22 research outputs found
Spin Nomenclature for Semiconductors and Magnetic Metals
The different conventions used in the semiconductor and magnetic metals
communities can cause confusion in the context of spin polarization and
transport in simple heterostructures. In semiconductors, terminology is based
on the orientation of the electron spin, while in magnetic metals it is based
on the orientation of the moment. In the rapidly expanding field of
spintronics, where both semiconductors and metallic metals are important, some
commonly used terms ("spin-up," "majority spin") can have different meanings.
Here, we clarify nomenclature relevant to spin transport and optical
polarization by relating the common physical observables and "definitions" of
spin polarization to the fundamental concept of conservation of angular
momentum within a well-defined reference frame.Comment: 3 pages, 2 figures, 16 reference
Electron Spin Injection at a Schottky Contact
We investigate theoretically electrical spin injection at a Schottky contact
between a spin-polarized electrode and a non-magnetic semiconductor. Current
and electron density spin-polarizations are discussed as functions of barrier
energy and semiconductor doping density. The effect of a spin-dependent
interface resistance that results from a tunneling region at the
contact/semiconductor interface is described. The model can serve as a guide
for designing spin-injection experiments with regard to the interface
properties and device structure.Comment: 4 pages, 4 figure
Optoelectric spin injection in semiconductor heterostructures without ferromagnet
We have shown that electron spin density can be generated by a dc current
flowing across a junction with an embedded asymmetric quantum well. Spin
polarization is created in the quantum well by radiative electron-hole
recombination when the conduction electron momentum distribution is shifted
with respect to the momentum distribution of holes in the spin split valence
subbands. Spin current appears when the spin polarization is injected from the
quantum well into the -doped region of the junction. The accompanied
emission of circularly polarized light from the quantum well can serve as a
spin polarization detector.Comment: 2 figure
Spin-Polarized Electron Transport at Ferromagnet/Semiconductor Schottky Contacts
We theoretically investigate electron spin injection and spin-polarization
sensitive current detection at Schottky contacts between a ferromagnetic metal
and an n-type or p-type semiconductor. We use spin-dependent continuity
equations and transport equations at the drift-diffusion level of
approximation. Spin-polarized electron current and density in the semiconductor
are described for four scenarios corresponding to the injection or the
collection of spin polarized electrons at Schottky contacts to n-type or p-type
semiconductors. The transport properties of the interface are described by a
spin-dependent interface resistance, resulting from an interfacial tunneling
region. The spin-dependent interface resistance is crucial for achieving spin
injection or spin polarization sensitivity in these configurations. We find
that the depletion region resulting from Schottky barrier formation at a
metal/semiconductor interface is detrimental to both spin injection and spin
detection. However, the depletion region can be tailored using a doping density
profile to minimize these deleterious effects. For example, a heavily doped
region near the interface, such as a delta-doped layer, can be used to form a
sharp potential profile through which electrons tunnel to reduce the effective
Schottky energy barrier that determines the magnitude of the depletion region.
The model results indicate that efficient spin-injection and spin-polarization
detection can be achieved in properly designed structures and can serve as a
guide for the structure design.Comment: RevTex
Electric-field dependent spin diffusion and spin injection into semiconductors
We derive a drift-diffusion equation for spin polarization in semiconductors
by consistently taking into account electric-field effects and nondegenerate
electron statistics. We identify a high-field diffusive regime which has no
analogue in metals. In this regime there are two distinct spin diffusion
lengths. Furthermore, spin injection from a ferromagnetic metal into a
semiconductor is enhanced by several orders of magnitude and spins can be
transported over distances much greater than the low-field spin diffusion
length.Comment: 5 pages, 3 eps figure
Spin-polarized Tunneling in Hybrid Metal-Semiconductor Magnetic Tunnel Junctions
We demonstrate efficient spin-polarized tunneling between a ferromagnetic
metal and a ferromagnetic semiconductor with highly mismatched conductivities.
This is indicated by a large tunneling magnetoresistance (up to 30%) at low
temperatures in epitaxial magnetic tunnel junctions composed of a ferromagnetic
metal (MnAs) and a ferromagnetic semiconductor (GaMnAs) separated by a
nonmagnetic semiconductor (AlAs). Analysis of the current-voltage
characteristics yields detailed information about the asymmetric tunnel
barrier. The low temperature conductance-voltage characteristics show a zero
bias anomaly and a V^1/2 dependence of the conductance, indicating a
correlation gap in the density of states of GaMnAs. These experiments suggest
that MnAs/AlAs heterostructures offer well characterized tunnel junctions for
high efficiency spin injection into GaAs.Comment: 14 pages, submitted to Phys. Rev.
First-principles study of nucleation, growth, and interface structure of Fe/GaAs
We use density-functional theory to describe the initial stages of Fe film
growth on GaAs(001), focusing on the interplay between chemistry and magnetism
at the interface. Four features appear to be generic: (1) At submonolayer
coverages, a strong chemical interaction between Fe and substrate atoms leads
to substitutional adsorption and intermixing. (2) For films of several
monolayers and more, atomically abrupt interfaces are energetically favored.
(3) For Fe films over a range of thicknesses, both Ga- and As-adlayers
dramatically reduce the formation energies of the films, suggesting a
surfactant-like action. (4) During the first few monolayers of growth, Ga or As
atoms are likely to be liberated from the interface and diffuse to the Fe film
surface. Magnetism plays an important auxiliary role for these processes, even
in the dilute limit of atomic adsorption. Most of the films exhibit
ferromagnetic order even at half-monolayer coverage, while certain
adlayer-capped films show a slight preference for antiferromagnetic order.Comment: 11 two-column pages, 12 figures, to appear in Phys. Rev.
Spin oscillations in transient diffusion of a spin pulse in n-type semiconductor quantum wells
By studying the time and spatial evolution of a pulse of the spin
polarization in -type semiconductor quantum wells, we highlight the
importance of the off-diagonal spin coherence in spin diffusion and transport.
Spin oscillations and spin polarization reverse along the the direction of spin
diffusion in the absence of the applied magnetic field are predicted from our
investigation.Comment: 5 pages, 4 figures, accepted for publication in PR
Coherent spin valve phenomena and electrical spin injection in ferromagnetic/semiconductor/ferromagnetic junctions
Coherent quantum transport in ferromagnetic/ semiconductor/ ferromagnetic
junctions is studied theoretically within the Landauer framework of ballistic
transport. We show that quantum coherence can have unexpected implications for
spin injection and that some intuitive spintronic concepts which are founded in
semi-classical physics no longer apply: A quantum spin-valve (QSV) effect
occurs even in the absence of a net spin polarized current flowing through the
device, unlike in the classical regime. The converse effect also arises, i.e. a
zero spin-valve signal for a non-vanishing spin-current. We introduce new
criteria useful for analyzing quantum and classical spin transport phenomena
and the relationships between them. The effects on QSV behavior of
spin-dependent electron transmission at the interfaces, interface Schottky
barriers, Rashba spin-orbit coupling and temperature, are systematically
investigated. While the signature of the QSV is found to be sensitive to
temperature, interestingly, that of its converse is not. We argue that the QSV
phenomenon can have important implications for the interpretation of
spin-injection in quantum spintronic experiments with spin-valve geometries.Comment: 15 pages including 11 figures. To appear in PR
Spin injection into a ballistic semiconductor microstructure
A theory of spin injection across a ballistic
ferromagnet-semiconductor-ferromagnet junction is developed for the Boltzmann
regime. Spin injection coefficient is suppressed by the Sharvin
resistance of the semiconductor , where is the
Fermi-surface cross-section. It competes with the diffusion resistances of the
ferromagnets , and in the absence of contact
barriers. Efficient spin injection can be ensured by contact barriers. Explicit
formulae for the junction resistance and the spin-valve effect are presented.Comment: 5 pages, 2 column REVTeX. Explicit prescription relating the results
of the ballistic and diffusive theories of spin injection is added. To this
end, some notations are changed. Three references added, typos correcte