Upper Limits of Magnetization, Curie Temperature, and Magnetic Anisotropy of Permanent Magnet Materials*

Calculation based on LDA/GGA is performed for the purpose of estimating possible upper limits of saturation magnetic polarization J_S, Curie temperature T_C, and magnetocrystaline anisotropy constant K₁ of permanent magnet materials. It was concluded that the upper limits of J_S, T_C, and K₁ could be ~2.7 T, ~2000 K, and ~1000 MJm^-3, respectively. K₁ obtained assuming that the 4f orbitals are bound to the lattice seems too large: one order of magnitude smaller than that might be plausible.
Despite the simplyfied model where bcc or L1₀ lattice structure and fictitious atoms with a fractional atomic number are assumed, the essential points concluded here are rather general.
From the results, it is suggested that the best strategy to realize materials suitable for high-performance permanent magnets is to design Fe-based magnets containing a small amount of light rare earths with additional elements including V, Cr, Co, Ni, and possibly some interstitial typical elements. The idea of realizing high-performance permanent magnets without using rare earths seems not realistic.