XVIth International Workshop on
Quantum Systems in
Chemistry and Physics
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Ab-initio calculations of magnetic nanostructures
László Udvardi, László Balogh, László Szunyogh
Department of Theoretical Physics, Budapest University of Technology and Economics, Hungary
As the magnetic storage devices approach a physical limit of a single atom the investigation of nanoclusters has become one of the important subjects in magnetism. Recent developments in nano technology permit to construct clusters with well-controlled structures and enable the measurement of various magnetic properties in atomic resolution. Ab-initio calculations on magnetic nano-structures are necessary for a clear interpretation of experimental results and to attain full understanding of the underlying physical phenomena.

Our calculations are based on the local density approximation (LDA) of the density functional theory (DFT). The effective one-particle problem is solved by means of the fully relativistic version of the Korringa-Kohn-Rostoker method using the multiple scattering formalism1. For thin magnetic films surface Green's function technique has been applied to treat the semi infinite substrate and embedded-cluster Green's function method2 is used for the proper description of deposited magnetic clusters. In order to find the magnetic ground-state of nano-clusters and to analyse their stability
a new method is developed based on the gradients and second order derivatives of the band energy with respect of the transverse magnetization. The capability of the approach is demonstrated on a magnetic domain wall through a point contact.

Multiscale approaches are extremely useful tools to explore the magnetic properties of systems which would be too large to treat them with the standard methods of the density functional theory. In the most widely applied approximations the energy of a magnetic system is mapped onto a classical Heisenberg model. For the inclusion of relativistic effects such as the magnetic anisotropy and Dzyaloshinsky-Moriya interaction the scalar exchange coupling between the spins must be replaced by tensorial coupling and the model must be extended by terms responsible for the on-site anisotropy. In the framework of the multiple scattering theory the exchange interactions can easily be calculated by the torque method proposed by Liechtenstein3. With the relativistic version of the torque method4 we are able to determine the full coupling tensor. By means of Monte Carlo simulations and atomistic spin dynamics complicated spin-spiral ground state configuration which has been detected experimentally by spin polarized STM measurements5
could be reproduced using the calculated exchange couplings.

[1] L Szunyogh and B Ujfalussy and P Weinberger and J Kollar, Phys. Rev. B, 49, 2721 (1994)
[2] Lazarovits, B. and Szunyogh, L. and Weinberger, P., Phys. Rev. B, 65, 104441 (2002)
[3] A. I. Liechtenstein and M. I. Katsnelson and V. P. Antropov and V. A. Gubanov, JMMM 67, 65 (1987)
[4] Udvardi, L. and Szunyogh, L. and Palotas, K. and Weinberger, P., Phys. Rev. B, 68, 104436 (2003)
[5] Bode, M. and Heide, M. and von Bergmann, K. and Ferriani, P. and Heinze, S. and Bihlmayer, G. and Kubetzka, A. and Pietzsch, O. and Bl\"ugel, S. and Wiesendanger, R., Nature 447, 190 (2007)

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