Plenarvorträge - DPG-Tagungen
Plenarvorträge - DPG-Tagungen
Plenarvorträge - DPG-Tagungen
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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