Plenarvorträge - DPG-Tagungen

Halbleiterphysik Mittwoch
2D system. Obviously it is not possible to have a “second quantization
axis of angular momentum” on top of the perpendicular quantization
axis due to HH-LH splitting. We will thus discuss how spin splitting and
spin polarization in quasi 2D HH systems compete with HH-LH splitting.
We will compare the results of calculations and experiments showing the
depopulation of the upper spin subband in quasi 2D HH systems as a
function of an in-plane magnetic field. We will demonstrate theoretically
and experimentally that the shape of the confinement potential greatly
affects the spin subband depopulation field.
HL 29.3 Mi 15:45 H13
Strukturelle und Magnetische Eigenschaften von epitaktischen
NiMnSb-Schichten — •P. Bach 1 , A. Bader 1 , C. Gould 1 , G.
Schmidt 1 , L.W. Molenkamp 1 , R. Urban 2 , G. Woltersdorf 2 und
B. Heinrich 2 — 1 Physikalisches Institut, Experimentelle Physik III,
Universität Würzburg, Am Hubland, 97074 Würzburg — 2 Simon Fraser
University, 8888 University Drive, Burnaby, British Columbia, V5AIS6,
Canada
Halbmetallische Ferromagneten wie die Halb-Heusler-Legierung
NiMnSb, die 100% Spinpolarisation bei Raumtemperatur aufweisen,
sind ideale Kandidaten für Spininjektionsbauelemente.
Es wurden qualitativ hochwertige NiMnSb-Schichten auf
In0.53Ga0.47As/InP mittels MBE gewachsen. Hochauflösende
Röntgendiffraktometrie (HRXRD) zeigt die hervorragende strukturelle
Qualität des NiMnSb und gibt Aufschluss über das Relaxationsverhalten
dieser Schichten.
Die Magnetischen Eigenschaften wurden mit Hilfe von Magnetooptischem
Kerr Effekt (MOKE), Ferromagnetischer Resonanz (FMR) und
SQUID Magnetometrie untersucht. Abhängig von der Schichtdicke wurden
verschiedene uniaxiale und vierfache Anisotropien beobachtet.
HL 29.4 Mi 16:00 H13
Very large Magnetoresistance in ferromagnetic (Ga,Mn)As
wires with Nanoconstrictions — •Christian Rüster, Tatjana
Borzenko, Charles Gould, Georg Schmidt, and Laurens
Molenkamp — Physikalisches Institut, Universität Würzburg, Am
Hubland, D-97074 Würzburg
It was recently pointed out by Flatté and Vignale [APL 78, (2001)
1273] that a very large magnetoresistance should be observable from domain
walls in (Ga,Mn)As. We have now fabricated, using e-beam lithography
and dry etching, (Ga,Mn)As based nanostructures where domain
walls can be pinned by sub-20 nm constrictions. Controlled by shape
anisotropy we can switch the regions on either side of the constriction to
either parallel or antiparallel magnetization. All samples exhibit a positive
magnetoresistance when a domain wall is trapped at the constriction.
We clearly observe the exponential I/V -characteristics predicted by
Flatté and Vignale. For samples in the diffusive transport regime, we find
a magnetoresistance up to 8%, while in samples where, due to depletion
at the constriction, a tunnel barrier is formed, we observe magnetoresistance
up to 2000%. Preliminary data on the manipulation of domain
walls using electrical current is also presented.
HL 29.5 Mi 16:15 H13
Growth and Characterization of GaMnAs on GaAs (001) and
(311)A — •M. Reinwald, U. Wurstbauer, M. Döppe, K. Wagenhuber,
P. Tranitz, W. Wegscheider, and D. Weiss — Institut für
Experimentelle und Angewandte Physik, Universität Regensburg, 93040
Regensburg, Germany
Ga1−xMnxAs-layers with manganese content x of about 5% have been
grown by molecular beam epitaxy on GaAs (001) and (311)A substrates.
SQUID measurements on the as-grown and on annealed samples show
that samples on both growth surfaces are ferromagnetic, but differ both
in the curie temperature and the magnetic anisotropy. Magnetotransport
measurements with in plane magnetic field show a planar hall effect, so
that it is possible to study the angular dependence of the switching fields
by rotating the sample in the field.
Although many GaMnAs samples have been grown, it is still possible
to grow GaAs/AlGaAs-heterostructures with high electron mobilities in
the same growth chamber. PL measurements on these heterostructures
reveal, that no manganese was incorporated into these samples. A second
growth chamber specially designed for highest electron mobilities, allows
the combination of ferromagnetic layers and high electron mobility structures,
to carry out spin injection experiments.
This work is supported by BMBF Verbundprojekt ” Spinelektronik und
Spinoptoelektronik in Halbleitern“
HL 29.6 Mi 16:30 H13
Rashba effect induced localization in quantum networks
— •Dario Bercioux 1 , Michele Governale 2 , Vittorio
Cataudella 1 , and Vincenzo Marigliano Ramaglia 1 —
1 Coherentia-INFM and Dipartimento di Fisica Universitá Federico II
Napoli Italy — 2 NEST-INFM and Scuola Normale Superiore Pisa Italy
Quantum networks are graphs of one-dimensional wires connected at
nodes. It has been shown recently that in a particular two-dimensional
quantum network, the so called T∋, quantum interference due to the
Aharon-Bohm effect and to the topology of the network can induce strong
localization [1,2]. For two-dimensional networks made up of quantum
wires realized in semiconductor heterostructures, spin-orbit coupling due
to the structural inversion asymmetry (Rashba effect[3]) may play an
important role. We study a quantum network extending in only onedimension
(chain of square loops connected at one vertex) made up of
quantum wires with Rashba spin-orbit coupling. This system is a simplified
version of a T∋, and exhibits the same kind of localization when a
magnetic field is applied. We show that the Rashba effect may give rise to
a localization phenomenon similar to the one induced by magnetic flux.
This localization effect can be attributed to the spin precession due to
the Rashba effect. Furthermore, we study the effect of disorder on the
transport properties of this network .
[1] J. Vidal, R. Mosseri and B. Douçot, Phys. Rev. Lett. 81, 5888
(1998). [2] C. Naud et al., Physica E 12, 190 (2001). [3] Yu A. Bychkov
and E.I. Rashba, J. Phys. C: Solid State Phys. 17, 6039 (1984).
HL 29.7 Mi 16:45 H13
Molecular-beam epitaxy of (Zn,Mn)Se on Si(100) for spintronics.
— •T. Slobodskyy, D. Keller, A. Slobodskyy, C. Gould,
P. Grabs, G. Schmidt , and L.W. Molenkamp — Physikalisches Institut,
Universität Würzburg, Am Hubland, 97074 Würzburg, Germany
Experiments on spin injection into silicon are very important for industrial
applications of spintronics devices. Due to the conductance mismatch[1],
diluted magnetic semiconductors like (Zn,Mn)Se are required
for spin injection experiments in the diffusive transport regime.
(Zn,Mn)Se films were grown by molecular beam epitaxy on Si(100)
substrates. A low temperature surface cleaning technique, in combination
with subsequent Arsenic passivation, yields a surface suitable for
II-VI epitaxy. A low temperature growth start involving atomic layer
epitaxy and molecular enhanced epitaxy has been optimized.
The structural properties of the films were determined by high resolution
X-ray diffraction and optical measurements. The surface investigated
using both optical and scanning electron microscopy. SQUID and
magneto-optical measurements show the expected magnetic properties
of the films. Preliminary transport measurements have also been carried
out.
[1] G. Schmidt, L. W. Molenkamp, A. T. Filip, and B. J. van Wees,
Phys. Rev. B. 62, R4790 (2000).
HL 29.8 Mi 17:00 H13
Resonant tunneling diodes with magnetic emitters for spin injection
experiments — •A. Slobodskyy, A. Gröger, C. Gould,
T. Slobodskyy, P. Grabs, G. Schmidt, and L. W. Molenkamp
— Physikalisches Institut der Universität Würzburg EP3, Am Hubland,
D-97074 Würzburg, Germany
All II-VI magnetic resonant tunneling diodes (RTD) were grown by
Molecular Beam Epitaxy (MBE). The emitter of the ZnSe/(Zn,Be)Se
RTD contains 6% manganese, making it suitable for the injection of spinpolarized
electrons at low temperatures and moderate magnetic fields.
Using this type of structure, spin injection into resonant tunneling diodes
(and quantum dots when patterned to sub-micron size) can be studied.
After patterning, the samples were inserted into a He bath cryostat
equipped with a 16 T superconducting magnet. The I/V characteristics
were measured with magnetic field applied either perpendicular to, or
in the plane of the quantum well. The I/V curves of the device show
characteristic resonant tunneling diode behavior. When subjected to an
external magnetic field the resonance shifts in agreement with the Zeeman
splitting that we expect from the DMS layer.