4,948 research outputs found
Transition temperature of ferromagnetic semiconductors: a dynamical mean field study
We formulate a theory of doped magnetic semiconductors such as
GaMnAs which have attracted recent attention for their possible use
in spintronic applications. We solve the theory in the dynamical mean field
approximation to find the magnetic transition temperature as a function
of magnetic coupling strength and carrier density . We find that
is determined by a subtle interplay between carrier density and magnetic
coupling.Comment: 4 pages, 4 figure
Polaron percolation in diluted magnetic semiconductors
We theoretically study the development of spontaneous magnetization in
diluted magnetic semiconductors as arising from a percolation of bound magnetic
polarons. Within the framework of a generalized percolation theory we derive
analytic expressions for the Curie temperature and the magnetization, obtaining
excellent quantitative agreement with Monte Carlo simulation results and good
qualitative agreement with experimental results.Comment: 5 page
Magnetic properties of the Ag-In-rare-earth 1/1 approximants
We have performed magnetic susceptibility and neutron scattering measurements
on polycrystalline Ag-In-RE (RE: rare-earth) 1/1 approximants. In the magnetic
susceptibility measurements, for most of the RE elements, inverse
susceptibility shows linear behaviour in a wide temperature range, confirming
well localized isotropic moments for the RE ions. Exceptionally for the
light RE elements, such as Ce and Pr, non-linear behaviour was observed,
possibly due to significant crystalline field splitting or valence fluctuation.
For RE = Tb, the susceptibility measurement clearly shows a bifurcation of the
field-cooled and zero-field-cooled susceptibility at ~K,
suggesting a spin-glass-like freezing. On the other hand, neutron scattering
measurements detect significant development of short-range antiferromagnetic
spin correlations in elastic channel, which accompanied by a broad peak at
~meV in inelastic scattering spectrum. These features have
striking similarity to those in the Zn-Mg-Tb quasicrystals, suggesting that the
short-range spin freezing behaviour is due to local high symmetry clusters
commonly seen in both the systems.Comment: 14 pages, 12 figure
Spin-polarized current amplification and spin injection in magnetic bipolar transistors
The magnetic bipolar transistor (MBT) is a bipolar junction transistor with
an equilibrium and nonequilibrium spin (magnetization) in the emitter, base, or
collector. The low-injection theory of spin-polarized transport through MBTs
and of a more general case of an array of magnetic {\it p-n} junctions is
developed and illustrated on several important cases. Two main physical
phenomena are discussed: electrical spin injection and spin control of current
amplification (magnetoamplification). It is shown that a source spin can be
injected from the emitter to the collector. If the base of an MBT has an
equilibrium magnetization, the spin can be injected from the base to the
collector by intrinsic spin injection. The resulting spin accumulation in the
collector is proportional to , where is the proton
charge, is the bias in the emitter-base junction, and is the
thermal energy. To control the electrical current through MBTs both the
equilibrium and the nonequilibrium spin can be employed. The equilibrium spin
controls the magnitude of the equilibrium electron and hole densities, thereby
controlling the currents. Increasing the equilibrium spin polarization of the
base (emitter) increases (decreases) the current amplification. If there is a
nonequilibrium spin in the emitter, and the base or the emitter has an
equilibrium spin, a spin-valve effect can lead to a giant magnetoamplification
effect, where the current amplifications for the parallel and antiparallel
orientations of the the equilibrium and nonequilibrium spins differ
significantly. The theory is elucidated using qualitative analyses and is
illustrated on an MBT example with generic materials parameters.Comment: 14 PRB-style pages, 10 figure
Solid-Liquid Phase Diagrams for Binary Metallic Alloys: Adjustable Interatomic Potentials
We develop a new approach to determining LJ-EAM potentials for alloys and use
these to determine the solid-liquid phase diagrams for binary metallic alloys
using Kofke's Gibbs-Duhem integration technique combined with semigrand
canonical Monte Carlo simulations. We demonstrate that it is possible to
produce a wide-range of experimentally observed binary phase diagrams (with no
intermetallic phases) by reference to the atomic sizes and cohesive energies of
the two elemental materials. In some cases, it is useful to employ a single
adjustable parameter to adjust the phase diagram (we provided a good choice for
this free parameter). Next, we perform a systematic investigation of the effect
of relative atomic sizes and cohesive energies of the elements on the binary
phase diagrams. We then show that this approach leads to good agreement with
several experimental binary phase diagrams. The main benefit of this approach
is not the accurately reproduction of experimental phase diagrams, but rather
to provide a method by which material properties can be continuously changed in
simulations studies. This is one of the keys to the use of atomistic
simulations to understand mechanisms and properties in a manner not available
to experiment
Saturated Ferromagnetism and Magnetization Deficit in Optimally Annealed (Ga,Mn)As Epilayers
We examine the Mn concentration dependence of the electronic and magnetic
properties of optimally annealed Ga1-xMnxAs epilayers for 1.35% < x < 8.3%. The
Curie temperature (Tc), conductivity, and exchange energy increase with Mn
concentration up to x ~ 0.05, but are almost constant for larger x, with Tc ~
110 K. The ferromagnetic moment per Mn ion decreases monotonically with
increasing x, implying that an increasing fraction of the Mn spins do not
participate in the ferromagnetism. By contrast, the derived domain wall
thickness, an important parameter for device design, remains surprisingly
constant.Comment: 8 pages, 4 figures, submitted for Rapid Communication in Phys Rev
Spin-polarized transport in inhomogeneous magnetic semiconductors: theory of magnetic/nonmagnetic p-n junctions
A theory of spin-polarized transport in inhomogeneous magnetic semiconductors
is developed and applied to magnetic/nonmagnetic p-n junctions. Several
phenomena with possible spintronic applications are predicted, including
spinvoltaic effect, spin valve effect, and giant magnetoresistance. It is
demonstrated that only nonequilibrium spin can be injected across the
space-charge region of a p-n junction, so that there is no spin injection (or
extraction) at low bias.Comment: Minor Revisions. To appear in Phys. Rev. Let
Spin Diode Based on Fe/MgO Double Tunnel Junction
We demonstrate a spin diode consisting of a semiconductor free nano-scale
Fe/MgO-based double tunnel junction. The device exhibits a near perfect
spin-valve effect combined with a strong diode effect. The mechanism consistent
with our data is resonant tunneling through discrete states in the middle
ferromagnetic layer sandwiched by tunnel barriers of different spin-dependent
transparency. The observed magneto-resistance is record high, ~4000%,
essentially making the structure an on/off spin-switch. This, combined with the
strong diode effect, ~100, offers a new device that should be promising for
such technologies as magnetic random access memory and re-programmable logic.Comment: 14 page
Noncollinear Ferromagnetism in (III,Mn)V Semiconductors
We investigate the stability of the collinear ferromagnetic state in kinetic
exchange models for (III,Mn)V semiconductors with randomly distributed Mn ions
>. Our results suggest that {\em noncollinear ferromagnetism} is commom to
these semiconductor systems. The instability of the collinear state is due to
long-ranged fluctuations invloving a large fraction of the localized magnetic
moments. We address conditions that favor the occurrence of noncollinear
groundstates and discuss unusual behavior that we predict for the temperature
and field dependence of its saturation magnetization.Comment: 5 pages, one figure included, presentation of technical aspects
simplified, version to appear in Phys. Rev. Let
Duplication-divergence model of protein interaction network
We show that the protein-protein interaction networks can be surprisingly
well described by a very simple evolution model of duplication and divergence.
The model exhibits a remarkably rich behavior depending on a single parameter,
the probability to retain a duplicated link during divergence. When this
parameter is large, the network growth is not self-averaging and an average
vertex degree increases algebraically. The lack of self-averaging results in a
great diversity of networks grown out of the same initial condition. For small
values of the link retention probability, the growth is self-averaging, the
average degree increases very slowly or tends to a constant, and a degree
distribution has a power-law tail.Comment: 8 pages, 13 figure
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