L 1 ₀ ordering of ultrasmall fept nanoparticles revealed by TEM in situ annealing

The transformation of ≤4 nm equiatomic FePt nanoparticles from the disordered cubic A1 to the ordered tetragonal L1 ₀ phase was studied by means of high-resolution transmission electron microscopy coupled with in situ heating experiments. In accordance with ex situ annealing experiments, a transition temperature of around 500 °C was determined. Diffusion is enhanced at surfaces and plays a dominant role in the ordering process. Hence, the ordering of the crystallographic structure starts at the surface of the nanoparticles and propagates toward their center, resulting in complete ordering within some minutes, for temperatures above 600 °C. Unlike the generally assumed lower limit of ordering (3.5 nm), we demonstrate that ultrasmall (less than 3 nm) FePt nanoparticles can also be fully transformed to the L1 ₀ phase. The well-controlled and precise stoichiometry (Fe _50at.%Pt ₅₀ at.%) and the homogeneous composition of these particles both play a major role in their successful phase transformation.