Size effect on the ordering of L10 FePt nanoparticles

We have studied the size effect on the L10 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 [1̄1l] (l=4-6), the critical size for L10 ordering has been evaluated to be d=1.5-2nm 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∼3nm and decreases to zero for d<2nm, the result is almost consistent with thermodynamic calculations previously reported. The present work unambiguously shows that the ordering of L10 FePt is entirely inhibited when its size is less than d∼2nm. 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<2nm where the L10 FePt would be magnetically unstable due to severe thermal agitation.