Fe oxidation versus Pt segregation in FePt nanoparticles and thin films

Metallic nanoparticles containing 3d elements are generally susceptible to oxidation leading to a deterioration of desired properties. Here, the oxidation behavior of differently sized FePt nanoparticles is experimentally studied by x-ray photoelectron spectroscopy (XPS) and compared to a FePt reference film. For all as-prepared metallic samples the common features are the formation of Fe3+, becoming detectable for exposures to pure oxygen above 10(6) langmuir whereas under identical conditions the Pt-0 signal is conserved. Most notably, these features are independent of particle size. Annealing at 650 degrees C, however, affects small and large FePt particles differently. While large particles as well as the reference film show a 100-1000 times enhanced resistance against oxidation, small FePt particles (diameter 5 nm) exhibit no such enhancement due to the thermal treatment. Additional XPS intensity analysis in combination with model calculations leads to an explanation of this observation in terms of Pt segregating to the surface. In large particles and films the thickness of the resulting Pt layer is sufficient to strongly impede oxidation, while in small particles this layer is incomplete and no longer provides protection against oxidation.