722 research outputs found
A model for vortex formation in magnetic nanodots
We use Monte Carlo simulation to study the vortex nucleation on magnetic
nanodots at low temperature. In our simulations, we have considered a simple
microscopic two-dimensional anisotropic Heisenberg model with term to describe
the anisotropy due to the presence of the nanodot edge. We have considered the
thickness of the edge, which was not considered in previous works, introducing
a term that controls the energy associated to the edge. Our results clearly
show that the thickness of the edge has a considerable influence in the vortex
nucleation on magnetic nanodots. We have obtained the hysteresis curve for
several values of the surface anisotropy and skin depth parameter (). The
results are in excellent agreement with experimental data
Estudo ab initio do sistema molecular HCO de interesse atmosférico.
Neste trabalho foram feitos cálculos ab initio de alto nível usando determinantes CASSCF para uma função MRCI e uma base ACV5Z-DK, juntamente com um ajuste preliminar de uma função de energia potencial a esses pontos. Até a data da defesa dessa dissertação foi possível realizar um estudo do mínimo global da superfície de energia potencial do HCO, mínimos locais e alguns estados de transição.
O objetivo final do trabalho aqui iniciado é a construção de uma superfície de energia potencial global para o estado fundamental da molécula HCO
Magnetic friction due to vortex fluctuation
We use Monte Carlo and molecular dynamics simulation to study a magnetic
tip-sample interaction. Our interest is to understand the mechanism of heat
dissipation when the forces involved in the system are magnetic in essence. We
consider a magnetic crystalline substrate composed of several layers
interacting magnetically with a tip. The set is put thermally in equilibrium at
temperature T by using a numerical Monte Carlo technique. By using that
configuration we study its dynamical evolution by integrating numerically the
equations of motion. Our results suggests that the heat dissipation in this
system is closed related to the appearing of vortices in the sample.Comment: 6 pages, 41 figure
Vortex behavior near a spin vacancy in 2D XY-magnets
The dynamical behavior of anisotropic two dimensional Heisenberg models is
still a matter of controversy. The existence of a central peak at all
temperatures and a rich structure of magnon peaks are not yet understood. It
seems that the central peaks are related, in some way, to structures like
vortices. In order to contribute to the discussion of the dynamical behavior of
the model we use Monte Carlo and spin dynamics simulations as well analytical
calculations to study the behavior of vortices in the presence of nonmagnetic
impurities. Our simulations show that vortices are attracted and trapped by the
impurities. Using this result we show that if we suppose that vortices are not
very much disturbed by the presence of the impurities, then they work as an
attractive potential to the vortices explaining the observed behavior in our
simulations.Comment: 4 pages, 6 figure
Monte Carlo study of the critical temperature for the planar rotator model with nonmagnetic impurities
We performed Monte Carlo simulations to calculate the
Berezinskii-Kosterlitz-Thouless (BKT) temperature for the
two-dimensional planar rotator model in the presence of nonmagnetic impurity
concentration . As expected, our calculation shows that the BKT
temperature decreases as the spin vacancies increase. There is a critical
dilution at which . The effective interaction
between a vortex-antivortex pair and a static nonmagnetic impurity is studied
analytically. A simple phenomenological argument based on the pair-impurity
interaction is proposed to justify the simulations.Comment: 5 pages, 5 figures, Revetex fil
Nonequilibrium molecular dynamics simulation of rapid directional solidification
We present the results of non-equilibrium molecular dynamics simulations for
the growth of a solid binary alloy from its liquid phase. The regime of high
pulling velocities, , for which there is a progressive transition from
solute segregation to solute trapping, is considered. In the segregation
regime, we recover the exponential form of the concentration profile within the
liquid phase. Solute trapping is shown to settle in progressively as is
increased and our results are in good agreement with the theoretical
predictions of Aziz [J. Appl. Phys. {\bf 53}, 1158 (1981)]. In addition, the
fluid advection velocity is shown to remain directly proportional to , even
at the highest velocities considered here (ms).Comment: Submitted to Phys. Rev.
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