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Dresden 2006 – scientific programme

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MA: Magnetismus

MA 32: Magnetic Particles / Clusters

MA 32.9: Talk

Thursday, March 30, 2006, 17:15–17:30, HSZ 401

Phase transformation of FePt nanoparticles from fcc to fct as probed by XMCD — •Ulf Wiedwald1, Birgit Kern1, Kai Fauth2, Andreas Klimmer1, Luyang Han1, Hans-Gerd Boyen1, and Paul Ziemann11Abteilung Festkörperphysik, Universität Ulm, Albert-Einstein-Allee 11, 89069 Ulm, Germany — 2Max-Planck-Institut für Metallforschung, Heisenbergstrasse 3, 70569 Stuttgart, Germany

FePt alloy nanoparticles show huge magnetic anisotropy energy in the chemically ordered L10 phase. The ordered phase is typically obtained by annealing at 600-800C starting from fcc, chemically disordered FePt nanoparticles. Nowadays, wet-chemical approaches like the synthesis of ligand-stabilized colloidal particles or the plasma-induced nucleation of metal salt loaded reverse micelles allow the self-organized formation in regular arrays onto various substrates. Hereby, the colloidal approach gives short interparticle spacing of 2-3 nm and the heat treatment at elevated temperatures is likely to form unwanted larger agglomerates of particles. By employing the micellar preparation route, the particle separation can be tuned between 20-100 nm. These larger distances permit us to study the phase transformation towards L10 ordered particles without any agglomeration, loss of the array quality or magnetostatic coupling between particles. We investigated FePt nanoparticles (4 nm and 9 nm) with spacing of 28 nm and 64 nm by XMCD. The phase transformation is tracked by hysteresis loops at various temperatures. In case of 9 nm particles we observe a coercive field of µ0H = 0.2 T at 340 K. The magnetic anisotropy rises by more than one order of magnitude due to annealing, while the total magnetic moment remains nearly constant.

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