|
|
|
Diluted
magnetic semiconductors (DMS) based on III-V alloys doped with
Mn have attracted a great interest recently, due to their relatively
high Curie temperatures Tc (150 K for Ga1-xMnxAs.), below which they exhibit
ferromagnetic order. In the ferromagnetic state, the charge carriers
are spin polarized, making these materials ideal sources of spin-polarized
currents.
|
GaAs. The
manganese atoms substitute randomly the Galium atoms.
|
Pressure-induced
ferromagnetism in (In,Mn)Sb DMS
M. Csotos,
G. Mihály, B. Jankó,
T. Wojtowicz, X.
Liu, , J.
K. Furdyna
Recent
advances in III1-xMnxV ferromagnetic semiconductors (for example
in Ga1-xMnxAs) have demonstrated that electrical control of
their spin properties can be used for manipulation and detection
of magnetic signals. The Mn2+ ions in these alloys provide magnetic
moments, and at the same time act as a source of valence-band
holes that mediate the Mn2+-Mn2+ interactions. This coupling
results in the ferromagnetic phase. In earlier work, it was
shown that the ferromagnetic state can be enhanced or suppressed
by varying the carrier density. Here we demonstrate that, by
using hydrostatic pressure to continuously tune the wavefunction
overlap, one can control the strength of ferromagnetic coupling
without any change in the carrier concentration. Tuning the
exchange coupling by this process increases the magnetization
spectacularly, and can even induce the ferromagnetic phase in
an initially paramagnetic alloy. These results may open new
directions for strain-engineering of nanodevices.
Hall
resistivity as a function of magnetic field taken at several
hydrostatic pressures.
The effect of hydrostatic pressure on the magnetic behaviour
of (In,Mn)Sb was studied in a pressure cell developed for electric
transport measurements. We assume that the pressure leads to
an isotropic reduction of the lattice constant, as the lattice
mismatch between the magnetic layer and the InSb buffer is small
(<0.05%), and the mismatch-induced strain relaxes on a length
scale (3-5 nm) that is much shorter than the sample thickness.
Measurement of the temperature dependence of the resistivity
is a simple and convenient way to detect the onset temperature
of the ferromagnetic ordering (TC). In addition, measurement
of the Hall resistivity RH provides the means for deducing the
field dependence of the magnetization M(B).
For details,
see Nature
Materials doi:10.1038/nmat1388
Theory
of Manganese-Manganese interaction in GaMnAs
P.
Redlinski, G.
Zarand, B. Jankó
We investigate the interaction of two Mn ions in the dilute
magnetic semiconductor GaMnAs using the variational envelope
wave function approach within the framework of six band model
of the valence band. We find that the effective interaction
between the Mn core spins at a typical separation d is strongly
anisotropic for active Mn concentrations less than x = 1.3%,
but it is almost isotropic for shorter distances (d < 13A).
As a result, in unannealed and strongly compensated samples
strong frustration effects must be present. We also verify that
an effective Hamiltonian description can be used in the dilute
limit, x < 1.3%, and extract the parameters of this effective
Hamiltonian.
Energies
of the eight lowest lying states of the Mn dimer vs. the angle
of an effective model (lines) and full calculations (circles)
at R=19 Angstroms . The upper panel corresponds to exchange
configuration while the lower panel shows results for the'anisotropy
configuration'.
For details,
see condmat/0505038
Scaling
theory of magneto-resistance in disordered local moment ferromagnets
Gergely
Zarand, Catalin Pascu Moca,
Boldizsar Jankó
We present
a scaling theory of magneto-transport in Anderson-localized
disordered ferromagnets. Within our framework a pronounced magnetic-field-sensitive
resistance peak emerges naturally for temperatures near the
magnetic phase transition. We find that the resistance anomaly
is a direct consequence of the change in localization length
caused by the magnetic transition. For increasing values of
the external magnetic field, the resistance peak is gradually
depleted and pushed towards higher temperatures. Our results
are in good agreement with magneto-resistance measurements on
a variety of disordered magnets.
Resistivity
for the localized phase. The resistivity curves were computed
deep in the localized phase . A peak appears at T_c due to
the interplay of magnetic ordering and localization, and is
shifted to higher temperatures upon application of magnetic
field.
For details,
see condmat/0410003
(to be published in PRL)
Positional
Disorder, Spin-Orbit Coupling and Frustration in GaMnAs
Gregory
A. Fiete, Gergely
Zarand, Boldizsar
Jankó, Pawel
Redlinski, C. Pascu Moca
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.
For details,
see
PRB 71, 115202 (2005)
|
|
|
|
Anomalous
behavior of spin wave resonances in Ga1-xMnxAs
thin films
T. G. Rappoport,
P. Redlínski ,X.
Liu, G. Zaránd,
J.
K. Furdyna, B. Jankó
We
did, in a close collaboration with the experimental group of Professor
Furdyna, a theoretical effort in understanding
the magnetic properties of the most prominent diluted magnetic semiconductor,
the Ga1-xMnxAs.
Given the fact that DMSs are synthesized in
film form by using molecular-beam epitaxy
(MBE), ferromagnetic resonance spectroscopy (FMR) is the most suitable
experimental probe for studying the dynamics of the ferromagnetic order
parameter, which also allows for the spectroscopy of the spin wave excitations.
We observed such spin wave resonance in Ga1-xMnxAs.
These experiments provided a direct proof for true long-ranged ferromagnetic
in Ga1-xMnxAs. Surprisingly, the spin waves
which were observed exhibit a somewhat unusual behavior: spin wave dispersion
which is linear in mode number, in qualitative contrast
with the quadratic dispersion expected for homogeneous samples. We
performed a detailed numerical analysis of the experimental data and
provide analytical calculations to demonstrate that such a linear dispersion
is incompatible with uniform magnetic parameters. Our theoretical analysis
of the ferromagnetic resonance data, combined with the knowledge that
strain-induced anisotropy is definitely present in these films, suggests
that a spatially dependent magnetic anisotropy is the most likely reason
behind the anomalous behavior observed.
Since the variation of elastic and/or magnetic properties across
the Ga1-xMnxAs film can have important consequences
in its future spintronics applications, these
observations are fundamental for the improvement in the fabrication
of the magnetic semiconductors and their hybrids.
Schematic
diagram of the experimental geometry. The Ga1-xMnxAs
film is grown on a thick GaAs substrate. A large constant magnetic
field is applied in the z direction: H0 || z. This field
produces a large magnetization M(z) which points predominantly in
the z direction. Additionally, a small external microwave magnetic
field h perpendicular to H0 produces a small perturbation
in magnetization m z .
Comparision
of the experimental data for the SWR and the theoretical calculations
for two different boundary conditions for the spin waves.
For
details, see PRB
69, 125213 (2004)
Ga1-xMnxAs:
A Frustrated Ferromagnet
Gergely
Zaránd and Boldizsár
Jankó
We
have calculated the detailed structure of the ferromagnetic ground state
in the DMS side of our system, by taking into account the inherent orientational
frustration present in these materials. Starting from a microscopic
description of the exchange interaction in Ga1-xMnxAs,
we derived an effective Mn-Mn interaction. Because of the strong spin-orbit coupling
in the valence band, this effective interaction is highly anisotropic
and has a spatial structure somewhat similar to dipolar interactions.
We found an intrinsic frustration in the system that persists all the
way down to zero temperature. The corresponding ground state has a finite
magnetization but is intrinsically spin disordered even at zero temperature.
This is illustrated by our Monte Carlo simulation
of a spin system with such anisotropy, shown in Fig. 4. The main figure
shows the probability density function of the spin orientation at zero
temperature. The peak near cos q = 1 indicates
that there still is a tendency for some fraction of the spins to point
in the same direction as the magnetization, as one would expect from
a ferromagnet. However,
there are many other spins whose orientation is very different from
that of the net magnetization! Surprisingly, we even find a finite number
of spins that are antiparallel to the magnetization, clearly indicating
the presence of frustration in the system. The numerically measured
magnetization (inset) qualitatively reproduces the magnetization measurements
by a variety of experimental groups. The main features, such as an almost
linear rise in magnetization near Tc,
as well as the absence of saturation at low temperature, are well reproduced.
Probability density function of the spin orientation at
zero temperature. Inset: Numerically calculated magnetization as
a function of temperature.
For details, Phys.
Rev. Lett. 89, 047201 (2002)
|
|
