Atomic ordering reaction and associated variation of magnetic coercivity of oriented L10-FePt nanoparticles

Atomically ordered FePt nanoparticles (L1₀-type structure) covered with amorphous (a-) Al₂O₃ have been fabricated. In this process, Fe particles were deposited on Pt "seed" particles which were epitaxially grown on (100) NaCl or MgO substrates. Annealing the a-Al₂O₃/Fe/Pt films at temperatures higher than 773 K leads to a formation of ordered nanoparticles with mutual fixed orientation in a monolayer form. Three variant ordered domains of the tetragonal L1₀ structure coexisted in a single nm-sized FePt particle, even in a particle as small as 7 nm. According to in-situ electron diffraction study, the degree of order of the ordered structure started to increase on annealing at 773 K and almost saturated on annealing at 873 K for 16 h. The magnetic coercivity varied depending on the particle size and the degree of order in the L1₀ structure formation. The perpendicular coercivity exceeded the in-plane one during the annealing. The in-plane coercivities of FePt nanoparticles measured both parallel to [100]_MgO and [010]_MgO directions were almost equal in numerical value. These results reflect the ordered domain formation process and the volume fraction of the domains. Remanent magnetization decay measured for the in-plane magnetization revealed a magnetic relaxation with the type of magnetic dipolar interaction between the FePt particles.