ultra-fast spin dynamics, where following a femtosecond laser pulse a sample
is demagnetized due to a sudden rise of the electron temperature. While
different model assumptions do not affect the simulational results
qualitatively, their details do depend on the nature of the modelling.Comment: 8 pages, 6 figure
A Characterization of Deterministic Sampling Patterns for Low-Rank Matrix Completion
Low-rank matrix completion (LRMC) problems arise in a wide variety of
applications. Previous theory mainly provides conditions for completion under
missing-at-random samplings. This paper studies deterministic conditions for
completion. An incomplete matrix is finitely rank- completable
if there are at most finitely many rank- matrices that agree with all its
observed entries. Finite completability is the tipping point in LRMC, as a few
additional samples of a finitely completable matrix guarantee its unique
completability. The main contribution of this paper is a deterministic sampling
condition for finite completability. We use this to also derive deterministic
sampling conditions for unique completability that can be efficiently verified.
We also show that under uniform random sampling schemes, these conditions are
satisfied with high probability if entries per column are
observed. These findings have several implications on LRMC regarding lower
bounds, sample and computational complexity, the role of coherence, adaptive
settings and the validation of any completion algorithm. We complement our
theoretical results with experiments that support our findings and motivate
future analysis of uncharted sampling regimes.Comment: This update corrects an error in version 2 of this paper, where we
erroneously assumed that columns with more than r+1 observed entries would
yield multiple independent constraint
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Heat and mass transfer under an infant radiant warmer – Development of a numerical model
This is the post-print version of the final paper published in Medical Engineering & Physics. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2010 Elsevier B.V.The main objectives of this paper are to present a procedure of how to create and set up a model for the physical processes that take place within an infant radiant warmer and to validate that Computational Fluid Dynamics (CFD) can be used to resolve such problems. In this study, the results are obtained for a simplified model, both in terms of the geometry employed and the prescribed boundary conditions. The results were numerically verified in terms of the convergence history, monitor data and the physical correctness. This study shows that the physical situation is unsteady and the results tend to oscillate, almost periodically, around a mean value. The results presented in the paper are found to be in qualitative agreement with the experimental data. This gives us confidence that the techniques employed in this paper are appropriate and form the starting point for the inclusion of more realistic effects, e.g. real shape of the newborn and radiant lamp, heat generated inside the newborn, moisture transport, etc.European Union Marie Curie Fellowship programm
The Sample Complexity of Search over Multiple Populations
This paper studies the sample complexity of searching over multiple
populations. We consider a large number of populations, each corresponding to
either distribution P0 or P1. The goal of the search problem studied here is to
find one population corresponding to distribution P1 with as few samples as
possible. The main contribution is to quantify the number of samples needed to
correctly find one such population. We consider two general approaches:
non-adaptive sampling methods, which sample each population a predetermined
number of times until a population following P1 is found, and adaptive sampling
methods, which employ sequential sampling schemes for each population. We first
derive a lower bound on the number of samples required by any sampling scheme.
We then consider an adaptive procedure consisting of a series of sequential
probability ratio tests, and show it comes within a constant factor of the
lower bound. We give explicit expressions for this constant when samples of the
populations follow Gaussian and Bernoulli distributions. An alternative
adaptive scheme is discussed which does not require full knowledge of P1, and
comes within a constant factor of the optimal scheme. For comparison, a lower
bound on the sampling requirements of any non-adaptive scheme is presented.Comment: To appear, IEEE Transactions on Information Theor
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