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MA: Magnetismus
MA 14: Poster: Schichten(1-31), Spinabh.Trsp.(32-47), Exch.Bias(48-54), Spindyn.(55-64), Mikromag.(65-76), Partikel(77-88), Oflmag.(89-92), Spinelektr.(93-98), Elektr.Theo.(99-103), Mikromag+PhasÜ+Aniso.(104-122), MagnMat.(123-134), Messm+Mol-Mag.(135-139), Kondo(140-151)
MA 14.84: Poster
Tuesday, March 25, 2003, 15:15–19:15, Zelt
Anisotropic Superparamagnetism of Monodispersive Cobalt-Platinum Nanocrystals — •J. Kötzler1, F. Wiekhorst1, E. Shevchenko2, and H. Weller2 — 1Institut für Angewandte Physik Universität Hamburg, D-20355 Hamburg — 2Institut für Physikalische Chemie, Universität Hamburg, D-20146 Hamburg
Based on the high-temperature organometallic route (Sun et al. Science 287, 1989 (2000)), we have synthesized powders containing CoPt3 single crystals with mean diameters of 3.3(2) nm and 6.0(2) nm. In the entire temperature range,5 K to 400 K, the zero-field cooled susceptibility χ(T) displays significant deviations from ideal superparamagnetism. Approaching the Curie temperature of 450(10) K, the deviations arise from the (mean-field) type reduction of the ferromagnetic moments. Below the blocking temperature Tb, χ(T) is suppressed by the presence of energy barriers, the distributions of which scale with the particle volumes obtained by TEM. This indication for volume anisotropy is supported by the shape of the magnetic absorption χ″(T,ω). Above 200 K, the magnetization isotherms M(H,T) display Langevin behavior providing 2.5(1) µB per CoPt3 in agreement with reports on bulk and thin film CoPt3. The non-Langevin shape of the magnetization curves at lower temperatures is for the first time interpreted as anisotropic superparamagnetism by taking into account an anisotropy energy of the nanoparticles EA(T). The hysteresis loops below Tb are quantitatively described by a blocking model, which also ignores particle interactions, but takes the size distributions from TEM into account.