Effect of the Ag evolution process on ordering the transition for L10-FePt nanoparticles synthesized by Ag addition

Abstract

As a typical element, Ag can effectively promote the ordering transition in the direct synthesis of L1₀-FePt nanoparticles (NPs). However, the role of Ag in the ordering process and the mechanism of ordering transition are still unclear. In this work, the L1₀-FePt NPs with a high coercivity (7.75 kOe) and ordering degree (0.887) have been synthesized by Ag addition. The evolution process of Ag in the ordering transition was firstly reported. This evolution process of Ag contains three stages. At the initial stage, the Ag and Pt precursors were reduced and formed smaller AgPt NPs. With the reaction progressing, the Fe atoms were reduced and diffused into the NPs to form the FePt phase. Meanwhile, the solubility of Ag in the FePt phase is low, and the excess Ag atoms migrated out of the NPs to form an Ag-rich area on the surface because of their lower surface energy. Finally, the Ag-rich area falls off the pure L1₀-FePt phase. The evolution behavior of Ag is important for understanding the mechanism of ordering transition. The diffusivity of Ag is much higher than that of Fe. The difference of diffusivity can generate a large number of vacancies in the lattice during the evolution process of Ag. The vacancies are beneficial to the ordering rearrangement of the Fe and Pt atoms, and lead to the form of a high structural ordering. This work revealed the evolution process and role of Ag in the ordering transition, which is significant to the direct synthesis of L1₀-FePt NPs on the third-element selection strategy.