The transformation of ≤4 nm equiatomic FePt nanoparticles from the disordered cubic A1 to the ordered tetragonal L10 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 L10 phase. The well-controlled and precise stoichiometry (Fe50 at.%Pt50 at.%) and the homogeneous composition of these particles both play a major role in their successful phase transformation.
The authors visualize a phase change from fcc to L10 in FePt nanoparticles. L10 FePt nanoparticles show strong uniaxial magnetocrystalline anisotropy, and show promise for a new type of data storage media. The phase change is shown in high resolution
using a JEOL 2200FS Cs corrected STEM.