Phase stability and coercivity in La 2 Fe 14 B magnet

Abstract

Critical rare-earth free La 2 Fe 14 B (2:14:1) has the potential to be a gap permanent magnet. However, La 2 Fe 14 B decomposes into La, α-Fe, and La 4 Fe 4 phases below 1067 K. The phase stability and coercivity have been studied in La 2 Fe 14 B magnet using first principles DFT (density functional theory) calculation and micromagnetic simulation. For a perfect La 2 Fe 14 B cube (edge length of 256 nm) without any structural defects and soft magnetic secondary phases, the coercivity (8.5 kOe) is reduced to ~40% of its magnetocrystalline anisotropy field ( H A = 20 kOe). Further, the coercivity sharply reduces to 3.2 kOe upon forming a thin layer (2 nm) of α-Fe on the surface of the La 2 Fe 14 B cube particle. The DFT calculations indicate that a partial replacement of La by other rare-earth (RE) elements can enhance the structural stability of 2:14:1. The gains in cohesive energy are 0.75, 0.10, and 0.33 eV per formula unit in (La 0.5 RE 0.5 ) 2 Fe 14 B with RE = Ce, Pr, and Nd, respectively. Stabilizing the 2:14:1 structure and mitigating the formation of soft magnetic structural defects or impurity phases such as α-Fe is necessary to develop La 2 Fe 14 B based magnet, which can be moderately achieved via partial substitution of La by other rare earth elements such as Ce, Pr, and Nd.