Chemically disordered wet-chemically synthesised FePt nanoparticles [1,2] with diameters around 6nm were self-assembled in 2D islands on Si substrates. The magnetic moments of the Fe and Pt atoms of naturally oxidised particles were measured before and after removal of the oleylamine and oleic acid ligands by means of x-ray magnetic circular dichroism (XMCD). We find by XMCD at the Fe L_3,2 edges that the particles contain a number of Fe atoms corresponding to less than one monolayer equivalent which are in an oxidic environment like α-Fe₂O₃. No influence of the ligand shell on the formation of oxides or on the magnetic properties was detected. After removal of the oxide shell by a soft hydrogen plasma treatment, the particles are transformed into a pure metallic state with an effective spin (orbital) magnetic moment per atom of 2.48µ_B (0.056µ_B) at the Fe sites and 0.41µ_B (0.054µ_B) at the Pt sites. Compared to the oxidised state, the values of the spin magnetic moments per unoccupied d-state at the Fe sites increased by a factor of 4, whereas at the Pt sites no significant changes are observed - confirming the initial guess of an antiferromagnetic Fe oxide. The possibility to identify a Pt enriched particles surface by high-resolution transmission electron microscopy (HR-TEM) analyses of the lattice expansion and the z-contrast will be discussed.

This work was supported by Deutsche Forschungsgemeinschaft (SFB 445), the Marie Curie Research Training Network “SyntOrbMag”, the BMBF (05 ES3XBA/5) and the ESRF.

[1] C. Antoniak, J. Lindner and M. Farle; Europhysics Letters 70 (2005) 250
[2] M. Ulmeanu, C. Antoniak, U. Wiedwald, M. Farle, Z. Frait, and S. Sun; Phys. Rev. B 69 (2004) 054417

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