Disorder-order transition in FePt nanoparticles mediated by Bi doping: a combined first-principles and experimental...

This study systematically investigates the effects of varying Bi doping concentrations on the magnetic properties and disorder-order transition of FePt nanoparticles (NPs) by combining first-principles calculations with experimental validation. The calculations indicate that a low concentration of Bi can be effectively doped into the FePt lattice, whereas the contribution of vacancy-assisted diffusion becomes increasingly important at higher Bi concentrations. Bi doping significantly reduces the vacancy formation energy and lowers the atomic diffusion barrier from 1.07 eV to 0.76 eV, thereby accelerating the disorder-order transition and enhancing magnetic properties. Experimental results confirm that the appropriate level of Bi doping promotes L1₀ ordering. FePt NPs with an ordering degree of S = 0.925 and a coercivity (H_c) of 11.05 kOe were obtained with the addition of 0.3 mmol Bi. However, excessive Bi doping induces particle aggregation and reduces their magnetic performance. Our findings reveal that the ordering transition in FePt NPs is governed by the synergistic interplay between Bi-induced lattice strain and vacancy-assisted diffusion kinetics. By elucidating these dual regulatory pathways, this study provides a viable paradigm for the controllable synthesis of L1₀-FePt nanomaterials with high structural ordering.