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Plenarvorträge - DPG-Tagungen

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Magnetismus Freitag<br />

formed magnetic circular dichroism (MCD) measurements in the visible<br />

spectral range of light. The MCD spectra of all samples show peak structures<br />

around 1.5eV and a very broad structure around 2.5eV with a superimposed<br />

substructure. In addition, the paramagnetic samples exhibit<br />

a peak around 1.8eV. According to [2], the peak at 1.5eV can be assigned<br />

to an excited neutral acceptor state and the peak at 1.8eV to a transition<br />

from Mn 3+ to Mn 2+ , while according to [3] the peak at 1.5eV corresponds<br />

to the charged acceptor state. Supported by BMBF/FKZ01BM160.<br />

[1] S. Dhar et al., Appl. Phys. Lett. 82, 2077 (2003)<br />

[2] T. Graf et al., Appl. Phys. Lett. 81, 5159 (2002)<br />

[3] R.Y. Korotkov et al., Appl. Phys. Lett. 80, 1731 (2002)<br />

MA 31.5 Fr 11:45 H22<br />

Which Mn is ferromagnetic in Ga1−xMnxAs? — •O. Rader 1 ,<br />

K. Fauth 2 , G. Schott 2 , G. Schmidt 2 , L. Molenkamp 2 , G.<br />

Schütz 3 , F. Kronast 1 , and W. Gudat 1,4 — 1 BESSY, 12489 Berlin<br />

— 2 Physikalisches Institut, Universität Würzburg, 97074 Würzburg<br />

— 3 MPI für Metallforschung, 70569 Stuttgart — 4 Institut für Physik,<br />

Universität Potsdam, 14415 Potsdam<br />

The origin of ferromagnetism in the diluted magnetic semiconductor<br />

Ga1−xMnxAs is a matter of controversy. In spite of important contributions<br />

from electron spectroscopy, understanding the electronic structure<br />

has been hampered by an indistinguishability of Mn atoms. We combine<br />

sensitivity to atomic number, chemical shifts, and to magnetic order in a<br />

field-dependent (0-2.4 T) magnetic circular x-ray dichroism study at the<br />

Mn L-edge and observe three constituents of distinct magnetic behavior.<br />

We find that ferromagnetic Mn deviates more strongly from a 3d 5 configuration<br />

than assumed previously, that superparamagnetic clusters of<br />

20µB are formed, and that interstitial Mn, previously considered the key<br />

to highest TC’s, does not contribute to the ferromagnetism.<br />

MA 31.6 Fr 12:00 H22<br />

Nonequilibrium spin transport in ballistic ferromagnetic - semiconductor<br />

structures — •Dmitri Ryndyk — Institut für Theoretische<br />

Physik, Universität Regensburg, 93040 Regensburg<br />

We consider spin injection and spin accumulation in ferromagnetic(F)<br />

- semiconductor(Sc) structures. In particular, spin-dependent<br />

transport phenomena can be observed in tunneling through a (un-<br />

doped) semiconductor barrier and in double F-I-Sc-I-F tunnel junctions.<br />

Using nonequilibrium Green function technique, nonequilibrium<br />

spin states, and crossover from coherent to incoherent quantum transport<br />

in ferromagnetic-semiconductor-ferromagnetic junctions are studied<br />

theoretically beyond the Landauer framework of ballistic transport.<br />

MA 31.7 Fr 12:15 H22<br />

Transport properties of Fe/MgO/Fe interfaces — •Bogdan Yavorsky,<br />

Peter Zahn, and Ingrid Mertig — MLU Halle-Wittenberg,<br />

Fachbereich Physik, D-06099 Halle, Germany<br />

The transport properties and tunneling magnetoresistance (TMR) of<br />

an Fe/MgO/Fe tunnel junction have been studied by means of an ab initio<br />

electronic structure calculation. The electronic structure was calculated<br />

within a tight-binding Korringa-Kohn-Rostoker (TB-KKR) method. The<br />

influence of structural relaxation, formation of an FeO - layer and noncollinear<br />

magnetic order at the interface on the conductance and TMR<br />

are discussed.<br />

MA 31.8 Fr 12:30 H22<br />

Nanosecond Pulse Characterization of Magnetic Tunnel Junctions<br />

— •Roman Adam 1 , Jakob Schelten 1 , Michael Siegel 2 , and<br />

Hermann Kohlstedt 3 — 1 Institute of Thin Films and Interfaces, Research<br />

Center Jülich, D-52425 Jülich, Germany — 2 Institute of Solid<br />

State Research, Research Center Jülich, D-52425 Jülich, Germany —<br />

3 Institut für Elektrotechnische Grundlagen der Informatik, Universtät<br />

Karlsruhe D-76187 Karlsruhe, Germany<br />

We fabricated and tested Co/Al2O3/Co magnetic tunnel junctions<br />

(MTJ) embedded in coplanar strip (CPS) transmission lines designed for<br />

high-frequency device measurements. MTJs were defined by photolithography<br />

and by Ar ion-beam etching resulting in device area ranging from<br />

2 × 6µm 2 to 3 × 18µm 2 . After MTJ and CPS fabrication, selected chip<br />

area was covered with SiO2 thin film for isolation. Control lines passing<br />

in 400 nm distance above MTJs were finally defined by lift-off. Our MTJs<br />

show a magnetoresistance effect up to 17.5 % at room temperature. To<br />

characterize device high-frequency response, MTJs were excited by 5- to<br />

50-ns-long electrical pulses applied to the control line. Device response<br />

due to the MTJ top electrode magnetization reversal was observed experimentally<br />

and compared with simulations.

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