FePt nanoparticles formed in Al2O3 by ion beam synthesis: Annealing environment effects

The properties of FePt nanoparticles formed by the implantation of Fe+Pt into c-axis-oriented Al2O3 single crystals followed by thermal annealing are shown to be strongly dependent on the annealing environment. Annealing in a reducing environment (flowing Ar+4% H2, or ultrahigh vacuum) gives rise to ferromagnetic FePt nanoparticles with the L10 structure and very high magnetic coercivity (greater than 20 kOe). FePt alloy formation does not occur during annealing in an oxidizing environment. Instead, the implanted Pt precipitates out forming oriented Pt nanoparticles and the implanted Fe redistributes with ∼40% segregating to the surface where it forms epitaxial α-Fe2O3 precipitates at the surface; the remainder of the implanted Fe remains in the bulk, most likely in solid solution in the matrix. Results obtained by sequential annealing of Fe+Pt implanted samples in reducing (oxidizing) environments followed by annealing in an oxidizing (reducing) environment suggest that equilibrium, rather than kinetic, effects are responsible for the observed microstructures.