961 research outputs found
Anisotropy-based mechanism for zigzag striped patterns in magnetic thin films
In this work we studied a two dimensional ferromagnetic system using Monte
Carlo simulations. Our model includes exchange and dipolar interactions, a
cubic anisotropy term, and uniaxial out-of-plane and in-plane ones. According
to the set of parameters chosen, the model including uniaxial out-of-plane
anisotropy has a ground-state which consists of a canted state with stripes of
opposite out-of-plane magnetization. When the cubic anisotropy is introduced
zigzag patterns appear in the stripes at fields close to the remanence. An
analysis of the anisotropy terms of the model shows that this configuration is
related to specific values of the ratio between the cubic and the effective
uniaxial anisotropy. The mechanism behind this effect is related to particular
features of the anisotropy's energy landscape, since a global minima transition
as a function of the applied field is required in the anisotropy terms. This
new mechanism for zigzags formation could be present in monocrystal
ferromagnetic thin films in a given range of thicknesses.Comment: 910 pages, 10 figure
Stability as a natural selection mechanism on interacting networks
Biological networks of interacting agents exhibit similar topological
properties for a wide range of scales, from cellular to ecological levels,
suggesting the existence of a common evolutionary origin. A general
evolutionary mechanism based on global stability has been proposed recently [J
I Perotti, O V Billoni, F A Tamarit, D R Chialvo, S A Cannas, Phys. Rev. Lett.
103, 108701 (2009)]. This mechanism is incorporated into a model of a growing
network of interacting agents in which each new agent's membership in the
network is determined by the agent's effect on the network's global stability.
We show that, out of this stability constraint, several topological properties
observed in biological networks emerge in a self organized manner. The
influence of the stability selection mechanism on the dynamics associated to
the resulting network is analyzed as well.Comment: 10 pages, 9 figure
Inverse transition in the two dimensional dipolar frustrated ferromagnet
We show that the mean field phase diagram of the dipolar frustrated
ferromagnet in an external field presents an inverse transition in the
field-temperature plane. The presence of this type of transition has recently
been observed experimentally in ultrathin films of Fe/Cu(001). We study a
coarse-grained model Hamiltonian in two dimensions. The model supports stripe
and bubble equilibrium phases, as well as the paramagnetic phase. At variance
with common expectations, already in a single mode approximation, the model
shows a sequence of paramagnetic-bubbles-stripes-paramagnetic phase transitions
upon lowering the temperature at fixed external field. Going beyond the single
mode approximation leads to the shrinking of the bubbles phase, which is
restricted to a small region near the zero field critical temperature. Monte
Carlo simulations results with a Heisenberg model are consistent with the mean
field results.Comment: 8 pages, 6 figure
The exchange bias phenomenon in uncompensated interfaces: Theory and Monte Carlo simulations
We performed Monte Carlo simulations in a bilayer system composed by two thin
films, one ferromagnetic (FM) and the other antiferromagnetic (AFM). Two
lattice structures for the films were considered: simple cubic (sc) and a body
center cubic (bcc). In both lattices structures we imposed an uncompensated
interfacial spin structure, in particular we emulated a FeF2-FM system in the
case of the (bcc) lattice. Our analysis focused on the incidence of the
interfacial strength interactions between the films J_eb and the effect of
thermal fluctuations on the bias field H_EB. We first performed Monte Carlo
simulations on a microscopic model based on classical Heisenberg spin
variables. To analyze the simulation results we also introduced a simplified
model that assumes coherent rotation of spins located on the same layer
parallel to the interface. We found that, depending on the AFM film anisotropy
to exchange ratio, the bias field is either controlled by the intrinsic pinning
of a domain wall parallel to the interface or by the stability of the first AFM
layer (quasi domain wall) near the interface.Comment: 18 pages, 11 figure
BRST quantization of quasi-symplectic manifolds and beyond
We consider a class of \textit{factorizable} Poisson brackets which includes
almost all reasonable Poisson structures. A particular case of the factorizable
brackets are those associated with symplectic Lie algebroids. The BRST theory
is applied to describe the geometry underlying these brackets as well as to
develop a deformation quantization procedure in this particular case. This can
be viewed as an extension of the Fedosov deformation quantization to a wide
class of \textit{irregular} Poisson structures. In a more general case, the
factorizable Poisson brackets are shown to be closely connected with the notion
of -algebroid. A simple description is suggested for the geometry underlying
the factorizable Poisson brackets basing on construction of an odd Poisson
algebra bundle equipped with an abelian connection. It is shown that the
zero-curvature condition for this connection generates all the structure
relations for the -algebroid as well as a generalization of the Yang-Baxter
equation for the symplectic structure.Comment: Journal version, references and comments added, style improve
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