Ab initio calculations of the Curie temperature of complex permanent-magnet materials: Sm₂Fe₁₆A (A=Ga, Si)

A low concentration of substitutional impurities such as Ga or Si in R₂Fe₁₇ (R=rare earth) raises its Curie temperature. Self-consistent spin-polarized electronic-structure calculations are performed and exchange parameters (J_ij) are derived from first-principles for pure Sm₂Fe₁₇ and Sm₂Fe₁₆A (A=Ga, Si). The changes in exchange parameters with doping are related to the magnetovolume effect and the lowering of hybridization of the iron atoms. Monte Carlo simulations have been performed using the calculated J_ij's to derive the Curie temperature of phases containing Ga and Si impurities. The results are in very good agreement with the experimental data.