Direct synthesis of Pt based L1 ₀ structured nanoparticles (invited)

Equiatomic FePt and CoPt nanoparticles with the ordered L 10 structure are attractive as ultrahigh density magnetic recording media. In a recent work, chemically synthesized fcc-FePt nanoparticles with narrow size distribution and their self-assembled array with close-packed microstructure has been achieved successfully. However, the particles coalesced during the subsequent annealing step necessary to obtain L 10 FePt nanoparticles. In the present study, we have successfully demonstrated the direct synthesis of L 10 FePt nanoparticles at low temperature of 553 K using the "modified polyol method" without subsequent annealing, whose diameter is 5-10 nm and intrinsic magnetocrystalline anisotropy field (Hk) is 31 kOe. This indicates that precisely controlling the reaction kinetics, especially low reduction rate through optimizing the polyol/Pt mole ratio and type of polyol are very important for directly synthesizing the L 10 FePt nanoparticles. Furthermore, we investigated the size, morphology and composition dependence of the magnetic properties of FePt nanoparticles in order to clarify the L 10 ordering mechanism. As a result, clear evidence of the existence of the critical diameter for the thermodynamical L 10 ordering is not observed in the size range above 2 nm. Furthermore, the recrystallization and sintering process can be a driving force for promoting the L 10 ordering, and hence, Fe and Pt atom diffusion at the grain boundary plays an important role to the L 10 ordering of FePt nanoparticles.