540 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
Phase diagram as a function of temperature and magnetic field for magnetic semiconductors
Using an extension of the Nagaev model of phase separation (E.L. Nagaev, and
A.I. Podel'shchikov, Sov. Phys. JETP, 71 (1990) 1108), we calculate the phase
diagram for degenerate antiferromagnetic semiconductors in the T-H plane for
different current carrier densities. Both, wide-band semiconductors and
'double-exchange' materials, are investigated.Comment: 5 pages, 6 figures, RevTex, Accepted for publication in PR
Absence of magnetic ordering in NiGa_2S_4
Triangular-layered NiGa2S4, contrary to intuitive expectation, does not form
a noncollinear antiferromagnetic structure, as do isoelectronic NaCrO2 and
LiCrO2. Instead, the local magnetic moments remain disordered down to the
lowest measured temperature. To get more insight into this phenomenon, we have
performed first principles calculations of the first, second end third
neighbors exchange interactions, and found that the second neighbor exchange is
negligible, while the first and the third neighbor exchanges are comparable and
antiferromagnetic. Both are rapidly suppressed by the on-site Hubbard
repulsion.Comment: 4 pages, 4 figure
Stabilization of magnetic polarons in antiferromagnetic semiconductors by extended spin distortions
We study the problem of a magnetic polaron in an antiferromagnetic
semiconductor (ferron). We obtain an analytical solution for the distortion
produced in the magnetic structure of the d-spins due to the presence of a
charge carrier bound to an impurity. The region in which the charge carrier is
trapped is of the order of the lattice constant (small ferron) but the
distortion of the magnetic structure extends over much larger distance. It is
shown that the presence of this distortion makes the ferron more stable, and
introduces a new length scale in the problem.Comment: 5 pages, 1 figure, RevTex 4, submitted to PRB; v2: one reference
added, minor changes in the experiment discussion; v3: minor changes in tex
Magnetic polarons in doped 1D antiferromagnetic chain
The structure of magnetic polarons (ferrons) is studied for an 1D
antiferromagnetic chain doped by non-magnetic donor impurities. The conduction
electrons are assumed to be bound by the impurities. Such a chain can be
described as a set of ferrons at the antiferromagnetic background. We found
that two types of ferrons can exist in the system. The ground state of the
chain corresponds to the ferrons with the sizes of the order of the
localization length of the electron near the impurity. The ferrons of the
second type produce a more extended distortion of spins in the chain. They are
stable within a finite domain of the system parameters and can be treated as
excitations above the ground state. The ferrons in the excited states can
appear in pairs only. The energy of the excited states decreases with the
growth in density of impurities. This can be interpreted as a manifestation of
an attractive interaction between ferrons.Comment: 6 pages, 5 figures, RevTex4, submitted to PR
Current fluctuations in a spin filter with paramagnetic impurities
We analyze the frequency dependence of shot noise in a spin filter consisting
of a normal grain and ferromagnetic electrodes separated by tunnel barriers.
The source of frequency-dependent noise is random spin-flip electron scattering
that results from spin-orbit interaction and magnetic impurities. Though the
latter mechanism does not contribute to the average current, it contributes to
the noise and leads to its dispersion at frequencies of the order of the
Korringa relaxation rate. Under nonequilibrium conditions, this rate is
proportional to the applied bias , but parametrically smaller than
.Comment: 6 pages, 2 figure
Impurity Conduction and Magnetic Polarons in Antiferromagnetic Oxides
Low-temperature transport and magnetization measurements for the
antiferromagnets SrMnO(3) and CaMnO(3) identify an impurity band of mobile
states separated by energy E from electrons bound in Coulombic potentials. Very
weak electric fields are sufficient to excite bound electrons to the impurity
band, increasing the mobile carrier concentration by more than three orders of
magnitude. The data argue against the formation of self-trapped magnetic
polarons (MPs) predicted by theory, and rather imply that bound MPs become
stable only for kT<<E.Comment: 4 pp., 4 fig
Small-scale phase separation in doped anisotropic antiferromagnets
We analyze the possibility of the nanoscale phase separation manifesting
itself in the formation of ferromagnetic (FM) polarons (FM droplets) in the
general situation of doped anisotropic three- and two-dimensional
antiferromagnets. In these cases, we calculate the shape of the most
energetically favorable droplets. We show that the binding energy and the
volume of a FM droplet in the three-dimensional (3D) case depend only upon two
universal parameters and , where and are effective
antiferromagnetic (AFM) exchange and hopping integrals, respectively. In the
two-dimensional (2D) case, these parameters have the form and . The most favorable shape of a
ferromagnetic droplet corresponds to an ellipse in the 2D case and to an
ellipsoid in the 3D case.Comment: 6 pages, 1 figure, RevTe
The evolution with temperature of magnetic polaron state in an antiferromagnetic chain with impurities
The thermal behavior of a one-dimensional antiferromagnetic chain doped by
donor impurities was analyzed. The ground state of such a chain corresponds to
the formation of a set of ferromagnetically correlated regions localized near
impurities (bound magnetic polarons). At finite temperatures, the magnetic
structure of the chain was calculated simultaneously with the wave function of
a conduction electron bound by an impurity. The calculations were performed
using an approximate variational method and a Monte Carlo simulation. Both
these methods give similar results. The analysis of the temperature dependence
of correlation functions for neighboring local spins demonstrated that the
ferromagnetic correlations inside a magnetic polaron remain significant even
above the N\'eel temperature implying rather high stability of the
magnetic polaron state. In the case when the electron-impurity coupling energy
is not too high (for lower that the electron hopping integral ), the
magnetic polaron could be depinned from impurity retaining its magnetic
structure. Such a depinning occurs at temperatures of the order of . At
even higher temperatures () magnetic polarons disappear and the chain
becomes completely disordered.Comment: 17 pages, 5 figures, RevTe
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