Size effect on the ordering of FePt granular films

We have investigated the ${\text{A1→L1}}_0$ ordering process of FePt granular films with different particle sizes. Although FePt particles larger than ∼7 nm ordered by annealing at 600 °C, the ordering did not proceed when the particle size was smaller than ∼4 nm in diameter. This suggests that there is a size dependence on the ordering of FePt particles. Calculation of the Helmholtz free energy using the Lennard-Jones potential suggested that $T_c$ decreases below the annealing temperature when the particle size is decreased to a few nanometers suggesting that experimental observation is physically feasible.