2,791 research outputs found
Magnitude and crystalline anisotropy of hole magnetization in (Ga,Mn)As
Theory of hole magnetization Mc in zinc-blende diluted ferromagnetic
semiconductors is developed relaxing the spherical approximation of earlier
approaches. The theory is employed to determine Mc for (Ga,Mn)As over a wide
range of hole concentrations and a number of crystallographic orientations of
Mn magnetization. It is found that anisotropy of Mc is practically negligible
but the obtained magnitude of Mc is significantly greater than that determined
in the spherical approximation. Its sign and value compares favorably with the
results of available magnetization measurements and ferromagnetic resonance
studies.Comment: 5 pages, 3 figure
Developing Universal Postal Services in Latin America – an Economic Perspective
The paper first analyzes if there is a need to develop universal postal services in developing countries. We conclude that postal services serve vital functions in economies now and for the foreseeable future. We then discuss regulatory remedies that will foster the evolution of universal postal services in developing countries.Developing Countries, Universal Postal Services
Anomalous Hall effect in field-effect structures of (Ga,Mn)As
The anomalous Hall effect in metal-insulator-semiconductor structures having
thin (Ga,Mn)As layers as a channel has been studied in a wide range of Mn and
hole densities changed by the gate electric field. Strong and unanticipated
temperature dependence, including a change of sign, of the anomalous Hall
conductance has been found in samples with the highest Curie
temperatures. For more disordered channels, the scaling relation between
and , similar to the one observed previously for
thicker samples, is recovered.Comment: 5 pages, 5 figure
Origin of ferromagnetism in (Zn,Co)O from magnetization and spin-dependent magnetoresistance
In order to elucidate the nature of ferromagnetic signatures observed in
(Zn,Co)O we have examined experimentally and theoretically magnetic properties
and spin-dependent quantum localization effects that control low-temperature
magnetoresistance. Our findings, together with a through structural
characterization, substantiate the model assigning spontaneous magnetization of
(Zn,Co)O to uncompensated spins at the surface of antiferromagnetic nanocrystal
of Co-rich wurtzite (Zn,Co)O. The model explains a large anisotropy observed in
both magnetization and magnetoresistance in terms of spin hamiltonian of Co
ions in the crystal field of the wurtzite lattice.Comment: 6 pages, 6 figure
Liberalization and Regulation of the Swiss Letter Market
The paper analyzes the impact of different regulatory models on competition and welfare in the Swiss letter market. We conclude that the US system worksharing yields the best results.Regulation, Liberalization, Universal Service, Worksharing
Domain-wall resistance in ferromagnetic (Ga,Mn)As
A series of microstructures designed to pin domain-walls (DWs) in (Ga,Mn)As
with perpendicular magnetic anisotropy has been employed to determine extrinsic
and intrinsic contributions to DW resistance. The former is explained
quantitatively as resulting from a polarity change in the Hall electric field
at DW. The latter is one order of magnitude greater than a term brought about
by anisotropic magnetoresistance and is shown to be consistent with
disorder-induced misstracing of the carrier spins subject to spatially varying
magnetization
Bound Magnetic Polaron Interactions in Insulating Doped Diluted Magnetic Semiconductors
The magnetic behavior of insulating doped diluted magnetic semiconductors
(DMS) is characterized by the interaction of large collective spins known as
bound magnetic polarons. Experimental measurements of the susceptibility of
these materials have suggested that the polaron-polaron interaction is
ferromagnetic, in contrast to the antiferromagnetic carrier-carrier
interactions that are characteristic of nonmagnetic semiconductors. To explain
this behavior, a model has been developed in which polarons interact via both
the standard direct carrier-carrier exchange interaction (due to virtual
carrier hopping) and an indirect carrier-ion-carrier exchange interaction (due
to the interactions of polarons with magnetic ions in an interstitial region).
Using a variational procedure, the optimal values of the model parameters were
determined as a function of temperature. At temperatures of interest, the
parameters describing polaron-polaron interactions were found to be nearly
temperature-independent. For reasonable values of these constant parameters, we
find that indirect ferromagnetic interactions can dominate the direct
antiferromagnetic interactions and cause the polarons to align. This result
supports the experimental evidence for ferromagnetism in insulating doped DMS.Comment: 11 pages, 7 figure
The enhancement of ferromagnetism in uniaxially stressed diluted magnetic semiconductors
We predict a new mechanism of enhancement of ferromagnetic phase transition
temperature in uniaxially stressed diluted magnetic semiconductors (DMS)
of p-type. Our prediction is based on comparative studies of both Heisenberg
(inherent to undistorted DMS with cubic lattice) and Ising (which can be
applied to strongly enough stressed DMS) models in a random field approximation
permitting to take into account the spatial inhomogeneity of spin-spin
interaction. Our calculations of phase diagrams show that area of parameters
for existence of DMS-ferromagnetism in Ising model is much larger than that in
Heisenberg model.Comment: Accepted for publication in Phys. Rev.
Prospect for room temperature tunneling anisotropic magnetoresistance effect: density of states anisotropies in CoPt systems
Tunneling anisotropic magnetoresistance (TAMR) effect, discovered recently in
(Ga,Mn)As ferromagnetic semiconductors, arises from spin-orbit coupling and
reflects the dependence of the tunneling density of states in a ferromagnetic
layer on orientation of the magnetic moment. Based on ab initio relativistic
calculations of the anisotropy in the density of states we predict sizable TAMR
effects in room-temperature metallic ferromagnets. This opens prospect for new
spintronic devices with a simpler geometry as these do not require
antiferromagnetically coupled contacts on either side of the tunnel junction.
We focus on several model systems ranging from simple hcp-Co to more complex
ferromagnetic structures with enhanced spin-orbit coupling, namely bulk and
thin film L1-CoPt ordered alloys and a monatomic-Co chain at a Pt surface
step edge. Reliability of the predicted density of states anisotropies is
confirmed by comparing quantitatively our ab initio results for the
magnetocrystalline anisotropies in these systems with experimental data.Comment: 4 pages, 2 figure
A new magnetic field dependence of Landau levels on a graphene like structure
We consider a tight-binding model on the honeycomb lattice in a magnetic
field. For special values of the hopping integrals, the dispersion relation is
linear in one direction and quadratic in the other. We find that, in this case,
the energy of the Landau levels varies with the field B as E_n(B) ~
[(n+\gamma)B]^{2/3}. This result is obtained from the low-field study of the
tight-binding spectrum on the honeycomb lattice in a magnetic field (Hofstadter
spectrum) as well as from a calculation in the continuum approximation at low
field. The latter links the new spectrum to the one of a modified quartic
oscillator. The obtained value is found to result from the
cancellation of a Berry phase.Comment: 4 pages, 4 figure
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