1,659 research outputs found
Aspects of Defect Topology in Smectic Liquid Crystals
We study the topology of smectic defects in two and three dimensions. We give
a topological classification of smectic point defects and disclination lines in
three dimensions. In addition we describe the combination rules for smectic
point defects in two and three dimensions, showing how the broken translational
symmetry of the smectic confers a path dependence on the result of defect
addition.Comment: 19 pages, 13 figure
Magnetization Switching in Nanowires: Monte Carlo Study with Fast Fourier Transformation for Dipolar Fields
For the investigations of thermally activated magnetization reversal in
systems of classical magnetic moments numerical methods are desirable. We
present numerical studies which base on time quantified Monte Carlo methods
where the long-range dipole-dipole interaction is calculated with the aid of
fast Fourier transformation. As an example, we study models for ferromagnetic
nanowires comparing our numerical results for the characteristic time of the
reversal process also with numerical data from Langevin dynamics simulations
where the fast Fourier transformation method is well established. Depending on
the system geometry different reversal mechanism occur like coherent rotation,
nucleation, and curling.Comment: 7 pages, 5 figures, submitted to J. Magn. Magn. Ma
Integral Relaxation Time of Single-Domain Ferromagnetic Particles
The integral relaxation time \tau_{int} of thermoactivating noninteracting
single-domain ferromagnetic particles is calculated analytically in the
geometry with a magnetic field H applied parallel to the easy axis. It is shown
that the drastic deviation of \tau_{int}^{-1} from the lowest eigenvalue of the
Fokker-Planck equation \Lambda_1 at low temperatures, starting from some
critical value of H, is the consequence of the depletion of the upper potential
well. In these conditions the integral relaxation time consists of two
competing contributions corresponding to the overbarrier and intrawell
relaxation processes.Comment: 8 pages, 3 figure
Scaling relations for magnetic nanoparticles
A detailed investigation of the scaling relations recently proposed by [J.
d'Albuquerque e Castro, D. Altbir, J. C. Retamal, and P. Vargas, Phys. Rev.
Lett. 88, 237202 (2002)] to study the magnetic properties of nanoparticles is
presented. Analytical expressions for the total energy of three characteristic
internal configurations of the particles are obtained, in terms of which the
behavior of the magnetic phase diagram for those particles upon scaling of the
exchange interaction is discussed. The exponent in scaling relations is
shown to be dependent on the geometry of the vortex core, and results for
specific cases are presented.Comment: 6 pages, 4 figure
Stabilization of magnetic polarons in antiferromagnetic semiconductors by extended spin distortions
We study the problem of a magnetic polaron in an antiferromagnetic
semiconductor (ferron). We obtain an analytical solution for the distortion
produced in the magnetic structure of the d-spins due to the presence of a
charge carrier bound to an impurity. The region in which the charge carrier is
trapped is of the order of the lattice constant (small ferron) but the
distortion of the magnetic structure extends over much larger distance. It is
shown that the presence of this distortion makes the ferron more stable, and
introduces a new length scale in the problem.Comment: 5 pages, 1 figure, RevTex 4, submitted to PRB; v2: one reference
added, minor changes in the experiment discussion; v3: minor changes in tex
Magnetostatic bias in multilayer microwires: theory and experiments
The hysteresis curves of multilayer microwires consisting of a soft magnetic
nucleus, intermediate non-magnetic layers, and an external hard magnetic layer
are investigated. The magnetostatic interaction between magnetic layers is
proved to give rise to an antiferromagnetic-like coupling resulting in a
magnetostatic bias in the hysteresis curves of the soft nucleus. This
magnetostatic biasing effect is investigated in terms of the microwire
geometry. The experimental results are interpreted considering an analytical
model taking into account the magnetostatic interaction between the magnetic
layers.Comment: 6 pages, 7 figure
Magnetic properties of bi-phase micro- and nanotubes
The magnetic configurations of bi-phase micro- and nanotubes consisting of a
ferromagnetic internal tube, an intermediate non-magnetic spacer, and an
external magnetic shell are investigated as a function of their geometry. Based
on a continuum approach we obtained analytical expressions for the energy which
lead us to obtain phase diagrams giving the relative stability of
characteristic internal magnetic configurations of the bi-phase tubes
Magnetisation switching in a ferromagnetic Heisenberg nanoparticle with uniaxial anisotropy: A Monte Carlo investigation
We investigate the thermal activated magnetisation reversal in a single
ferromagnetic nanoparticle with uniaxial anisotropy using Monte Carlo
simulations. The aim of this work is to reproduce the reversal magnetisation by
uniform rotation at very low temperature in the high energy barrier hypothesis,
that is to realize the N\'eel-Brown model. For this purpose we have considered
a simple cubic nanoparticle where each site is occupied by a classical
Heisenberg spin. The Hamiltonian is the sum of an exchange interaction term, a
single-ion anisotropy term and a Zeeman interaction term. Our numerical data of
the thermal variation of the switching field are compared to an approximated
expression and previous experimental results on Co nanoparticles
Vortex core size in interacting cylindrical nanodot arrays
The effect of dipolar interactions among cylindrical nanodots, with a
vortex-core magnetic configuration, is analyzed by means of analytical
calculations. The cylinders are placed in a N x N square array in two
configurations - core oriented parallel to each other and with antiparallel
alignment between nearest neighbors. Results comprise the variation in the core
radius with the number of interacting dots, the distance between them and dot
height. The dipolar interdot coupling leads to a decrease (increase) of the
core radius for parallel (antiparallel) arrays
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