Abstract: L1₀-FePt nanoparticles (NPs) are urgently anticipated because of their promising applications. However, the preparation of the NPs with both of high ordering degree and super-fine size is still a challenge. Inspired by recent studies on the effect of vacancy defects on structural ordering, we proposed an intentional vacancy defect design strategy for directly synthesizing highly ordered FePt NPs. In the present work, we used the first-principle calculations to investigate the influence of doping typical elements (Cu, Ag, and Pb) on the vacancy formation energy (E_vac) of FePt NPs. The vacancy defects were effectively formed by introducing elements of larger atomic radii and higher propensity for segregation into the FePt lattice, facilitating the diffusion of Fe and Pt atoms. The Pb doping showed remarkable efficacy in promoting the ordering transition. Experimentally, wet-chemical synthesis confirmed the success of the proposed strategy in achieving highly ordered L1₀-FePt NPs with exceptional magnetic properties and super-fine size (ordering degree of 0.896, impressive coercivity of 21.74 kOe, and small particle size of 9.02 nm). Additionally, we have deduced a diffusion model elucidating the formation process of the ordered FePt NPs, focusing on the migration of Pb atoms from the center to the surface of the particles. This migration is demonstrated to generate more vacancies and promote the transition to the ordered L1₀-FePt phase. The findings of this research offer valuable insights into synthesizing highly ordered and ultrafine L1₀-type nanomaterials.

Key words: L 10 -FePt nanoparticles, Vacancy defect, Phase segregation, Ordering transition, First-principle calculation