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Scientific Report 2007-2009
Condensed matter physics and biophysics
C37. Quantum engineering and self-organization
in hybrid semiconductor/magnetic metal nanostructures:
perspectives for spintronics
Spin polarized electron transport is drawing increasing
attention. The discovery and exploitation of giant
magnetoresistance in magnetic multilayers was the first
remarkable achievement in spin electronics (spintronics).
The second breakthrough was the observation of
spin injection in structures containing layers of ferromagnetic
metal (FM), the so-called δ layers, separated
by a spacer of non-magnetic semiconductor (NS). Realization
of both phenomena in the same FM/NS structures
will lead to an integration of spintronics and conventional
semiconductor technology, opening the way
to wide-range applications. The optimization of materials
and geometry is crucial. Indeed, spin injection
from transition (Fe,Co,Mn) or rare-earth (Gd) magnetic
metals into common semiconductors (A IV =Si,Ge;
A III B V =GaAs,GaSb,GaN) is conditioned by the large
difference of carrier concentration on the FM and NS
sides, leading to a wide depletion region in the NS. The
formation of spin-polarized local states reduces spin injection,
even in an ideal contact, and the situation in
real structures is even worse, due to the roughness of
the interface. Various solutions were proposed to enhance
spin injection, including a variety of tunnel barriers
between FM and NS, or heavy doping of the surface
region on the NS side, strongly narrowing the depletion
region. Obviously, these methods have also significant
drawbacks. Tunneling through an insulating layer
may be the dominant transport mechanism only at low
temperature, while heavy doping increases the recombination
rate of injected carriers in the depletion region,
reducing their lifetime. The simplest FM/NS layered
system is a tunnel junction of two FM layers separated
by a NS spacer. A periodic chain of tunnel junctions
forms a multilayer [digital magnetic alloy (DMA)] with
complicated transport and magnetic properties. Obviously,
each FM/NS interface of the multilayer may be
treated as almost magnetically independent [1,2] only in
the case of a thick spacer. For a thin spacer, the different
interfaces have to be considered as correlated, so
the additional problem of their effective interaction and
competition between parallel or anti-parallel configuration
of their magnetic moments arises [3].
The promising way to avoid the above mentioned
problems and improve spin injection consists in using a
dilute magnetic semiconductor (DMS). Multiple studies
reveal that magnetism may not be the intrinsic property
of DMS, but rather arises from magnetic peculiarities
of (self-organized) clusters enriched in transition metals
contaminants containing, e.g., germanides or silicides
of the transition metal in the NS matrix, which appear
due to the phase separation [4]. Systems containing FM
nanoparticles or layers incorporated inside DMS matrix
Figure 1: Density of states of the majority- and minorityspin
two-dimensional bound-state bands formed in the vicinity
of a δ layer, below the band edge of the semiconductor
(located at ω > 0), illustrating a half-metallic behavior [1].
(FM/DMS nanocomposites or multilayers) are the most
promising candidates. Recently, a quite new approach to
spin polarized electron transport has been opened using
FM/NS and FM/DMS DMA, grown by molecular-beam
epitaxy. The structure of these systems consists of alternated
FM submonolayers built inside a NS matrix. Depending
on the FM coverage and on the growing regime,
an intra-submonolayer ferromagnetic ordering has been
found with the Curie point far above room temperature.
The study of DMA has just started and many problems,
solved already for conventional FM/NS structures, are
still under discussion for the DMA.
Figure 2: Structure of the domain wall in the case of antiferromagnetic
order of impurity spins near a δ layer [(x, y)
plane]. The ideal Néel structure for the sublattice magnetizations
α and β is only recovered far off the δ layer [2].
References
1. S. Caprara, et al., Europhys. Lett. 85, 27006 (2009).
2. V. N. Men’shov, et al., Phys. Rev. B 80, 035315 (2009).
3. V. N. Men’shov, et al., Phys. Rev. B 78, 024438 (2008).
4. S. Caprara, et al., Phys. Rev. B 79, 035202 (2009).
Authors
S. Caprara
http://theprestige.phys.uniroma1.it/clc/
Sapienza Università di Roma 90 Dipartimento di Fisica