312 research outputs found
Ab initio study of the interface properties of Fe/GaAs(110)
We have investigated the initial growth of Fe on GaAs(110) by means of
density functional theory. In contrast to the conventionally used (001)-surface
the (110)-surface does not reconstruct. Therefore, a flat interface and small
diffusion can be expected, which makes Fe/GaAs(110) a possible candidate for
spintronic applications. Since experimentally, the actual quality of the
interface seems to depend on the growth conditions, e.g., on the flux rate, we
simulate the effect of different flux rates by different Fe coverages of the
semiconductor surface. Systems with low coverages are highly diffusive. With
increasing amount of Fe, i.e., higher flux rates, a flat interface becomes more
stable. The magnetic structure strongly depends on the Fe coverage but no
quenching of the magnetic moments is observed in our calculations.Comment: 9 pages, 8 figure
Modelling Electron Spin Accumulation in a Metallic Nanoparticle
A model describing spin-polarized current via discrete energy levels of a
metallic nanoparticle, which has strongly asymmetric tunnel contacts to two
ferromagnetic leads, is presented.
In absence of spin-relaxation, the model leads to a spin-accumulation in the
nanoparticle, a difference () between the chemical potentials of
spin-up and spin-down electrons, proportional to the current and the Julliere's
tunnel magnetoresistance. Taking into account an energy dependent
spin-relaxation rate , as a function of bias
voltage () exhibits a crossover from linear to a much weaker dependence,
when equals the spin-polarized current through the
nanoparticle. Assuming that the spin-relaxation takes place via electron-phonon
emission and Elliot-Yafet mechanism, the model leads to a crossover from linear
to dependence. The crossover explains recent measurements of the
saturation of the spin-polarized current with in Aluminum nanoparticles,
and leads to the spin-relaxation rate of in an Aluminum
nanoparticle of diameter , for a transition with an energy difference of
one level spacing.Comment: 37 pages, 7 figure
Canted Magnetization Texture in Ferromagnetic Tunnel Junctions
We study the formation of inhomogeneous magnetization texture in the vicinity
of a tunnel junction between two ferromagnetic wires nominally in the
antiparallel configuration and its influence on the magnetoresistance of such a
device. The texture, dependent on magnetization rigidity and crystalline
anisotropy energy in the ferromagnet, appears upon an increase of ferromagnetic
inter-wire coupling above a critical value and it varies with an external
magnetic field.Comment: 5 pages, 4 figure
Electronic Phase Separation in Manganite/Insulator Interfaces
By using a realist microscopic model, we study the electric and magnetic
properties of the interface between a half metallic manganite and an insulator.
We find that the lack of carriers at the interface debilitates the double
exchange mechanism, weakening the ferromagnetic coupling between the Mn ions.
In this situation the ferromagnetic order of the Mn spins near the interface is
unstable against antiferromagnetic CE correlations, and a separation between
ferromagnetic/metallic and antiferromagnetic/insulator phases at the interfaces
can occur. We obtain that the insertion of extra layers of undoped manganite at
the interface introduces extra carriers which reinforce the double exchange
mechanism and suppress antiferromagnetic instabilities.Comment: 8 pages, 7 figures include
Half-metallic ferromagnets for magnetic tunnel junctions
Using theoretical arguments, we show that, in order to exploit half-metallic
ferromagnets in tunneling magnetoresistance (TMR) junctions, it is crucial to
eliminate interface states at the Fermi level within the half-metallic gap;
contrary to this, no such problem arises in giant magnetoresistance elements.
Moreover, based on an a priori understanding of the electronic structure, we
propose an antiferromagnetically coupled TMR element, in which interface states
are eliminated, as a paradigm of materials design from first principles. Our
conclusions are supported by ab-initio calculations
Magnetic and orbital blocking in Ni nanocontacts
We address the fundamental question of whether magneto-resistance (MR) of
atomic-sized contacts of Nickel is very large because of the formation of a
domain wall (DW) at the neck. Using {\em ab initio} transport calculations we
find that, as in the case of non-magnetic electrodes, transport in Ni
nanocontacts depends very much on the orbital nature of the electrons. Our
results are in agreement with several experiments in the average value of the
conductance. On the other hand, contrary to existing claims, DW scattering does
{\em not} account for large MR in Ni nanocontacts.Comment: 5 pages, 3 Figure
Magnetization dependent current rectification in (Ga,Mn)As magnetic tunnel junctions
We have found that the current rectification effect in triple layer (double
barrier) (Ga,Mn)As magnetic tunnel junctions strongly depends on the
magnetization alignment. The direction as well as the amplitude of the
rectification changes with the alignment, which can be switched by
bi-directional spin-injection with very small threshold currents. A possible
origin of the rectification is energy dependence of the density of states
around the Fermi level. Tunneling density of states in (Ga,Mn)As shows
characteristic dip around zero-bias indicating formation of correlation gap,
the asymmetry of which would be a potential source of the energy dependent
density of states
Effects of different geometries on the conductance, shot noise and tunnel magnetoresistance of double quantum dots
The spin-polarized transport through a coherent strongly coupled double
quantum dot (DQD) system is analyzed theoretically in the sequential and
cotunneling regimes. Using the real-time diagrammatic technique, we analyze the
current, differential conductance, shot noise and tunnel magnetoresistance
(TMR) as a function of both the bias and gate voltages for double quantum dots
coupled in series, in parallel as well as for T-shaped systems. For DQDs
coupled in series, we find a strong dependence of the TMR on the number of
electrons occupying the double dot, and super-Poissonian shot noise in the
Coulomb blockade regime. In addition, for asymmetric DQDs, we analyze transport
in the Pauli spin blockade regime and explain the existence of the leakage
current in terms of cotunneling and spin-flip cotunneling-assisted sequential
tunneling. For DQDs coupled in parallel, we show that the transport
characteristics in the weak coupling regime are qualitatively similar to those
of DQDs coupled in series. On the other hand, in the case of T-shaped quantum
dots we predict a large super-Poissonian shot noise and TMR enhanced above the
Julliere value due to increased occupation of the decoupled quantum dot. We
also discuss the possibility of determining the geometry of the double dot from
transport characteristics. Furthermore, where possible, we compare our results
with existing experimental data on nonmagnetic systems and find qualitative
agreement.Comment: 15 pages, 12 figures, accepted in Phys. Rev.
Spin splitting and Kondo effect in quantum dots coupled to noncollinear ferromagnetic leads
We study the Kondo effect in a quantum dot coupled to two noncollinear
ferromagnetic leads. First, we study the spin splitting
of an energy level
in the quantum dot by tunnel couplings to the ferromagnetic leads, using the
Poor man's scaling method. The spin splitting takes place in an intermediate
direction between magnetic moments in the two leads. , where is the spin
polarization in the leads, is the angle between the magnetic moments,
and is an asymmetric factor of tunnel barriers (). Hence the spin
splitting is always maximal in the parallel alignment of two ferromagnets
() and minimal in the antiparallel alignment (). Second,
we calculate the Kondo temperature . The scaling calculation
yields an analytical expression of as a function of
and , , when .
is a decreasing function with respect to
. When is
relevant, we evaluate using the
slave-boson mean-field theory. The Kondo resonance is split into two by finite
, which results in the spin accumulation in the quantum dot and
suppression of the Kondo effect.Comment: 11 pages, 8 figures, revised versio
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