Europhys. Lett.
Volume 70, Number 2, April 2005
Page(s) 250 - 256
Section Condensed matter: electronic structure, electrical, magnetic, and optical properties
Published online 18 March 2005
Europhys. Lett., 70 (2), pp. 250-256 (2005)
DOI: 10.1209/epl/i2004-10485-9

Magnetic anisotropy and its temperature dependence in iron-rich $\chem{Fe}$x$\chem{Pt}$1-x nanoparticles

C. Antoniak, J. Lindner and M. Farle

Fachbereich Physik, Universität Duisburg-Essen Lotharstr. 1, D-47048 Duisburg, Germany

received 17 November 2004; accepted in final form 2 March 2005
published online 18 March 2005

The effective magnetic anisotropy Keff of chemically disordered $\chem{Fe_{70}Pt_{30}}$ particles with a mean diameter of 2.3$\un{nm}$ is shown to be temperature dependent between 50$\un{K}$ and 350$\un{K}$. From the determination of the blocking temperatures by field-cooled and zero-field-cooled magnetisation measurements and ferromagnetic resonance experiments, that is in two different time windows, we find $K_{eff}=(8.4\pm0.9)\times10^5$ $\un{J/m^3}$ at 23$\un{K}$. This is found to be one order of magnitude larger than the bulk material value for the disordered phase. This value is confirmed by quantitative simulations of the experimentally determined zero-field-cooled magnetisation and can be explained by the large contribution of surface anisotropy, small deviations from a spherical shape and the presence of an approximately one monolayer thick iron oxide shell.

76.50.+g - Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance.
81.07.Bc - Nanocrystalline materials.
75.30.Gw - Magnetic anisotropy.

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