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Dresden 2006 – scientific programme

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MA: Magnetismus

MA 33: Spin-Dynamics, Magnetization Reversal III

MA 33.4: Talk

Thursday, March 30, 2006, 16:00–16:15, HSZ 403

Decrease of entropy in magnetic particles — •H.J. Elmers1, A. Krasyuk1, F. Wegelin1, S.A. Nepijko1, A. Conca1, G. Schönhense1, M. Bolte2, and C.M. Schneider31Johannes Gutenberg-Universität Mainz, Institut für Physik, Staudingerweg 7, D-55099 Mainz — 2Universität Hamburg, Institut für Angewandte Physik, Jungiusstrasse 11, D-20355 Mainz — 3Forschungszentrum Jülich GmbH, Institut für Festkörperforschung IFF-6, D-52425 Jülich

In a closed system entropy maximization tends to decrease order. An open system with a constant throughput of energy, however, allows for an increase of local order. Exciting micron-sized permalloy particles with an oscillating external field we found an example for this phenomenon. The external oscillating field is the energy source while the internal damping plays the role of the sink. In a rectangular platelet (16 µm×32 µm×10 nm) the equilibrium magnetization state is formed by a symmetric flux-closure domain pattern comprising two equally-sized domains separated by a 180 degree domain wall. The external oscillating field is applied off-resonance along the short side of the platelet, thus exciting a precessional motion of the magnetization. The system reacts by increasing one magnetic domain at the expense of the others. The final state has uniform magnetization and is thus completely ordered. The basic mechanism, revealed by both stroboscopic imaging with time-resolved photoelectron emission microscopy and computer simulation, is the decrease of the resonance frequency in the larger domain. This leads to larger energy dissipation in the system, allowing increasing order with increasing entropy.

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