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Plenarvorträge - DPG-Tagungen

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Halbleiterphysik Donnerstag<br />

Py-Elektroden (Länge: 150 µm, Breiten: 2 µm, 0.4 µm, Abstand: 200 nm,<br />

Dicke: 30 nm) werden mittels Elektronenstrahllithographie, Elektronenstrahlverdampfung<br />

und anschließendem Lift-off über dieses Kontaktfenster<br />

gelegt. Erste Ergebnisse zum winkelabhängigen Magnetowiderstand<br />

werden präsentiert.<br />

HL 44.3 Do 16:30 Poster A<br />

Tunneling Through Single-Crystal GaAs(001) Barriers —<br />

•Marcus Zenger, Jürgen Moser, Peifeng Chen, Stephan<br />

Kreuzer, Werner Wegscheider, and Dieter Weiss —<br />

Universität Regensburg<br />

We investigate transport through epitaxial GaAs(001) barriers (6-10<br />

nm) sandwiched between evaporated polycrystalline metal films. To realize<br />

a metal/GaAs(001)/metal sandwich we use the epoxy-bond-andstop-etch-technique.<br />

Previous experiments have shown a non-linear I-Vcharacteristic<br />

and a small but clear TMR ratio of 0.2-0.5%. To increase<br />

the TMR ratio we reduced the temperatures during the fabrication process.<br />

This leads to a higher TMR ratio of about 1.7% which corresponds<br />

to a polarization of about 9% in the Julliére model. A negative MR which<br />

is observed at high magnetic fields together with the still low degree of polarization<br />

suggests that spin-flip scattering is important at the Fe/GaAs<br />

interface. The negative MR in Fe/GaAs/Fe junctions will be compared<br />

to the corresponding high-field MR of nonmagnetic sandwiches, which<br />

grows quadratically. If this nearly temperature independent positive contribution<br />

is subtracted from our high-field TMR data a linear negative<br />

MR results. The latter seems to contain information about the spin-flip<br />

scattering process.<br />

HL 44.4 Do 16:30 Poster A<br />

Präparation von Aluminiumoxid-Tunnelbarrieren für den spinpolarisierten<br />

Transport — •Christine M. S. Johnas, Alexander<br />

van Staa, Toru Matsuyama und Guido Meier — Institut für Angewandte<br />

Physik und Zentrum für Mikrostrukturforschung, Jungiusstraße<br />

11, 20355 Hamburg<br />

Im Bereich magnetischer Tunnelelemente sind Al2O3-Barrieren bereits<br />

etabliert und werden industriell eingesetzt. Für den spinpolarisierten<br />

Transport in Hybridsystemen aus Ferromagneten und Halbleitern ist<br />

theoretisch vorhergesagt, dass Tunnelbarrieren an der Grenzfläche zwischen<br />

Ferromagnet und Halbleiter eine Erhöhung der Spininjektionsrate<br />

bewirken [1,2]. In diesem Beitrag befassen wir uns mit der Herstellung<br />

und Charakterisierung von Al2O3 als Tunnelbarriere. Es werden in situ<br />

oxidierte Au/Al2O3/Au-Kontakte und Ni80Fe20/Al2O3/Normalleiter-<br />

Kontakte untersucht. Hierzu variieren wir die Oxidationszeit, die<br />

Al2O3-Schichtdicke und die Kontaktgeometrie im Größenbereich von<br />

1 × 1µm 2 bis 8 × 8µm 2 . In zukünftigen Experimenten sollen die<br />

Al2O3-Barrieren auch an Ferromagnet-Halbleiter-Grenzflächen erforscht<br />

werden. Als Halbleiter wird p-Typ InAs verwendet werden, das an der<br />

Oberfläche ein zweidimensionales Elektronengas ausbildet und eine<br />

starke, abstimmbare Spin-Bahn-Wechselwirkung aufweist [3].<br />

[1] C.-M. Hu and T. Matsuyama, Phys. Rev. Lett. 87, 066803 (2001).<br />

[2] E. I. Rashba, Phys. Rev. B 62, 16267 (2000).<br />

[2] T. Matsuyama et al., Phys. Rev. B 61, 15588 (2000).<br />

HL 44.5 Do 16:30 Poster A<br />

Magnetotransport in (GaMn)As/MnAs and (GaInMn)As/MnAs<br />

paramagnetic-ferromagnetic hybrids — •S. Ye, P.J. Klar, W.<br />

Heimbrodt, M. Lampalzer, S. Hara, and W. Stolz — FB Physik<br />

und WZMW, Philipps-Universität, Marburg<br />

The (GaMn)As/MnAs and (GaInMn)As/MnAs paramagneticferromagnetic<br />

granular hybrids were grown by metal organic vapour<br />

phase epitaxy. By varying the growth conditions different cluster sizes<br />

and different orientations of the MnAs clusters with respect to the matrix<br />

can be obtained. We have performed magnetoresistance (MR) and Hall<br />

measurements in magnetic fields up to 10T and hydrostatic pressures up<br />

to 15kbar at temperatures between 1.6 and 300K. The (GaMn)As/MnAs<br />

system exhibits a large MR (≈ 160%) at 10T, and changes from a negative<br />

to a positive MR with increasing temperature. The MR depends<br />

strongly on the cluster size, the chosen transport geometry and an the applied<br />

pressure. However the observed effects for the (GaInMn)As/MnAs<br />

are considerably smaller even for comparable cluster sizes and similar<br />

layer thickness. It is suggested that the interaction between MnAs ferromagnetic<br />

clusters and paramagnetic matrix plays an important role for<br />

the magnetotransport in such magnetic semicondutor hybrids, which can<br />

be adjusted by the cluster size, the chosen transport geometry and the<br />

type of paramagnetic matrix material.<br />

HL 44.6 Do 16:30 Poster A<br />

Laser threshold reduction in a room-temperature spintronic<br />

device — •Jörg Rudolph 1 , Joachim Fritzsche 1 , Wolfgang<br />

Stolz 2 , Daniel Hägele 1 , and Michael Oestreich 1 — 1 Universität<br />

Hannover, Institut für Festkörperphysik, Abteilung Nanostrukturen,<br />

Appelstr. 2, 30167 Hannover — 2 Wissenschaftliches Zentrum für Materialwissenschaften,<br />

Philipps-Universität Marburg, Hans-Meerwein-Strasse,<br />

35032 Marburg<br />

Despite the progress in the field of spintronics, commercial applications<br />

are still missing. We demonstrate that the injection of spin polarized electrons<br />

into a V ertical Cavity Surface Emitting Laser (VCSEL) leads via<br />

the optical selection rules to a reduction of the laser threshold which is<br />

a key figure for semiconductor laser applications.<br />

We formerly demonstrated this threshold-reduction only for low temperatures<br />

[1]. Now we present time-resolved photoluminescence experiments<br />

with a VCSEL-structure at elevated temperatures showing that<br />

the threshold reduction takes place even at room-temperature. The<br />

spintronic-VCSEL has therefore the potential to become a real-world<br />

spintronic device.<br />

[1] J. Rudolph, D. Hägele, H.M. Gibbs, G. Khitrova, and M. Oestreich,<br />

Appl. Phys. Lett. 82, 4516 (2003)<br />

HL 44.7 Do 16:30 Poster A<br />

Magnetotransportuntersuchen an (311)a-(Ga,Mn)As —<br />

•Matthias Döppe, Ursula Wurstbauer, Matthias Reinwald,<br />

Werner Wegscheider und Dieter Weiss — Institut für Experimentelle<br />

und Angewandte Physik, Universität Regensburg, D-93040<br />

Regensburg<br />

In den letzten Jahren rückte das bei tiefen Temperaturen (Tc ≈ 80K)<br />

ferromagnetische (001)-(Ga,Mn)As als potentielles Material für Spininjektionsexperimente<br />

in den Mittelpunkt experimenteller Untersuchungen.<br />

Effekte wie der Anisotrope Magnetowiderstand (AMR), das stark negative<br />

Magnetowiderstandsverhalten bei hohen Feldern und der GPHE<br />

(Giant Planar Hall Effekt [1]) sind kennzeichnend für dieses Materialsystem,<br />

das bislang hauptsächlich in (001)-Orientierung untersucht wurde.<br />

Im Rahmen dieses Projektes untersuchen wir Magnetotransport in unterschiedlichen<br />

kristallographischen Richtungen von (Ga,Mn)As Schichten<br />

auf (311)A. Das in-plane Magnetfeld wurde hierbei in Schritten von<br />

6 ◦ bzw. 12 ◦ variiert. Gemessen wurde das in-plane Schaltverhalten der<br />

Schichten mittels AMR und planarem Halleffekt.<br />

[1] H. X. Tang, R. K. Kawakami, D. D. Awschalom and M. L. Roukes,<br />

Phys. Rev. Lett. 90, 107201 (2003)<br />

HL 44.8 Do 16:30 Poster A<br />

Spin-orbit interaction in quantum wires in transverse magnetic<br />

field — •S. Debald 1 , T. Brandes 2 , and B. Kramer 1 — 1 I. Institut<br />

für Theoretische Physik, Universität Hamburg — 2 Dept. of Physics,<br />

UMIST, PO Box 88, Manchester M60 1QD, UK<br />

The effect of spin-orbit interaction on spectral and transport properties<br />

of quasi one-dimensional electron systems in a transverse magnetic<br />

field is investigated in the ballistic regime.<br />

Spin-orbit interaction previously has been reported to significantly<br />

change the structure of electronic subbands if the spin precession length<br />

is comparable to the width of the wire [1,2]. We extend these calculations<br />

by introducing a transverse magnetic field.<br />

We apply exact numerical and perturbative approaches to identify different<br />

regimes dependent on the relative strength in the interplay of confinement,<br />

spin-orbit coupling and magnetic field. This interplay leads to<br />

new effects like a finite subband-dependent spin-splitting at k = 0 being<br />

absent without magnetic field.<br />

[1] A.V. Moroz and C.H.W. Barnes, Phys. Rev. B 60, 14272 (1999).<br />

[2] M. Governale and U. Zülicke, Phys. Rev. B 66, 073311 (2002).<br />

HL 44.9 Do 16:30 Poster A<br />

Mn Doped ZnO: A Diluted Magnetic Semiconductor? — •Karl-<br />

Wilhelm Nielsen 1 , Stefanie Wagner 1 , Daniel Reisinger 1 , Jan<br />

Boris Philipp 1 , Petra Majewski 1 , Lambert Alff 1 , Jürgen Simon<br />

2 , and Rudolf Gross 1 — 1 Walther-Meißner-Institut, Walther-<br />

Meißner-Str. 8, 85748 Garching — 2 Institut für Anorganische Chemie,<br />

Universität Bonn, Römerstraße 164, 53117 Bonn<br />

Over the last years the interest in spintronics has grown continuously.<br />

The use of magnetic semiconductors is of particular interest since it would<br />

allow for all semiconductor spintronic devices. In diluted magnetic semiconductors<br />

the semiconductor ions are partly replaced by transition metal

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