Plenarvorträge - DPG-Tagungen

Magnetismus Freitag
formed magnetic circular dichroism (MCD) measurements in the visible
spectral range of light. The MCD spectra of all samples show peak structures
around 1.5eV and a very broad structure around 2.5eV with a superimposed
substructure. In addition, the paramagnetic samples exhibit
a peak around 1.8eV. According to [2], the peak at 1.5eV can be assigned
to an excited neutral acceptor state and the peak at 1.8eV to a transition
from Mn 3+ to Mn 2+ , while according to [3] the peak at 1.5eV corresponds
to the charged acceptor state. Supported by BMBF/FKZ01BM160.
[1] S. Dhar et al., Appl. Phys. Lett. 82, 2077 (2003)
[2] T. Graf et al., Appl. Phys. Lett. 81, 5159 (2002)
[3] R.Y. Korotkov et al., Appl. Phys. Lett. 80, 1731 (2002)
MA 31.5 Fr 11:45 H22
Which Mn is ferromagnetic in Ga1−xMnxAs? — •O. Rader 1 ,
K. Fauth 2 , G. Schott 2 , G. Schmidt 2 , L. Molenkamp 2 , G.
Schütz 3 , F. Kronast 1 , and W. Gudat 1,4 — 1 BESSY, 12489 Berlin
— 2 Physikalisches Institut, Universität Würzburg, 97074 Würzburg
— 3 MPI für Metallforschung, 70569 Stuttgart — 4 Institut für Physik,
Universität Potsdam, 14415 Potsdam
The origin of ferromagnetism in the diluted magnetic semiconductor
Ga1−xMnxAs is a matter of controversy. In spite of important contributions
from electron spectroscopy, understanding the electronic structure
has been hampered by an indistinguishability of Mn atoms. We combine
sensitivity to atomic number, chemical shifts, and to magnetic order in a
field-dependent (0-2.4 T) magnetic circular x-ray dichroism study at the
Mn L-edge and observe three constituents of distinct magnetic behavior.
We find that ferromagnetic Mn deviates more strongly from a 3d 5 configuration
than assumed previously, that superparamagnetic clusters of
20µB are formed, and that interstitial Mn, previously considered the key
to highest TC’s, does not contribute to the ferromagnetism.
MA 31.6 Fr 12:00 H22
Nonequilibrium spin transport in ballistic ferromagnetic - semiconductor
structures — •Dmitri Ryndyk — Institut für Theoretische
Physik, Universität Regensburg, 93040 Regensburg
We consider spin injection and spin accumulation in ferromagnetic(F)
- semiconductor(Sc) structures. In particular, spin-dependent
transport phenomena can be observed in tunneling through a (un-
doped) semiconductor barrier and in double F-I-Sc-I-F tunnel junctions.
Using nonequilibrium Green function technique, nonequilibrium
spin states, and crossover from coherent to incoherent quantum transport
in ferromagnetic-semiconductor-ferromagnetic junctions are studied
theoretically beyond the Landauer framework of ballistic transport.
MA 31.7 Fr 12:15 H22
Transport properties of Fe/MgO/Fe interfaces — •Bogdan Yavorsky,
Peter Zahn, and Ingrid Mertig — MLU Halle-Wittenberg,
Fachbereich Physik, D-06099 Halle, Germany
The transport properties and tunneling magnetoresistance (TMR) of
an Fe/MgO/Fe tunnel junction have been studied by means of an ab initio
electronic structure calculation. The electronic structure was calculated
within a tight-binding Korringa-Kohn-Rostoker (TB-KKR) method. The
influence of structural relaxation, formation of an FeO - layer and noncollinear
magnetic order at the interface on the conductance and TMR
are discussed.
MA 31.8 Fr 12:30 H22
Nanosecond Pulse Characterization of Magnetic Tunnel Junctions
— •Roman Adam 1 , Jakob Schelten 1 , Michael Siegel 2 , and
Hermann Kohlstedt 3 — 1 Institute of Thin Films and Interfaces, Research
Center Jülich, D-52425 Jülich, Germany — 2 Institute of Solid
State Research, Research Center Jülich, D-52425 Jülich, Germany —
3 Institut für Elektrotechnische Grundlagen der Informatik, Universtät
Karlsruhe D-76187 Karlsruhe, Germany
We fabricated and tested Co/Al2O3/Co magnetic tunnel junctions
(MTJ) embedded in coplanar strip (CPS) transmission lines designed for
high-frequency device measurements. MTJs were defined by photolithography
and by Ar ion-beam etching resulting in device area ranging from
2 × 6µm 2 to 3 × 18µm 2 . After MTJ and CPS fabrication, selected chip
area was covered with SiO2 thin film for isolation. Control lines passing
in 400 nm distance above MTJs were finally defined by lift-off. Our MTJs
show a magnetoresistance effect up to 17.5 % at room temperature. To
characterize device high-frequency response, MTJs were excited by 5- to
50-ns-long electrical pulses applied to the control line. Device response
due to the MTJ top electrode magnetization reversal was observed experimentally
and compared with simulations.