644 research outputs found
Correlated defects, metal-insulator transition, and magnetic order in ferromagnetic semiconductors
The effect of disorder on transport and magnetization in ferromagnetic III-V
semiconductors, in particular (Ga,Mn)As, is studied theoretically. We show that
Coulomb-induced correlations of the defect positions are crucial for the
transport and magnetic properties of these highly compensated materials. We
employ Monte Carlo simulations to obtain the correlated defect distributions.
Exact diagonalization gives reasonable results for the spectrum of valence-band
holes and the metal-insulator transition only for correlated disorder. Finally,
we show that the mean-field magnetization also depends crucially on defect
correlations.Comment: 4 pages RevTeX4, 5 figures include
Self-compensation in manganese-doped ferromagnetic semiconductors
We present a theory of interstitial Mn in Mn-doped ferromagnetic
semiconductors. Using density-functional theory, we show that under the
non-equilibrium conditions of growth, interstitial Mn is easily formed near the
surface by a simple low-energy adsorption pathway. In GaAs, isolated
interstitial Mn is an electron donor, each compensating two substitutional Mn
acceptors. Within an impurity-band model, partial compensation promotes
ferromagnetic order below the metal-insulator transition, with the highest
Curie temperature occurring for 0.5 holes per substitutional Mn.Comment: 4 pages, 3 figures, to appear in Phys. Rev. Let
Positional Disorder, Spin-Orbit Coupling and Frustration in GaMnAs
We study the magnetic properties of metallic GaMnAs. We calculate the
effective RKKY interaction between Mn spins using several realistic models for
the valence band structure of GaAs. We also study the effect of positional
disorder of the Mn on the magnetic properties. We find that the interaction
between two Mn spins is anisotropic due to spin-orbit coupling within both the
so-called spherical approximation and in the more realistic six band model. The
spherical approximation strongly overestimates this anistropy, especially for
short distances between Mn ions. Using the obtained effective Hamiltonian we
carry out Monte Carlo simulations of finite and zero temperature magnetization
and find that, due to orientational frustration of the spins, non-collinear
states appear in both valence band approximations for disordered, uncorrelated
Mn impurities in the small concentration regime. Introducing correlations among
the substitutional Mn positions or increasing the Mn concentration leads to an
increase in the remnant magnetization at zero temperature and an almost fully
polarized ferromagnetic state.Comment: 17 Pages, 13 Figure
Reorientation Transition in Single-Domain (Ga,Mn)As
We demonstrate that the interplay of in-plane biaxial and uniaxial anisotropy
fields in (Ga,Mn)As results in a magnetization reorientation transition and an
anisotropic AC susceptibility which is fully consistent with a simple single
domain model. The uniaxial and biaxial anisotropy constants vary respectively
as the square and fourth power of the spontaneous magnetization across the
whole temperature range up to T_C. The weakening of the anisotropy at the
transition may be of technological importance for applications involving
thermally-assisted magnetization switching.Comment: 4 pages, 4 figure
Disorder, spin-orbit, and interaction effects in dilute
We derive an effective Hamiltonian for in
the dilute limit, where can be described in
terms of spin polarons hopping between the {\rm Mn} sites and coupled
to the local {\rm Mn} spins. We determine the parameters of our model from
microscopic calculations using both a variational method and an exact
diagonalization within the so-called spherical approximation. Our approach
treats the extremely large Coulomb interaction in a non-perturbative way, and
captures the effects of strong spin-orbit coupling and Mn positional disorder.
We study the effective Hamiltonian in a mean field and variational calculation,
including the effects of interactions between the holes at both zero and finite
temperature. We study the resulting magnetic properties, such as the
magnetization and spin disorder manifest in the generically non-collinear
magnetic state. We find a well formed impurity band fairly well separated from
the valence band up to for which finite size
scaling studies of the participation ratios indicate a localization transition,
even in the presence of strong on-site interactions, where is the fraction of magnetically active Mn. We study the
localization transition as a function of hole concentration, Mn positional
disorder, and interaction strength between the holes.Comment: 15 pages, 12 figure
Magnetic properties of nanosized diluted magnetic semiconductors with band splitting
The continual model of the nonuniform magnetism in thin films and wires of a
diluted magnetic semiconductor is considered with taking into account the
finite spin polarization of carriers responsible for the indirect interaction
of magnetic impurities (e.g. via RKKY mechanism). Spatial distributions (across
the film thickness or the wire radius) of the magnetizaton and carrier
concentrations of different spin orientations, as well as the temperature
dependence of the average magnetization are determined as the solution of the
nonlinear integral equation
Anomalous Hall effect in a two-dimensional electron gas with spin-orbit interaction
We discuss the mechanism of anomalous Hall effect related to the contribution
of electron states below the Fermi surface (induced by the Berry phase in
momentum space). Our main calculations are made within a model of
two-dimensional electron gas with spin-orbit interaction of the Rashba type,
taking into account the scattering from impurities. We demonstrate that such an
"intrinsic" mechanism can dominate but there is a competition with the
impurity-scattering mechanism, related to the contribution of states in the
vicinity of Fermi surface. We also show that the contribution to the Hall
conductivity from electron states close to the Fermi surface has the intrinsic
properties as well.Comment: 9 pages, 6 figure
Low voltage control of ferromagnetism in a semiconductor p-n junction
The concept of low-voltage depletion and accumulation of electron charge in
semiconductors, utilized in field-effect transistors (FETs), is one of the
cornerstones of current information processing technologies. Spintronics which
is based on manipulating the collective state of electron spins in a
ferromagnet provides complementary technologies for reading magnetic bits or
for the solid-state memories. The integration of these two distinct areas of
microelectronics in one physical element, with a potentially major impact on
the power consumption and scalability of future devices, requires to find
efficient means for controlling magnetization electrically. Current induced
magnetization switching phenomena represent a promising step towards this goal,
however, they relay on relatively large current densities. The direct approach
of controlling the magnetization by low-voltage charge depletion effects is
seemingly unfeasible as the two worlds of semiconductors and metal ferromagnets
are separated by many orders of magnitude in their typical carrier
concentrations. Here we demonstrate that this concept is viable by reporting
persistent magnetization switchings induced by short electrical pulses of a few
volts in an all-semiconductor, ferromagnetic p-n junction.Comment: 11 pages, 4 figure
Exploiting Locally Imposed Anisotropies in (Ga,Mn)As: a Non-volatile Memory Device
Progress in (Ga,Mn)As lithography has recently allowed us to realize
structures where unique magnetic anisotropy properties can be imposed locally
in various regions of a given device. We make use of this technology to
fabricate a device in which we study transport through a constriction
separating two regions whose magnetization direction differs by 90 degrees. We
find that the resistance of the constriction depends on the flow of the
magnetic field lines in the constriction region and demonstrate that such a
structure constitutes a non-volatile memory device
Systematic study of Mn-doping trends in optical properties of (Ga,Mn)As
We report on a systematic study of optical properties of (Ga,Mn)As epilayers
spanning the wide range of accessible substitutional Mn_Ga dopings. The growth
and post-growth annealing procedures were optimized for each nominal Mn doping
in order to obtain films which are as close as possible to uniform
uncompensated (Ga,Mn)As mixed crystals. We observe a broad maximum in the
mid-infrared absorption spectra whose position exhibits a prevailing blue-shift
for increasing Mn-doping. In the visible range, a peak in the magnetic circular
dichroism blue shifts with increasing Mn-doping. These observed trends confirm
that disorder-broadened valence band states provide a better one-particle
representation for the electronic structure of high-doped (Ga,Mn)As with
metallic conduction than an energy spectrum assuming the Fermi level pinned in
a narrow impurity band.Comment: 22 pages, 14 figure
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