Plenarvorträge - DPG-Tagungen

Magnetismus Donnerstag
magnon wave-vector and, therefore, on its energy. This dependence can
explain strongly non-monotonous inelastic tunneling spectra measured in
Refs.[1].
[1] J.S. Moodera, J. Nowak, and R.J.M. van de Veerdonk,
Phys.Rev.Lett. 80, 2941 (1998); Y. Ando, J. Murai, H. Kubota, and T.
Miyazaki, J. Appl. Phys. 87, 5209 (2000); J.H. Yu, H.M. Lee, Y. Ando,
and T. Miyazaki, Appl. Phys. Lett. 82, 4735 (2003).
[2] S. Zhang, P.M. Levy, A.C. Marley, and S.S.P. Parkin,
Phys.Rev.Lett. 79, 3744 (1997).
MA 27.13 Do 18:15 H22
Persistent Spin Currents in Helimagnets — •Jürgen König 1 ,
Jan Heurich 2 , and Allan H. MacDonald 3 — 1 Institut für Theoretische
Physik III, Ruhr-Universität Bochum — 2 Institut für Theoretische
Festkörperphysik, Universität Karlsruhe — 3 Department of Physics,
University of Texas at Austin
We study collective spin transport in helimagnets, e.g., in systems with
ground states that possess spiral magnetic order. It is demonstrated that
weak external magnetic fields generate dissipationless spin currents in
these systems, whereas the spiral ground state in the absence of a magnetic
field does not support collective spin transport. Our conclusions are
based on phenomenological considerations and on microscopic mean-field
theory calculations for an illustrative toy model [1]. We discuss similarities
and differences of these results to our recent analysis of spin supercurrents
in thin film ferromagnets [2], and we speculate on possible
applications of this effect in spintronic devices.
[1] J. Heurich, J. König, and A.H. MacDonald, Phys. Rev. B 68, 064406
(2003).
[2] J. König, M.C. Bønsager, and A.H. MacDonald, Phys. Rev. Lett. 87,
187202 (2001).
MA 27.14 Do 18:30 H22
Anomalous Hall effect and Berry Phase in Ferromagnets —
•Mathieu Taillefumier 1,2 , Vitalii Dugaev 1,3 , and Patrick
Bruno 1 — 1 Max Planck Institut für mikrostrukturphysik, Weinberg 2,
06120 Halle, Germany — 2 Laboratoire Louis Néel, CNRS, Boite Postale
166,, 38042 Grenoble Cedex 09, France — 3 Institute for Problems of
Materials Science, NASU, Vilde 5, 58001 Chernovtsy, Ukraine
We propose a new mechanism of anomalous Hall effect (AHE) in twodimensional
ferromagnets and magnetic nanostructures with inhomogeneous
magnetization. In contrast to the usual theories (side jump and
skew scattering mechanism) of AHE, the presented theory does not necessary
require spin-orbit interaction, and is induced by a Berry phase of
the wavefunctions of the electrons moving a inhomogeneous magnetization
field. We obtain a nonvanishing contribution to the Hall conductivity
provided that the topology of the magnetization field is nontrivial like in
the case of separated magnetic domains or periodic lattice of magnetic
domains.
We propose an experiment with a structure containing two-dimensional
electrons or holes of diluted magnetic semiconductor subject to the stray
field of a lattice of magnetic nanocylinders. The striking behavior predicted
for such a system (of which all revelant parameters are well known)
allows to distinguish it from the other mechanisms.
MA 28 Spindynamik / Ummagnetisierungsvorgänge
Zeit: Donnerstag 15:15–18:45 Raum: H23
MA 28.1 Do 15:15 H23
A new method for atomistic spin-dynamics simulations with abintio
accuracy — •Reinhard Singer, Ralf Drautz, and Manfred
Fähnle — MPI für Metallforschung, Heisenbergstr. 3, 70569 Stuttgart,
Germany
We propose a new method to simulate the adiabatic spin dynamics in
systems with magnetization singularities (vortex structures, Bloch points,
extremely narrow domain walls in nanostripes etc.) with ab-initio accuracy
but orders of magnitude faster than the conventional ab-initio spin
dynamics simulation. It is based on the same equation of motion as the
conventional simulation, i.e., a Gilbert like equation with a precission
term and a damping term. In the conventional simulation the effective
fields entering the precission term have to be calculated by the full abinitio
electon theory which renders the method very costly. In contrast,
we determine the effective fields by our recently proposed spin-clusterexpansion
technique which represents an analytical expression for the
magnetic energy of a spin system with arbitrary multi-spin interactions.
The expansion coefficients thereby are determined by the ab-initio density
functional electron theory. Therefore the construction of the expansion
is costly, but once a converged spin-cluster expansion is generated it can
be used to calculate the energy of any spin configuration very effectively
and probably with ab-initio accuracy.
MA 28.2 Do 15:30 H23
Atomic-scale model of thermal effects on magnetic anisotropy
and dynamic switching properties of FePt nano-particles —
•Ulrich Nowak 1,2 , Oleg Mryasov 1 , Konstantin Guslienko 1 , and
Roy Chantrell 1 — 1 Seagate Research, 1251 Waterfront Place, Pittsburgh,
PA, 15222, USA — 2 Institut für Physik, Universität Duisburg-
Essen, Germany
We propose a microscopic atomic-scale model of magnetic interactions
in ordered L10 FePt based on an effective, classical spin Hamiltonian
which is constructed with the use of first-principles calculations for noncollinear
magnetic configurations and site-resolved magneto-crystalline
anisotropy energies. Our effective Hamiltonian contains, in addition to
the isotropic exchange, an effective, anisotropic exchange due to the
strong single-ion anisotropy of the Pt atoms.
This model is studied numerically using Monte-Carlo methods as well
as Langevin dynamics. The model allows to describe correctly the observed
temperature dependence of the magnetic anisotropy energy of ordered
FePt films. In particular the dependence of the anisotropy constant
K1(T) on the reduced magnetization is found to be in excellent agree-
ment with experimental results. Furthermore, the model is used to study
the thermally induced switching behavior of FePt nano-particles. We find
a strong deviation from the Neel-Brown theory for mean switching times
due to the temperature dependence of the activation energy barrier.
MA 28.3 Do 15:45 H23
Zur Wechselwirkung von Magnetisierungswellen mit Néel-
Wänden — •Riccardo Hertel, Wulf Wulfhekel und Jürgen
Kirschner — Max-Planck-Institut für Mikrostrukturphysik, Weinberg
2, 06120 Halle
Magnetisierungswellen treten in ferromagnetischen Teilchen auf der
Zeitskala von Piko- bis Nanosekunden auf, wenn die magnetischen Teilchen
angeregt werden oder Energie freigesetzt wird, beispielsweise bei
einem Ummagnetisierungsprozess. Vor kurzem konnten lokalisierte Magnetiserungswellen,
die sich in einem Ferromagneten ausbreiten, resonant
erzeugt und untersucht werden [1]. Mit mikromagnetischen Simulationen
wird die Ausbreitung von Magnetisierungswellen in schmalen, dünnen
Permalloy-Streifen (6 nm x 36 nm x 300 nm) untersucht, die sich infolge
einer anfänglichen Störung des Gleichgewichts entlang des Streifens
ausbreiten. Die Simulationen zeigen, dass diese Magnetisierungswellen
eine Néelwand in den Streifen ungehindert passieren können. Dabei
ändert sich jedoch die Phase der Welle. Dieser Effekt wird anhand
eines ringförmigen Spinwelleninterferometers untersucht, bei dem Magnetisierungswellen
auf kontrollierte Weise zu konstruktiven oder destruktiven
Interferenzerscheinungen führen können, je nachdem ob sich
Domänenwände in den jeweiligen Zweigen des Interferometers befinden.
[1] Serga et al., J. Appl. Phys. 93 (10) 8585 (2003)
MA 28.4 Do 16:00 H23
Vortex-Dynamik in weichmagnetischen Dünnschichtelementen
— •Riccardo Hertel und Jürgen Kirschner — Max-Planck-
Institut für Mikrostrukturphysik, Weinberg 2, 06120 Halle
Ferromagnetische Nanoteilchen treten entweder in nahezu homogener
Magnetisierungsstruktur auf oder zerfallen in magnetische
Domänenstrukturen mit geschlossenem magnetischen Fluß. Solche
Domänenstrukturen beinhalten magnetische Wirbel (Vortex-
Strukturen), die vor kurzem mit hoher Auflösung abgebildet werden
konnten [1]. Mit Hilfe mikromagnetischer finite-Elemente Simulationen
werden die dynamischen Eigenschaften von Vortexstrukturen untersucht.
Dabei wird der Einfluß eines äußeren Wechselfelds auf die magnetische
Vortexstruktur einer Permalloy-Kreisscheibe von 300 nm Durchmesser
und 3 nm Dicke simuliert. Bei einer Frequenz von ca. 200 MHz tritt eine