Study on Ordering Transition of L10-FePt Promoted by Doping Bi with First Principles

Doping the third element is an important method to promote ordering transition of FePt and reduce the ordering transition temperature. Bi can greatly reduce the ordering transition temperature and directly synthesize highly ordered FePt nanoparticles at low temperature, but the mechanism of Bi promoting FePt ordering and the role in ordering transition remain a challenge. Therefore, this work investigated the effects of Bi element on the lattice constant, formation energy, differential charge density, saturation magnetization and ordering transition temperature of FePt with the first principle density functional theory. The results show that the substitution formation energy of Bi atom replacing Fe atom is lower than that of Pt atom. Bi atom is easier to replace Fe atom. The substitution formation energy of Bi atoms replacing near neighbors is lower than that of far neighbors. Bi atoms tend to aggregate in the lattice. Fe atoms are the main source of magnetism in FePt systems, and Bi atoms have little effect on the electronic structure and magnetic properties of FePt at the Pt lattice site. The ordering transition temperature of FePt is significantly reduced after the doping of Bi element. The lowest ordering transition temperature of the FePt system is 623.32 K under double-site substitution. The doping position has greater influence on the ordering transition temperature than the doping concentration. The vacancy mechanism plays a leading role in the ordering transition. Bi element reduces the vacancy formation energy of FePt, increases the vacancy concentration of Fe and Pt atoms in the system, and promotes the diffusion and migration of Fe and Pt atoms, so as to promote the ordering transition of FePt.