09.12.2012 Views

Plenarvorträge - DPG-Tagungen

Plenarvorträge - DPG-Tagungen

Plenarvorträge - DPG-Tagungen

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

Magnetismus Dienstag<br />

MA 13.87 Di 15:00 Bereich A<br />

Stochastic growth of magnetic alloy nanostructures on surfaces<br />

— •S. Heinrichs 1 , M. Kessler 1 , W. Dieterich 1 , Ph. Maass 2 , and<br />

A. Majhofer 3 — 1 Fachbereich Physik, Universität Konstanz, Germany<br />

— 2 Institut für Physik, Technische Universität Ilmenau, Germany —<br />

3 Institute of Experimental Physics, Warsaw University, Poland<br />

Nanostructures of certain magnetic fcc-alloys on surfaces can exhibit<br />

perpendicular magnetic anisotropy and hence are promising materials<br />

for magnetic storage technologies. In CoPt3 nanoclusters the magnetic<br />

anisotropy emerges from a frozen-in anisotropic short range order created<br />

during deposition at temperatures where bulk diffusion is negligible[1].<br />

By Monte Carlo simulation we investigate a structural model that is consistent<br />

with the observed chemical ordering in the bulk[2], surface facets<br />

and surface segregation, and in addition includes the van der Waals interaction<br />

with the substrate. We discuss the subtle connection of kinetic<br />

growth history with the emerging non-equilibrium structures and relate<br />

them to the parameters of a phenomenological magnetic model.<br />

[1] M. Albrecht et al., Europhys. Lett. 56, 884 (2001)<br />

[2] M. Kessler et al., Phys. Rev. B, 67, 134201 (2003)<br />

MA 13.88 Di 15:00 Bereich A<br />

Mass-filtered ferromagnetic alloy clusters on surfaces — •M.<br />

Getzlaff 1 , R.K. Gebhardt 1 , F. Bulut 1 , D. Schmitz 2 , E. Holub-<br />

Krappe 2 , H. Maletta 2 , A. Kleibert 3 , R.-P. Methling 3 , J.<br />

Bansmann 3 , and K.-H. Meiwes-Broer 3 — 1 Institut für Angewandte<br />

Physik, Uni Düsseldorf — 2 HMI Berlin — 3 Fachbereich Physik, Uni<br />

Rostock<br />

Clusters with their enlarged magnetic moments are promising materials<br />

for ferromagnetic thin film devices. Fe and FeCo clusters with diameters<br />

of 6...12 nm were prepared in a new developed, continuously<br />

working arc cluster ion source (ACIS) and subsequently mass filtered<br />

by means of an electrostatic quadrupole deflector. The clusters were deposited<br />

on ferromagnetic (Ni/W(110), Co/W(110)) and nonmagnetic surfaces<br />

(W(110), Si(001)). AFM measurements prove a height being smaller<br />

than the diameter for free particles which can be explained by particlesupport<br />

interaction. HRTEM measurements confirm that the stoichiometry<br />

in deposited FeCo clusters is very similar to the chosen bulk material.<br />

Magnetic properties were determined by X-ray magnetic circular<br />

dichroism (XMCD) with specificity to different elements at BESSY. We<br />

observed very large total magnetic moments of both Fe and Co when<br />

compared to their resp. bulk properties. Analogously to pure clusters<br />

in this size regime, the orbital moments ml in FeCo alloy clusters on<br />

Ni(111) are significantly enhanced with respect to bulk material. Thus,<br />

ml of FeCo clusters play the dominant role for reaching total magnetic<br />

moments above the known bulk value although their contributions are<br />

about one order of magnitude smaller than the spin moments ms.<br />

MA 13.89 Di 15:00 Bereich A<br />

Magnetic structures of 3d clusters on Ni(100) and Fe(110) — •S.<br />

Lounis 1,2 , Ph. Mavropoulos 2 , S. Blügel 2 , and P. H. Dederichs 2<br />

— 1 Fachbereich Physik, Universität Osnabrück, 49069Osnabrück. —<br />

2 Institut für Festkörperforschung, Forschungszentrum Jülich, D–52425<br />

Jülich.<br />

We present ab initio calculations on the magnetic properties of 3d<br />

transition-metal clusters at the (100) surface of Ni and the (110) surface<br />

of Fe. Adatoms, dimers and trimers are investigated. We focus on the<br />

local magnetic moments in the clusters and the substrates, the energetical<br />

stability of different magnetic configurations within the cluster and<br />

the coupling to the substrates. The magnetic hyperfine fields are also<br />

presented. The results are obtained using the Korringa–Kohn–Rostoker<br />

(KKR) Green’s function method within density functional theory (DFT)<br />

in the local spin density approximation (LSDA).<br />

MA 13.90 Di 15:00 Bereich A<br />

Monte Carlo Simulation of Temperature Dependence of<br />

Magnetic Properties of Small Metall Clusters — •Svitlana<br />

Polesya and Hubert Ebert — Dept. Chemie/Phys. Chemie,<br />

Universität München, Butenandtstr. 5-13,<br />

D-81377 München, Germany<br />

Temperature dependent magnetic properties of small clusters of 3delements<br />

and their compounds both unsupported and deposited on metallic<br />

surfaces have been studied using MC simulations together with the<br />

extended Heisenberg model. The exchange parameters of the systems under<br />

consideration have been obtained from ab initio spin-polarized KKR<br />

calculations of electronic structure. It is shown that the temperature<br />

TC of a transition from magnetically ordered to a disordered state for<br />

clusters consisting of few atoms is strongly determined by the magnetic<br />

anisotropy of the system. The dependence of TC on the cluster size is<br />

analyzed and compared to the value obtained for an infinite crystal.<br />

MA 13.91 Di 15:00 Bereich A<br />

Punktkontakt Andreev Spektroskopie und R(T)-Messungen an<br />

der Heuslerlegierung Ni2MnIn — •Jan M. Scholtyssek, Sebastian<br />

von Oehsen, Christian Pels, Malte Kurfiss, Guido Meier<br />

und Ulrich Merkt — Institut für Angewandte Physik und Zentrum<br />

für Mikrostrukturforschung, Universität Hamburg, Jungiusstr. 11, 20355<br />

Hamburg<br />

Für die Heuslerlegierung Ni2MnIn wird an der Grenzfläche zu InAs eine<br />

Spinpolarisation an der Fermi-Kante von 100 Prozent vorhergesagt [1].<br />

Sie ist deshalb von besonderem Interesse für die Spinelektronik. Die von<br />

uns untersuchten Ni2MnIn-Proben werden durch thermisches Verdampfen<br />

[2] und durch DC-Magnetron-Sputtern hergestellt. Zur Bestimmung<br />

der Spinpolarisation werden die Proben unterschiedlicher Schichtdicken<br />

mit Point-Contact-Andreev-Reflection (PCAR) bei verschiedenen Temperaturen<br />

untersucht. Der spezifische Widerstand der Proben wird temperaturabhängig<br />

in Vierpunkt-Geometrie gemessen. Diese R(T)-Daten<br />

liefern Hinweise auf die elektronische Struktur der Schichten.<br />

[1] K.A. Kilian and R.H. Victora, IEEE Trans. Mag. 37, 1976 (2001).<br />

[2] M. Kurfiss and R. Anton, J. Alloys and Compounds 361, 36 (2003).<br />

MA 13.92 Di 15:00 Bereich A<br />

Ni2MnIn-Schichten: Herstellung sowie elektrische und magnetische<br />

Charakterisierung — •Franziska Schultz, Malte Kurfiß,<br />

Rainer Anton, Christian Pels, Guido Meier und Ulrich<br />

Merkt — Universität Hamburg, Institut für Angewandte Physik und<br />

Zentrum für Mikrostrukturforschung, Jungiusstr.11, 20355 Hamburg<br />

Die Heusler-Legierung Ni2MnIn ist ein halbmetallischer Ferromagnet,<br />

für den 100% Spinpolarisation an der Grenzfläche zu InAs vorausgesagt<br />

wird [1]. In der Spinelektronik ist diese hohe Spinpolarisation für die Injektion<br />

von polarisierten Elektronen in Halbleiter von großer Bedeutung.<br />

Für die Herstellung der Schichten werden zwei Methoden verwendet:<br />

thermisches Koverdampfen im Ultrahochvakuum und DC-Magnetron-<br />

Sputtern. Als Substrate dienen InAs, Si und C. Das Materialsystem<br />

Ni2MnIn/InAs(100) bildet keine Schottky-Barriere an der Grenzfläche<br />

und besitzt eine vorzügliche Gitteranpassung. In Wachstumsreihen mit<br />

variierten Substrattemperaturen wird die Kristallstruktur optimiert. Die<br />

Stöchiometrie der Schichten wird mit Röntgenspektroskopie (EDX) im<br />

Transmissions- und Rasterelektronenmikroskop kontrolliert, die Struktur<br />

durch Elektronenbeugung. Die Schichten werden für die elektrische und<br />

magnetische Charakterisierung mikrostrukturiert. Die elektrische Charakterisierung<br />

geschieht durch R(T)-Messungen, die magnetischen Eigenschaften<br />

werden mit SQUID- und Hall-Mikromagnetometrie sowie mit<br />

dem Magnetkraftmikroskop untersucht. Mittels Punktkontakt-Andreev-<br />

Spektroskopie [2] wird der Grad der Spinpolarisation bestimmt.<br />

[1]K.A.Kilian und R.H.Victora, IEEE Trans.Mag. 37, 1976 (2001).<br />

[2]R.J.Soulen et al., Science 282, 85 (1998).<br />

MA 13.93 Di 15:00 Bereich A<br />

Transport Properties of Magnetite-Nb:SrTiO3 Interfaces — •M.<br />

Ziese, U. Köhler, R. Höhne, A. Bollero, and P. Esquinazi —<br />

Division of Superconductivity and Magnetism, University of Leipzig,<br />

Linnéstrasse 5, 04103 Leipzig.<br />

The magnetotransport properties of contacts between magnetite and<br />

Nb-doped SrTiO3 were studied. Magnetite films were deposited on metallic<br />

Nb(0.1%):SrTiO3 single crystal substrates by pulsed laser deposition.<br />

During deposition the substrate temperature was 430 ◦ C and oxygen partial<br />

pressure 10 −5 mbar. At these conditions epitaxial films with Verwey<br />

temperatures of 115 K were obtained. The magnetotransport properties<br />

at the interface were investigated by four probe measurements in<br />

current-perpendicular-to-plane configuration by recording current (I)–<br />

voltage (V ) characteristics at various temperatures (60 K ≤ T ≤ 300 K)<br />

and magnetic fields (µ0H ≤ 6 T). The I-V -characteristics showed rectifying<br />

behaviour, especially below the Verwey transition. The data could be<br />

successfully analyzed within a Schottky-barrier model; below the Verwey<br />

transition an energy barrier of about 0.1 eV was extracted, which vanished<br />

above the transition. The ideality factor n increases from a value<br />

near unity below 80 K to n ∼ 4 above 135 K. The magnetoresistance<br />

of this interface depends strongly on the bias voltage. When analyzed<br />

within thermionic emission theory taking a barrier potential shift due to

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!