Ordering process and size effect of FePt magnetic thin films

This study focuses on the relationship between the microstructure and the magnetic properties of in-situ ordered L1₀-FePt thin films. Results from selected area electron diffraction (SAED) and transmission electron microscopy (TEM) analyses show that the ordering in the thin films progresses by the abnormal grain growth of disordered nano-grains. The driving force of the abnormal grain growth is the grain boundary and strain energies that are accumulated in the films with the film thickness. When the particle size is smaller than 3.5 nm, the ordering does not progress even after prolonged annealing time at 600°C. The degree of order increases to 1 as the particle size increases and the critical size of the ordering is found to be about 4 nm. This critical size of ordering depends on the interfacial energy and it is also smaller than the superparamagnetic limit of the L1₀ ordered FePt particles.