42 research outputs found
Vortex States of a Superconducting Film from a Magnetic Dot Array
Using Ginzburg-Landau theory, we find novel configurations of vortices in
superconducting thin films subject to the magnetic field of a magnetic dot
array, with dipole moments oriented perpendicular to the film. Sufficiently
strong magnets cause the formation of vortex-antivortex pairs. In most cases,
the vortices are confined to dot regions, while the antivortices can form a
rich variety of lattice states. We propose an experiment in which the
perpendicular component of the dot dipole moments can be tuned using an
in-plane magnetic field. We show that in such an experiment the
vortex-antivortex pair density shows broad plateaus as a function of the dipole
strength. Many of the plateaus correspond to vortex configurations which break
dot lattice symmetries. In some of these states, the vortex cores are strongly
distorted. Possible experimental consequences are mentioned.Comment: 4 pages, 4 figure
Critical behavior of diluted magnetic semiconductors: the apparent violation and the eventual restoration of the Harris criterion for all regimes of disorder
Using large-scale Monte Carlo calculations, we consider strongly disordered
Heisenberg models on a cubic lattice with missing sites (as in diluted magnetic
semiconductors such as Ga_{1-x}Mn_{x}As). For disorder ranging from weak to
strong levels of dilution, we identify Curie temperatures and calculate the
critical exponents nu, gamma, eta, and beta finding, per the Harris criterion,
good agreement with critical indices for the pure Heisenberg model where there
is no disorder component. Moreover, we find that thermodynamic quantities (e.g.
the second moment of the magnetization per spin) self average at the
ferromagnetic transition temperature with relative fluctuations tending to zero
with increasing system size. We directly calculate effective critical exponents
for T > T_{c}, yielding values which may differ significantly from the critical
indices for the pure system, especially in the presence of strong disorder.
Ultimately, the difference is only apparent, and eventually disappears when T
is very close to T_{c}.Comment: 11 pages, 9 figure
Magnetic Percolation and the Phase Diagram of the Disordered RKKY model
We consider ferromagnetism in spatially randomly located magnetic moments, as
in a diluted magnetic semiconductor, coupled via the carrier-mediated indirect
exchange RKKY interaction. We obtain via Monte Carlo the magnetic phase diagram
as a function of the impurity moment density and the relative carrier
concentration . As evidenced by the diverging correlation length
and magnetic susceptibility, the boundary between ferromagnetic (FM) and
non-ferromagnetic (NF) phases constitutes a line of zero temperature critical
points which can be viewed as a magnetic percolation transition. In the dilute
limit, we find that bulk ferromagnetism vanishes for . We also
incorporate the local antiferromagnetic direct superexchange interaction
between nearest neighbor impurities, and examine the impact of a damping factor
in the RKKY range function.Comment: 5 pages, 3 figures; figure formatting modified, typos fixe