Disorder-Order Transformation and Local Structure Changes of FePt Nanoparticles Synthesized by Polyol Process

The ordering process of FePt nanoparticles synthesized in polyol solution at 383 K was investigated. Whereas X-ray diffraction peaks of the as-synthesized specimen corresponded to the fcc structure, the Fe-O correlation and Pt-O correlation peaks were observed in the radial structure functions of Fe and Pt atoms obtained by X-ray absorption spectroscopy. Thus, the as-synthesized specimen was composed of crystalline FePt alloy phase and poorly crystallized Fe- and Pt-oxide phase. In X-ray diffraction patterns of the specimens which were heat-treated at temperatures above 568 K in a mixture of 5%H₂ in Ar, diffraction peaks corresponding to superlattice lines were observed. Although the lattice constants of the as-synthesized specimen were much larger than that of the bulk fcc-FePt alloy with equiatomic composition, the lattice constants of the specimen heat-treated at 1173 K were very close to the bulk L1₀-FePt alloy. Additionally, in the radial structure function, peaks corresponding to the Fe-O and Pt-O correlations disappeared and peaks corresponding to Fe-Pt (or Fe) and Pt-Fe (Pt) correlations clearly appeared after heat treatment at temperatures above 568 K. Consequently, it is highly probable that the disorder-order transformation of FePt nanoparticles synthesized in polyol solution is accompanied by compositional homogenization due to the reduction of the poorly crystallized Fe- and Pt-oxide phase.