Synthesis of super-fine L10-FePt nanoparticles with high ordering degree by two-step sintering under high magnetic field

Abstract Super-fine L10-FePt nanoparticles (NPs) with high ordering degree were successfully prepared by a modified two-step sintering method, which includes low-temperature pre-sintering, and the high magnetic field (HMF) assisted post-sintering processes. The particle size of the L10-FePt NPs was obviously refined by lowering the sintering temperature. By applying the HMF during the post-sintering process, the fine size characteristics of L10-FePt NPs were retained, and the ordering degree was significantly improved. The L10-FePt NPs with sizes of about 4.5 nm, ordering degree of 0.940, and coercivity of 22.01 kOe were obtained by this two-step sintering under a magnetic field of 12 T. The mechanism investigation of HMF enhancing the ordering degree indicates that the HMF enhances lattice distortion and magnetization energy (Zeeman energy). The enhanced lattice distortions cause high stress existing in the lattice, which can effectively promote the disordered-order transition. When the magnetic field reaches to 3 T, the Zeeman energy of the NPs is higher than the thermal disturbing energy of the NPs, and the magnetization effect is stronger. Therefore, the HMF (higher than 3 T) can obviously improve the disorder-order transition by lowering the energy barrier and accelerating the orderly diffusions of atoms. The HMF is a promising assistant to synthesize the L10-phase NPs with both of high ordering degree and super-fine size.