532 research outputs found
Capillary Waves at Liquid/Vapor Interfaces: A Molecular Dynamics Simulation
Evidence for capillary waves at a liquid/vapor interface are presented from
extensive molecular dynamics simulations of a system containing up to 1.24
million Lennard-Jones particles. Careful measurements show that the total
interfacial width depends logarithmically on , the length of the
simulation cell parallel to the interface, as predicted theoretically. The
strength of the divergence of the interfacial width on depends
inversely on the surface tension . This allows us to measure
two ways since can also be obtained from the difference in the
pressure parallel and perpendicular to the interface. These two independent
measures of agree provided that the interfacial order parameter
profile is fit to an error function and not a hyperbolic tangent, as often
assumed. We explore why these two common fitting functions give different
results for
Hysteresis loop areas in kinetic Ising models: Effects of the switching mechanism
Experiments on ferromagnetic thin films have measured the dependence of the
hysteresis loop area on the amplitude and frequency of the external field,
=, and approximate agreement with numerical simulations of
Ising models has been reported. Here we present numerical and theoretical
calculations of in the low-frequency regime for two values of ,
which bracket a temperature and system-size dependent crossover field. Our
previous Monte Carlo studies have shown that the hysteretic response of the
kinetic Ising model is qualitatively different for amplitudes above and below
this crossover field. Using droplet theory, we derive analytic expressions for
the low-frequency asymptotic behavior of the hysteresis loop area. In both
field regimes, the loop area exhibits an extremely slow approach to an
asymptotic, logarithmic frequency dependence of the form . Our results are relevant to the interpretation of data
from experiments and simulations, on the basis of which power-law exponents for
the hysteresis-loop area have been reported.Comment: 9 pages including 3 figures. Submitted as a manuscript for the 7th
Joint MMM-Intermag conference. To be published in the Journal of Applied
Physics and the IEEE Transactions on Magnetics. Contains 1 updated figure and
revised tex
Kinetic Ising model in an oscillating field: Finite-size scaling at the dynamic phase transition
We study hysteresis for a two-dimensional, spin-1/2, nearest-neighbor,
kinetic Ising ferromagnet in an oscillating field, using Monte Carlo
simulations. The period-averaged magnetization is the order parameter for a
proposed dynamic phase transition (DPT). To quantify the nature of this
transition, we present the first finite-size scaling study of the DPT for this
model. Evidence of a diverging correlation length is given, and we provide
estimates of the transition frequency and the critical indices ,
and .Comment: Accepted by Physical Review Letters. 9 page
Nonequilibrium phase transition in the kinetic Ising model: Is transition point the maximum lossy point ?
The nonequilibrium dynamic phase transition, in the kinetic Ising model in
presence of an oscillating magnetic field, has been studied both by Monte Carlo
simulation (in two dimension) and by solving the meanfield dynamical equation
of motion for the average magnetization. The temperature variations of
hysteretic loss (loop area) and the dynamic correlation have been studied near
the transition point. The transition point has been identified as the
minimum-correlation point. The hysteretic loss becomes maximum above the
transition point. An analytical formulation has been developed to analyse the
simulation results. A general relationship among hysteresis loop area, dynamic
order parameter and dynamic correlation has also been developed.Comment: 8 pages Revtex and 4 Postscript figures; To appear in Phys. Rev.
Kinetic Ising System in an Oscillating External Field: Stochastic Resonance and Residence-Time Distributions
Experimental, analytical, and numerical results suggest that the mechanism by
which a uniaxial single-domain ferromagnet switches after sudden field reversal
depends on the field magnitude and the system size. Here we report new results
on how these distinct decay mechanisms influence hysteresis in a
two-dimensional nearest-neighbor kinetic Ising model. We present theoretical
predictions supported by numerical simulations for the frequency dependence of
the probability distributions for the hysteresis-loop area and the
period-averaged magnetization, and for the residence-time distributions. The
latter suggest evidence of stochastic resonance for small systems in moderately
weak oscillating fields.Comment: Includes updated results for Fig.2 and minor text revisions to the
abstract and text for clarit
Hysteresis and the dynamic phase transition in thin ferromagnetic films
Hysteresis and the non-equilibrium dynamic phase transition in thin magnetic
films subject to an oscillatory external field have been studied by Monte Carlo
simulation. The model under investigation is a classical Heisenberg spin system
with a bilinear exchange anisotropy in a planar thin film geometry with
competing surface fields. The film exhibits a non-equilibrium phase transition
between dynamically ordered and dynamically disordered phases characterized by
a critical temperature Tcd, whose location of is determined by the amplitude H0
and frequency w of the applied oscillatory field. In the presence of competing
surface fields the critical temperature of the ferromagnetic-paramagnetic
transition for the film is suppressed from the bulk system value, Tc, to the
interface localization-delocalization temperature Tci. The simulations show
that in general Tcd < Tci for the model film. The profile of the time-dependent
layer magnetization across the film shows that the dynamically ordered and
dynamically disordered phases coexist within the film for T < Tcd. In the
presence of competing surface fields, the dynamically ordered phase is
localized at one surface of the film.Comment: PDF file, 21 pages including 8 figure pages; added references,typos
added; to be published in PR
Spatial stochastic resonance in 1D Ising systems
The 1D Ising model is analytically studied in a spatially periodic and
oscillatory external magnetic field using the transfer-matrix method. For low
enough magnetic field intensities the correlation between the external magnetic
field and the response in magnetization presents a maximum for a given
temperature. The phenomenon can be interpreted as a resonance phenomenon
induced by the stochastic heatbath. This novel "spatial stochastic resonance"
has a different origin from the classical stochastic resonance phenomenon.Comment: REVTex, 5 pages, 3 figure
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