Abstract: In this paper, Fe$_{30}$Pt$_{70}$/Fe$_{3}$O$_{4}$ core/shell nanoparticles were synthesized by chemical routine and the layered polyethylenimine (PEI)-Fe$_{30}$Pt$_{70}$/Fe$_{3}$O$_{4 }$ structure was constructed by molecule-mediated self-assembly technique. The dimension of core/shell structured nanoparticles was that of 4nm core and 2 nm shell. After annealing under a flow of forming gas (50%Ar$_{2}$+30%H$_{2})$ for 1 h at or above 400℃, the iron oxide shell was reduced to Fe and diffused to Pt-rich core, which leaded to the formation of L1$_{0}$ phase FePt at low temperature. The x-ray diffraction results and magnetic properties measurement showed that the chemical ordering temperature of Fe$_{30}$Pt$_{70}$/Fe$_{3}$O$_{4}$ core/shell nanoparticles assembly can be reduced to as low as 400℃. The sample annealed at 400℃ showed the coercivity of 4 KOe with the applied field of 1.5T. The core/shell structure was suggested to be an effective way to reduce the ordering temperature obviously.   

Key words: FePt, core/shell structure, nanoparticles, magnetic properties