Plenarvorträge - DPG-Tagungen

Magnetismus Dienstag
MA 13.87 Di 15:00 Bereich A
Stochastic growth of magnetic alloy nanostructures on surfaces
— •S. Heinrichs 1 , M. Kessler 1 , W. Dieterich 1 , Ph. Maass 2 , and
A. Majhofer 3 — 1 Fachbereich Physik, Universität Konstanz, Germany
— 2 Institut für Physik, Technische Universität Ilmenau, Germany —
3 Institute of Experimental Physics, Warsaw University, Poland
Nanostructures of certain magnetic fcc-alloys on surfaces can exhibit
perpendicular magnetic anisotropy and hence are promising materials
for magnetic storage technologies. In CoPt3 nanoclusters the magnetic
anisotropy emerges from a frozen-in anisotropic short range order created
during deposition at temperatures where bulk diffusion is negligible[1].
By Monte Carlo simulation we investigate a structural model that is consistent
with the observed chemical ordering in the bulk[2], surface facets
and surface segregation, and in addition includes the van der Waals interaction
with the substrate. We discuss the subtle connection of kinetic
growth history with the emerging non-equilibrium structures and relate
them to the parameters of a phenomenological magnetic model.
[1] M. Albrecht et al., Europhys. Lett. 56, 884 (2001)
[2] M. Kessler et al., Phys. Rev. B, 67, 134201 (2003)
MA 13.88 Di 15:00 Bereich A
Mass-filtered ferromagnetic alloy clusters on surfaces — •M.
Getzlaff 1 , R.K. Gebhardt 1 , F. Bulut 1 , D. Schmitz 2 , E. Holub-
Krappe 2 , H. Maletta 2 , A. Kleibert 3 , R.-P. Methling 3 , J.
Bansmann 3 , and K.-H. Meiwes-Broer 3 — 1 Institut für Angewandte
Physik, Uni Düsseldorf — 2 HMI Berlin — 3 Fachbereich Physik, Uni
Rostock
Clusters with their enlarged magnetic moments are promising materials
for ferromagnetic thin film devices. Fe and FeCo clusters with diameters
of 6...12 nm were prepared in a new developed, continuously
working arc cluster ion source (ACIS) and subsequently mass filtered
by means of an electrostatic quadrupole deflector. The clusters were deposited
on ferromagnetic (Ni/W(110), Co/W(110)) and nonmagnetic surfaces
(W(110), Si(001)). AFM measurements prove a height being smaller
than the diameter for free particles which can be explained by particlesupport
interaction. HRTEM measurements confirm that the stoichiometry
in deposited FeCo clusters is very similar to the chosen bulk material.
Magnetic properties were determined by X-ray magnetic circular
dichroism (XMCD) with specificity to different elements at BESSY. We
observed very large total magnetic moments of both Fe and Co when
compared to their resp. bulk properties. Analogously to pure clusters
in this size regime, the orbital moments ml in FeCo alloy clusters on
Ni(111) are significantly enhanced with respect to bulk material. Thus,
ml of FeCo clusters play the dominant role for reaching total magnetic
moments above the known bulk value although their contributions are
about one order of magnitude smaller than the spin moments ms.
MA 13.89 Di 15:00 Bereich A
Magnetic structures of 3d clusters on Ni(100) and Fe(110) — •S.
Lounis 1,2 , Ph. Mavropoulos 2 , S. Blügel 2 , and P. H. Dederichs 2
— 1 Fachbereich Physik, Universität Osnabrück, 49069Osnabrück. —
2 Institut für Festkörperforschung, Forschungszentrum Jülich, D–52425
Jülich.
We present ab initio calculations on the magnetic properties of 3d
transition-metal clusters at the (100) surface of Ni and the (110) surface
of Fe. Adatoms, dimers and trimers are investigated. We focus on the
local magnetic moments in the clusters and the substrates, the energetical
stability of different magnetic configurations within the cluster and
the coupling to the substrates. The magnetic hyperfine fields are also
presented. The results are obtained using the Korringa–Kohn–Rostoker
(KKR) Green’s function method within density functional theory (DFT)
in the local spin density approximation (LSDA).
MA 13.90 Di 15:00 Bereich A
Monte Carlo Simulation of Temperature Dependence of
Magnetic Properties of Small Metall Clusters — •Svitlana
Polesya and Hubert Ebert — Dept. Chemie/Phys. Chemie,
Universität München, Butenandtstr. 5-13,
D-81377 München, Germany
Temperature dependent magnetic properties of small clusters of 3delements
and their compounds both unsupported and deposited on metallic
surfaces have been studied using MC simulations together with the
extended Heisenberg model. The exchange parameters of the systems under
consideration have been obtained from ab initio spin-polarized KKR
calculations of electronic structure. It is shown that the temperature
TC of a transition from magnetically ordered to a disordered state for
clusters consisting of few atoms is strongly determined by the magnetic
anisotropy of the system. The dependence of TC on the cluster size is
analyzed and compared to the value obtained for an infinite crystal.
MA 13.91 Di 15:00 Bereich A
Punktkontakt Andreev Spektroskopie und R(T)-Messungen an
der Heuslerlegierung Ni2MnIn — •Jan M. Scholtyssek, Sebastian
von Oehsen, Christian Pels, Malte Kurfiss, Guido Meier
und Ulrich Merkt — Institut für Angewandte Physik und Zentrum
für Mikrostrukturforschung, Universität Hamburg, Jungiusstr. 11, 20355
Hamburg
Für die Heuslerlegierung Ni2MnIn wird an der Grenzfläche zu InAs eine
Spinpolarisation an der Fermi-Kante von 100 Prozent vorhergesagt [1].
Sie ist deshalb von besonderem Interesse für die Spinelektronik. Die von
uns untersuchten Ni2MnIn-Proben werden durch thermisches Verdampfen
[2] und durch DC-Magnetron-Sputtern hergestellt. Zur Bestimmung
der Spinpolarisation werden die Proben unterschiedlicher Schichtdicken
mit Point-Contact-Andreev-Reflection (PCAR) bei verschiedenen Temperaturen
untersucht. Der spezifische Widerstand der Proben wird temperaturabhängig
in Vierpunkt-Geometrie gemessen. Diese R(T)-Daten
liefern Hinweise auf die elektronische Struktur der Schichten.
[1] K.A. Kilian and R.H. Victora, IEEE Trans. Mag. 37, 1976 (2001).
[2] M. Kurfiss and R. Anton, J. Alloys and Compounds 361, 36 (2003).
MA 13.92 Di 15:00 Bereich A
Ni2MnIn-Schichten: Herstellung sowie elektrische und magnetische
Charakterisierung — •Franziska Schultz, Malte Kurfiß,
Rainer Anton, Christian Pels, Guido Meier und Ulrich
Merkt — Universität Hamburg, Institut für Angewandte Physik und
Zentrum für Mikrostrukturforschung, Jungiusstr.11, 20355 Hamburg
Die Heusler-Legierung Ni2MnIn ist ein halbmetallischer Ferromagnet,
für den 100% Spinpolarisation an der Grenzfläche zu InAs vorausgesagt
wird [1]. In der Spinelektronik ist diese hohe Spinpolarisation für die Injektion
von polarisierten Elektronen in Halbleiter von großer Bedeutung.
Für die Herstellung der Schichten werden zwei Methoden verwendet:
thermisches Koverdampfen im Ultrahochvakuum und DC-Magnetron-
Sputtern. Als Substrate dienen InAs, Si und C. Das Materialsystem
Ni2MnIn/InAs(100) bildet keine Schottky-Barriere an der Grenzfläche
und besitzt eine vorzügliche Gitteranpassung. In Wachstumsreihen mit
variierten Substrattemperaturen wird die Kristallstruktur optimiert. Die
Stöchiometrie der Schichten wird mit Röntgenspektroskopie (EDX) im
Transmissions- und Rasterelektronenmikroskop kontrolliert, die Struktur
durch Elektronenbeugung. Die Schichten werden für die elektrische und
magnetische Charakterisierung mikrostrukturiert. Die elektrische Charakterisierung
geschieht durch R(T)-Messungen, die magnetischen Eigenschaften
werden mit SQUID- und Hall-Mikromagnetometrie sowie mit
dem Magnetkraftmikroskop untersucht. Mittels Punktkontakt-Andreev-
Spektroskopie [2] wird der Grad der Spinpolarisation bestimmt.
[1]K.A.Kilian und R.H.Victora, IEEE Trans.Mag. 37, 1976 (2001).
[2]R.J.Soulen et al., Science 282, 85 (1998).
MA 13.93 Di 15:00 Bereich A
Transport Properties of Magnetite-Nb:SrTiO3 Interfaces — •M.
Ziese, U. Köhler, R. Höhne, A. Bollero, and P. Esquinazi —
Division of Superconductivity and Magnetism, University of Leipzig,
Linnéstrasse 5, 04103 Leipzig.
The magnetotransport properties of contacts between magnetite and
Nb-doped SrTiO3 were studied. Magnetite films were deposited on metallic
Nb(0.1%):SrTiO3 single crystal substrates by pulsed laser deposition.
During deposition the substrate temperature was 430 ◦ C and oxygen partial
pressure 10 −5 mbar. At these conditions epitaxial films with Verwey
temperatures of 115 K were obtained. The magnetotransport properties
at the interface were investigated by four probe measurements in
current-perpendicular-to-plane configuration by recording current (I)–
voltage (V ) characteristics at various temperatures (60 K ≤ T ≤ 300 K)
and magnetic fields (µ0H ≤ 6 T). The I-V -characteristics showed rectifying
behaviour, especially below the Verwey transition. The data could be
successfully analyzed within a Schottky-barrier model; below the Verwey
transition an energy barrier of about 0.1 eV was extracted, which vanished
above the transition. The ideality factor n increases from a value
near unity below 80 K to n ∼ 4 above 135 K. The magnetoresistance
of this interface depends strongly on the bias voltage. When analyzed
within thermionic emission theory taking a barrier potential shift due to