37 research outputs found
Temperature dependence of surface magnetization in local-moment systems
We present a theory to study the temperature-dependent behavior of surface
states in a ferromagnetic semi-infinite crystal. Our approach is based on the
single-site approximation for the \emph{s-f} model. The effect of the
semi-infinite nature of the crystal is taken into account by a localized
perturbation method. Using the mean-field theory for the layer-dependent
magnetization, the local density of states and the electron-spin polarization
are investigated at different temperatures for ordinary and surface transition
cases. The results show that the surface magnetic properties may differ
strongly from those in the bulk and the coupling constant of atoms plays a
decisive role in the degree of spin polarization. In particular, for the case
in which the exchange coupling constant on the surface and between atoms in the
first and second layer is higher than the corresponding in the bulk, an
enhancement of surface Curie temperature and hence the spin polarization can be
obtained.Comment: 9 pages,8 figure
Tailoring ferromagnetic chalcopyrites
If magnetic semiconductors are ever to find wide application in real
spintronic devices, their magnetic and electronic properties will require
tailoring in much the same way that band gaps are engineered in conventional
semiconductors. Unfortunately, no systematic understanding yet exists of how,
or even whether, properties such as Curie temperatures and band gaps are
related in magnetic semiconductors. Here we explore theoretically these and
other relationships within 64 members of a single materials class, the Mn-doped
II-IV-V2 chalcopyrites, three of which are already known experimentally to be
ferromagnetic semiconductors. Our first-principles results reveal a variation
of magnetic properties across different materials that cannot be explained by
either of the two dominant models of ferromagnetism in semiconductors. Based on
our results for structural, electronic, and magnetic properties, we identify a
small number of new stable chalcopyrites with excellent prospects for
ferromagnetism.Comment: 6 pages with 4 figures, plus 3 supplementary figures; to appear in
Nature Material
Consistent model of magnetism in ferropnictides
The discovery of superconductivity in LaFeAsO introduced the ferropnictides
as a major new class of superconducting compounds with critical temperatures
second only to cuprates. The presence of magnetic iron makes ferropnictides
radically different from cuprates. Antiferromagnetism of the parent compounds
strongly suggests that superconductivity and magnetism are closely related.
However, the character of magnetic interactions and spin fluctuations in
ferropnictides, in spite of vigorous efforts, has until now resisted
understanding within any conventional model of magnetism. Here we show that the
most puzzling features can be naturally reconciled within a rather simple
effective spin model with biquadratic interactions, which is consistent with
electronic structure calculations. By going beyond the Heisenberg model, this
description explains numerous experimentally observed properties, including the
peculiarities of the spin wave spectrum, thin domain walls, crossover from
first to second order phase transition under doping in some compounds, and
offers new insight in the occurrence of the nematic phase above the
antiferromagnetic phase transition.Comment: 5 pages, 3 figures, revtex
All-optical manipulation and probing of the d-f exchange interaction in EuTe
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