62 research outputs found
On the absence of conduction electrons in the antiferromagnetic part of the phase-separated states in magnetic semiconductors
We have calculated the energies of the phase-separated states for degenerate
antiferromagnetic semiconductors including the possibility of the existence of
conduction electrons in the antiferromagnetic part of the phase-separated
states. It is demonstrated that, at T=0, the minimum energy corresponds to a
droplet phase with absence of electrons in the antiferromagnetic part.Comment: 13 pages, 4 figure
Electronic structure of VO: charge ordering, metal-insulator transition and magnetism
The low and high-temperature phases of VO have been studied by
\textit{ab initio} calculations. At high temperature, all V atoms are
electronically equivalent and the material is metallic. Charge and orbital
ordering, associated with the distortions in the V pseudo-rutile chains, occur
below the metal-insulator transition. Orbital ordering in the low-temperature
phase, different in V and V chains, allows to explain the
distortion pattern in the insulating phase of VO. The in-chain magnetic
couplings in the low-temperature phase turn out to be antiferromagnetic, but
very different in the various V and V bonds. The V dimers
formed below the transition temperature form spin singlets, but V ions,
despite dimerization, apparently participate in magnetic ordering.Comment: 10 pages, 6 figures, 2 table
Nonmonotonic Evolution of the Blocking Temperature in Dispersions of Superparamagnetic Nanoparticles
We use a Monte Carlo approach to simulate the influence of the dipolar
interaction on assemblies of monodisperse superparamagnetic
nanoparticles. We have identified a critical
concentration c*, that marks the transition between two different regimes in
the evolution of the blocking temperature () with interparticle
interactions. At low concentrations (c < c*) magnetic particles behave as an
ideal non-interacting system with a constant . At concentrations c > c*
the dipolar energy enhances the anisotropic energy barrier and
increases with increasing c, so that a larger temperature is required to reach
the superparamagnetic state. The fitting of our results with classical particle
models and experiments supports the existence of two differentiated regimes.
Our data could help to understand apparently contradictory results from the
literature.Comment: 13 pages, 7 figure
Homopolar bond formation in ZnVO close to a metal-insulator transition
Electronic structure calculations for spinel vanadate ZnVO show that
partial electronic delocalization in this system leads to structural
instabilities. These are a consequence of the proximity to the
itinerant-electron boundary, not being related to orbital ordering. We discuss
how this mechanism naturally couples charge and lattice degrees of freedom in
magnetic insulators close to such a crossover. For the case of ZnVO,
this leads to the formation of V-V dimers along the [011] and [101] directions
that readily accounts for the intriguing magnetic structure of ZnVO.Comment: 5 pages, 3 figures, 1 tabl
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