Plenarvorträge - DPG-Tagungen

Magnetismus Dienstag
MA 13.28 Di 15:00 Bereich A
Scaling of the Extraordinary Hall Effect in Manganite Films —
•M. Ziese — Division of Superconductivity and Magnetism, University
of Leipzig, Linnéstrasse 5, 04103 Leipzig.
There has been a long-standing debate about the anomalous Hall effect
in manganites. Whereas traditionalists explained the data in the
conventional side-jump und skew-scattering models, other groups argued
in favour of a new mechanism induced by the Berry phase. Strong support
for the latter came from reports of a one-parameter scaling of the
Hall effect with the reduced magnetization [1]. Using Hall effect data
measured on La0.7Ca0.3MnO3 and La0.7Ba0.3MnO3 films it is shown that
a one-parameter scaling does not hold; instead a two-parameter scaling
is observed. The scaling function can be derived within the basic double
exchange model assuming a side jump mechanism.
[1] Y. Lyanda-Geller, S. H. Chun, M. B. Salamon, P. M. Goldbart,
P. D. Han, Y. Tomioka, A. Asamitsu and Y. Tokura, Phys. Rev. B 63,
184426 (2001).
MA 13.29 Di 15:00 Bereich A
Properties of CMR-manganite films: Pr0.7Ca0.3MnO3 —
•Wilko Westhäuser, Karsten Guth, Harald Jarzina, and
Christian Jooss — Institut für Materialphysik, Universität Göttingen,
Tammannstrasse 1, 37077 Göttingen, Germany
The CMR-effect describes a colossal drop in resistance caused by
a phase transition from a charge-ordered antiferromagnetic state to a
charge-delocalized ferromagnetic state. As the charge-ordered state of
Pr0.7Ca0.3MnO3 is unstable under external perturbations, this transition
can be switched by applying an electrical field, a magnetic field or
using photon exposure.
Mixed powder of Pr6O11, CaCO3 and MnO3 with a defined ratio was
calcined in air at 1050C for 48h and then pressed into a 25-mm diameter
target. By using pulsed laser deposition, thin films were grown on singlecrystalline
SrTiO3-substrates (001). The calcined powder and the ablated
thin films were analysed by XRD, DSC, REM and EDX to determine the
crystallographic structure and the chemical composition.
Transport properties, in particular the metallic-insulator transition in
electrical fields, are analysed as a function of temperature.
MA 13.30 Di 15:00 Bereich A
Transport in nanocontacts: An ab initio description — •Michael
Czerner, Alexej Bagrets, and Ingrid Mertig — Martin-Luther-
University Halle-Wittenberg, Department of Physics, Von-Seckendorff-
Platz 1, 06120 Halle, Germany
We present ab-initio calculations of the transport properties of metallic
atomic-sized nanocontacts. The nanocontacts were modelled by atomic
chains of Cu and Co suspended between two semi-infinite leads. We used
the first principle screened Korringa-Kohn-Rostoker method to calculate
the electronic properties of the systems, treating the leads and atomic
chains on the same footing without any model parameters. The Landauer
formalism was applied to investigate the conductance. Quantum
size effects and the influence of impurity scattering is discussed in detail.
MA 13.31 Di 15:00 Bereich A
Fabrication and charakterization of monocrystalline
Fe/ZnSe/Fe tunnel contacts — •Martin Dumm and Günther
Bayreuther — Institut für Experimentelle und Angewandte Physik,
Universität Regensburg, 93040 Regensburg
Theoretical work predicts high magneto-resistance values for
monocrystalline tunnel contacts with semiconductor barriers.
Fe/ZnSe/Fe trilayers have been fabricated by molecular beam epitaxy
(MBE) using two separate MBE-Systems, where the samples can
be transferred from one chamber to the other without breaking the
vacuum using a portable UHV chamber. High resolution transmission
electron microscopy on cross sections and reflective high energy electron
diffraction prove that the whole system is grown epitaxially on the
GaAs(001) substrate. The 20nm thick Fe bottom layer has a dominating
fourfold magnetic anisotropy with the easy axes along [100] and [010],
whereas a uniaxial anisotropy with easy axis along [110] prevails in the
2nm thick Fe top layer. Magnetization loops for the Fe/ZnSe/Fe show
that the coercive field of the top layer is smaller than that of the bottom
layer between 300K and 80K, whereas below 80K both Fe films switch
at the same magnetic field. This means that at low temperature the
switching of the thinner top layer is triggered by the switching of the
thicker bottom layer.
Finally, first magneto-transport measurements across the tunnel barrier
at 100K are reported.
MA 13.32 Di 15:00 Bereich A
Characterization and optimization of Co/Al2O3/Co/CoO magnetic
tunnelling junctions — •Erik Verduijn and Kurt Westerholt
— Institut für Experimentalphysik/Festkörperphysik, Ruhr-
Universität Bochum, 44780 Bochum
Co/Al2O3/Co magnetic tunnel junctions (MTJs) are well established
in literature and ideally suited as a reference system for the development
and optimization of the techniques relative for the design and
fabrication of microstructured tunnel junctions. To this end we have
performed magnetotransport and SQUID magnetometry measurements
on Co/Al2O3/Co/CoO MTJs and atomic force microscopy studies of
Co/Al2O3/Co/CoO layers fabricated under a variety of different growth
conditions in order to optimize several critical MTJ parameters, in particular
the optimization of the tunnel barrier oxidation parameters, roughness
of the bottom ferromagnetic electrode and the magnetic switching
behaviour of the ferromagnetic electrodes. Furthermore we report on
the microlithographic technique based on ion beam etching and electron
beam lithography we use for the fabrication of micrometer scale MTJs.
MA 13.33 Di 15:00 Bereich A
Temperature and bias voltage dependent transport in magnetic
tunnel junctions with low energy Ar-ion irradiated barriers —
•Jan Schmalhorst and Guenter Reiss — University of Bielefeld,
Department of Physics, Nano Device Group, P.O. Box 100131, 33501
Bielefeld, Germany
Magnetic tunnel junctions (Mn83Ir17/Co70Fe30/AlOx/Ni80Fe20) are investigated,
whose barriers are irradiated by a low energy Ar + ion beam
immediately after plasma oxidation of the aluminium film. The tunneling
magnetoresistance prior to irradiation is up to 71% at 10K. The ion
irradiation increases the area resistance product up to a factor 40 for ion
energies up to 150V. Further, the tunneling magnetoresistance and the
dielectric stability is strongly reduced with increasing ion energies. From
the analysis of the temperature and the voltage dependence of the tunneling
magnetoresistance we conclude, that this is due to an irradiation
induced local change of the coordination of the Al- und O-atoms in the
barrier. This leads to a thicker barrier and an increase of the precursor
density for the dielectric breakdown. Further, an increase of hopping
conductance through localized states is discussed. At energies larger than
150V the resistance breaks down rapidly and the tunneling magnetoresistance
vanishes completely. This results from the enhanced intermixing
and sputtering of the barrier and electrode material. The results are also
supported by investigations of the magnetic and the noise properties of
the junctions and the Cu-Kα- reflectivity of AlOx multilayers.
MA 13.34 Di 15:00 Bereich A
Barrierenmodifikation von magnetischen Tunnelelementen —
•Marc Sacher, Jan Sauerwald, Jan Schmalhorst und Günter
Reiss — Universität Bielefeld, Fakultät für Physik, Universitätsstr. 25,
33615 Bielefeld
Es werden doppelt gepinnte, magnetische Tunnelelemente (Mn83Ir17/
Co / Al2O3/ Co/ Mn83Ir17) mit systematisch variierter Barrieren-
Grenzfläche hergestellt. Die Proben werden im Magnetfeld gesputtert,
und zeigen dadurch ohne anschließendes Tempern das Exchange-Bias. So
kann eine thermisch aktivierte Veränderung der Tunnelbarrieren ausgeschlossen
und auftretende Effekte auf die Bestrahlung zurückgeführt werden.
Die Barrierenmodifikation erfolgt mittels niederenergetischer Argonionen
(10...100eV) in zwei verschiedenen Experimenten:
1. Die Aluminiumschicht wird direkt nach der Oxidation bestrahlt. Der
Tunnelwiderstand wächst exponentiell mit der Bestrahlungsdauer auf das
400-fache (100s) an. Dabei ist der TMR bis zu einer Energie von 60eV
konstant auf dem Wert des Referenzelementes. Erst darüber sinkt er
leicht ab. Bei einer anschließenden Auslagerung der Probe reduziert sich
der Widerstand auf die Größenordnung des Referenzwertes.
2. Nach der Oxidation des Aluminiums wird zunächst eine dünne Kobaltschicht
(1...20˚A) aufgebracht, anschließend bestrahlt und erst danach
das restliche Kobalt (auf 6nm) gesputtert. Hier verschwindet der TMR
bei einer Kobaltdicke von 0,4nm, nimmt aber ab 1,5nm wieder zu.
Ziel dieser Versuche ist ein tieferes Verständnis der Einflüsse von z.B.
Grenzflächenrauigkeit auf den Tunnelprozess.