Dresden 2011 – scientific programme

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MA: Fachverband Magnetismus

MA 19: Poster I (Bio- and Molecular Magnetism/ Magnetic Particles and Clusters/ Micro- and Nanostructured Magnetic Materials/ Magnetic Materials/ Multiferroics/ Magnetic Shape Memory Alloys/ Electron Theory of Magntism/ Spincaloric Transport/ Magnetic Coupling and Exchange Bias/ Magnetization Dynamics/ Micromagnetism and Computational Magnetics)

MA 19.89: Poster

Tuesday, March 15, 2011, 10:45–13:00, P2

Limitations of the macro-spin model for magnetic nanoparticles — •Mohammad Sayad, Daniel Guetersloh, and Michael Potthoff — I. Institut für Theoretische Physik, Universität Hamburg, Jungiusstraße 9, 20355 Hamburg

Magnetization reversal of a magnetic nanoparticle or of a molecular magnet is often described by means of a macro-spin model assuming a strong exchange coupling between the individual spins. This model, however, is a phenomenological construct. Here we discuss the strict microscopic derivation of the macro-spin model in the limit of weak anisotropy. In addition the limitations of the model are worked out by studying linear chains as well as two- and three-dimensional clusters of ferromagnetically exchange-coupled Heisenberg quantum spins with single-site or coupling anisotropy. Magnetization profiles, correlation functions, excitation gaps and the tunneling barrier are computed by exact diagonalization and the Lanczos method for spin-S systems as a function of the system size L, the anisotropy strength and the system geometry. In the case of the classical Heisenberg model, we determine the transition between different reversal mechanisms as a function of system size, system geometry and applied external magnetic field.