Quantitative analysis of ordering of equiatomic FePt nanoparticles by electron diffraction

In the present work, we investigate the size effect on the L1₀ ordering of FePt [001] epitaxial particles. From the dark field images using superlattice 110 spots, we clarify the critical size for L1₀ ordering of FePt. Furthermore, we investigate the order parameter S of each FePt nanoparticle from its electron diffraction pattern taken by nanometer-size electron beam, and clarify the relationship between the particle size d and the order parameter S. The present work has unambiguously shown that the ordering of L1₀ FePt is entirely suppressed when its size is less than d ∼ 2 nm. Such size effect is not so serious for practical applications of FePt to permanent magnets or magnetic recording media, because the effect is significant only for d < 1.5 ∼ 2 nm which is much smaller than the superparamagnetic limit of L1₀ FePt.