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Plenarvorträge - DPG-Tagungen

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Magnetismus Donnerstag<br />

magnon wave-vector and, therefore, on its energy. This dependence can<br />

explain strongly non-monotonous inelastic tunneling spectra measured in<br />

Refs.[1].<br />

[1] J.S. Moodera, J. Nowak, and R.J.M. van de Veerdonk,<br />

Phys.Rev.Lett. 80, 2941 (1998); Y. Ando, J. Murai, H. Kubota, and T.<br />

Miyazaki, J. Appl. Phys. 87, 5209 (2000); J.H. Yu, H.M. Lee, Y. Ando,<br />

and T. Miyazaki, Appl. Phys. Lett. 82, 4735 (2003).<br />

[2] S. Zhang, P.M. Levy, A.C. Marley, and S.S.P. Parkin,<br />

Phys.Rev.Lett. 79, 3744 (1997).<br />

MA 27.13 Do 18:15 H22<br />

Persistent Spin Currents in Helimagnets — •Jürgen König 1 ,<br />

Jan Heurich 2 , and Allan H. MacDonald 3 — 1 Institut für Theoretische<br />

Physik III, Ruhr-Universität Bochum — 2 Institut für Theoretische<br />

Festkörperphysik, Universität Karlsruhe — 3 Department of Physics,<br />

University of Texas at Austin<br />

We study collective spin transport in helimagnets, e.g., in systems with<br />

ground states that possess spiral magnetic order. It is demonstrated that<br />

weak external magnetic fields generate dissipationless spin currents in<br />

these systems, whereas the spiral ground state in the absence of a magnetic<br />

field does not support collective spin transport. Our conclusions are<br />

based on phenomenological considerations and on microscopic mean-field<br />

theory calculations for an illustrative toy model [1]. We discuss similarities<br />

and differences of these results to our recent analysis of spin supercurrents<br />

in thin film ferromagnets [2], and we speculate on possible<br />

applications of this effect in spintronic devices.<br />

[1] J. Heurich, J. König, and A.H. MacDonald, Phys. Rev. B 68, 064406<br />

(2003).<br />

[2] J. König, M.C. Bønsager, and A.H. MacDonald, Phys. Rev. Lett. 87,<br />

187202 (2001).<br />

MA 27.14 Do 18:30 H22<br />

Anomalous Hall effect and Berry Phase in Ferromagnets —<br />

•Mathieu Taillefumier 1,2 , Vitalii Dugaev 1,3 , and Patrick<br />

Bruno 1 — 1 Max Planck Institut für mikrostrukturphysik, Weinberg 2,<br />

06120 Halle, Germany — 2 Laboratoire Louis Néel, CNRS, Boite Postale<br />

166,, 38042 Grenoble Cedex 09, France — 3 Institute for Problems of<br />

Materials Science, NASU, Vilde 5, 58001 Chernovtsy, Ukraine<br />

We propose a new mechanism of anomalous Hall effect (AHE) in twodimensional<br />

ferromagnets and magnetic nanostructures with inhomogeneous<br />

magnetization. In contrast to the usual theories (side jump and<br />

skew scattering mechanism) of AHE, the presented theory does not necessary<br />

require spin-orbit interaction, and is induced by a Berry phase of<br />

the wavefunctions of the electrons moving a inhomogeneous magnetization<br />

field. We obtain a nonvanishing contribution to the Hall conductivity<br />

provided that the topology of the magnetization field is nontrivial like in<br />

the case of separated magnetic domains or periodic lattice of magnetic<br />

domains.<br />

We propose an experiment with a structure containing two-dimensional<br />

electrons or holes of diluted magnetic semiconductor subject to the stray<br />

field of a lattice of magnetic nanocylinders. The striking behavior predicted<br />

for such a system (of which all revelant parameters are well known)<br />

allows to distinguish it from the other mechanisms.<br />

MA 28 Spindynamik / Ummagnetisierungsvorgänge<br />

Zeit: Donnerstag 15:15–18:45 Raum: H23<br />

MA 28.1 Do 15:15 H23<br />

A new method for atomistic spin-dynamics simulations with abintio<br />

accuracy — •Reinhard Singer, Ralf Drautz, and Manfred<br />

Fähnle — MPI für Metallforschung, Heisenbergstr. 3, 70569 Stuttgart,<br />

Germany<br />

We propose a new method to simulate the adiabatic spin dynamics in<br />

systems with magnetization singularities (vortex structures, Bloch points,<br />

extremely narrow domain walls in nanostripes etc.) with ab-initio accuracy<br />

but orders of magnitude faster than the conventional ab-initio spin<br />

dynamics simulation. It is based on the same equation of motion as the<br />

conventional simulation, i.e., a Gilbert like equation with a precission<br />

term and a damping term. In the conventional simulation the effective<br />

fields entering the precission term have to be calculated by the full abinitio<br />

electon theory which renders the method very costly. In contrast,<br />

we determine the effective fields by our recently proposed spin-clusterexpansion<br />

technique which represents an analytical expression for the<br />

magnetic energy of a spin system with arbitrary multi-spin interactions.<br />

The expansion coefficients thereby are determined by the ab-initio density<br />

functional electron theory. Therefore the construction of the expansion<br />

is costly, but once a converged spin-cluster expansion is generated it can<br />

be used to calculate the energy of any spin configuration very effectively<br />

and probably with ab-initio accuracy.<br />

MA 28.2 Do 15:30 H23<br />

Atomic-scale model of thermal effects on magnetic anisotropy<br />

and dynamic switching properties of FePt nano-particles —<br />

•Ulrich Nowak 1,2 , Oleg Mryasov 1 , Konstantin Guslienko 1 , and<br />

Roy Chantrell 1 — 1 Seagate Research, 1251 Waterfront Place, Pittsburgh,<br />

PA, 15222, USA — 2 Institut für Physik, Universität Duisburg-<br />

Essen, Germany<br />

We propose a microscopic atomic-scale model of magnetic interactions<br />

in ordered L10 FePt based on an effective, classical spin Hamiltonian<br />

which is constructed with the use of first-principles calculations for noncollinear<br />

magnetic configurations and site-resolved magneto-crystalline<br />

anisotropy energies. Our effective Hamiltonian contains, in addition to<br />

the isotropic exchange, an effective, anisotropic exchange due to the<br />

strong single-ion anisotropy of the Pt atoms.<br />

This model is studied numerically using Monte-Carlo methods as well<br />

as Langevin dynamics. The model allows to describe correctly the observed<br />

temperature dependence of the magnetic anisotropy energy of ordered<br />

FePt films. In particular the dependence of the anisotropy constant<br />

K1(T) on the reduced magnetization is found to be in excellent agree-<br />

ment with experimental results. Furthermore, the model is used to study<br />

the thermally induced switching behavior of FePt nano-particles. We find<br />

a strong deviation from the Neel-Brown theory for mean switching times<br />

due to the temperature dependence of the activation energy barrier.<br />

MA 28.3 Do 15:45 H23<br />

Zur Wechselwirkung von Magnetisierungswellen mit Néel-<br />

Wänden — •Riccardo Hertel, Wulf Wulfhekel und Jürgen<br />

Kirschner — Max-Planck-Institut für Mikrostrukturphysik, Weinberg<br />

2, 06120 Halle<br />

Magnetisierungswellen treten in ferromagnetischen Teilchen auf der<br />

Zeitskala von Piko- bis Nanosekunden auf, wenn die magnetischen Teilchen<br />

angeregt werden oder Energie freigesetzt wird, beispielsweise bei<br />

einem Ummagnetisierungsprozess. Vor kurzem konnten lokalisierte Magnetiserungswellen,<br />

die sich in einem Ferromagneten ausbreiten, resonant<br />

erzeugt und untersucht werden [1]. Mit mikromagnetischen Simulationen<br />

wird die Ausbreitung von Magnetisierungswellen in schmalen, dünnen<br />

Permalloy-Streifen (6 nm x 36 nm x 300 nm) untersucht, die sich infolge<br />

einer anfänglichen Störung des Gleichgewichts entlang des Streifens<br />

ausbreiten. Die Simulationen zeigen, dass diese Magnetisierungswellen<br />

eine Néelwand in den Streifen ungehindert passieren können. Dabei<br />

ändert sich jedoch die Phase der Welle. Dieser Effekt wird anhand<br />

eines ringförmigen Spinwelleninterferometers untersucht, bei dem Magnetisierungswellen<br />

auf kontrollierte Weise zu konstruktiven oder destruktiven<br />

Interferenzerscheinungen führen können, je nachdem ob sich<br />

Domänenwände in den jeweiligen Zweigen des Interferometers befinden.<br />

[1] Serga et al., J. Appl. Phys. 93 (10) 8585 (2003)<br />

MA 28.4 Do 16:00 H23<br />

Vortex-Dynamik in weichmagnetischen Dünnschichtelementen<br />

— •Riccardo Hertel und Jürgen Kirschner — Max-Planck-<br />

Institut für Mikrostrukturphysik, Weinberg 2, 06120 Halle<br />

Ferromagnetische Nanoteilchen treten entweder in nahezu homogener<br />

Magnetisierungsstruktur auf oder zerfallen in magnetische<br />

Domänenstrukturen mit geschlossenem magnetischen Fluß. Solche<br />

Domänenstrukturen beinhalten magnetische Wirbel (Vortex-<br />

Strukturen), die vor kurzem mit hoher Auflösung abgebildet werden<br />

konnten [1]. Mit Hilfe mikromagnetischer finite-Elemente Simulationen<br />

werden die dynamischen Eigenschaften von Vortexstrukturen untersucht.<br />

Dabei wird der Einfluß eines äußeren Wechselfelds auf die magnetische<br />

Vortexstruktur einer Permalloy-Kreisscheibe von 300 nm Durchmesser<br />

und 3 nm Dicke simuliert. Bei einer Frequenz von ca. 200 MHz tritt eine

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