Formation of Fe3Pt phase in FePt-based nanocomposite magnets

Atomic interdiffusion between FePt and Fe3O4 nanoparticles in annealed FePt-based nanocomposite magnets has been studied by means of structural and magnetic characterizations. The results show that the Fe3Pt phase is formed during the annealing only when the mass ratio x of Fe3O4/FePt is larger than 1/20. When x <= 1/20, only FePt single phase is formed. It is interesting to find that the coercivity of the annealed samples increases with a small addition of Fe3O4 before the formation of the Fe3Pt phase. This magnetic hardening behaviour indicates that the composition of the FePt phase can be further adjusted via the post annealing process. The characteristic of recoil loops and Henkel plots also give evidence for the transition from single-phase FePt magnets to nanocomposite magnets with the addition of Fe3O4.