Intrinsic Hard Magnetism and Thermal Stability of ThMn₁₂-Structure Permanent Magnet: Density Functional Theory and Monte Carlo Simulation

We propose a possible solution to realize an otherwise unstable ThMn₁₂-structure SmFe₁₂ permanent magnet through systematic full-potential density functional theory and Monte Carlo simulations on ternary Sm(Fe,M)₁₂ and quaternary Sm(Fe,Co,M)₁₂ compounds (M is a 3d or 3p metal substitute atom). Among the 11 metal substitutable elements (Ti–Ga and Al), only the simple metal Al, rather than the conventional transition metal substitute atoms, is predicted to be optimal. The presence of the Al substitute atoms not only stabilizes the ThMn₁₂ structure but also improves further the superior intrinsic magnetic properties to the state-of-the-art permanent magnet Nd₂Fe₁₄B. A quaternary Sm(Fe,Co,Al)₁₂ compound has the uniaxial magnetocrystalline anisotropy (MA) of 9.1 MJ·m^-3, anisotropy field of 19.2 T, and the magnetic hardness parameter of 2.8 at room temperature, and Curie temperature of 665K. Numerical results of MA and MA-driven hard magnetic properties can be described by the strong spin-orbit coupling and orbital angular momentum of the Sm 4f-electron orbitals. The other simple metal Ga, which is isoelectronic to Al, makes the present argument rather general, stabilizing the ThMn₁₂ structure while still preserving MA uniaxial.