We have studied the size effect on the $\mathrm{L}{1}_0$ ordering of FePt (001) nanoparticles epitaxially grown on MgO (001). From the dark field images using 110 superlattice spots excited by incident electron beam along $\left[\overline{1}1l\right]$ $(l=4–6)$, the critical size for $\mathrm{L}{1}_0$ ordering has been evaluated to be $d=1.5–2\mathrm{nm}$ below which no ordering occurs. Further, we have taken an electron diffraction pattern of each FePt nanoparticle using a nanometer-sized electron beam and determined the respective long-range order parameter $S$ by analyzing the superlattice/fundamental diffraction intensity ratio based on the multislice method. It is found that the order parameter $S$ sharply drops below $d\sim 3\mathrm{nm}$ and decreases to zero for $d<2\mathrm{nm}$, the result is almost consistent with thermodynamic calculations previously reported. The present work unambiguously shows that the ordering of $\mathrm{L}{1}_0$ FePt is entirely inhibited when its size is less than $d\sim 2\mathrm{nm}$. 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 $d<2\mathrm{nm}$ where the $\mathrm{L}{1}_0$ FePt would be magnetically unstable due to severe thermal agitation.

• Received 16 November 2004

DOI:https://doi.org/10.1103/PhysRevB.72.144419