Magnetic properties of TbCu7-type Sm-Fe-Co-W-M (M = Nb, Cu, and Cr) system nitrided compounds

This paper reports on experiments on the effect of composition, heat treatment, and nitriding conditions on the magnetic properties of Sm-Fe-Co-W-Nb (Cu, Cr) nitrides with TbCu₇-type structure. The optimum preparation conditions of the compounds were as follows: composition {Sm₁₀(Fe_0.95Co_0.05)_89.4W_0.3Nb_0.3}_{82 .7}N_17.3; roller velocity 50 m/s; heat treatment 675°C × 60 min in high-purity Ar gas; and nitriding condition 420°C × 15 h in high-purity N₂ gas. Typical magnetic properties of the obtained powders were as follows: J_r=0.96 T, H_cJ=773.0 kA/m, H_cB=540.4 kA/m, (BH)_max=141.3 kJ/m³ (17.8 MGOe), and T_c=493°C. Recoil loops of the hysteresis curve and X-ray analysis showed that the sample was an exchange spring magnet. The value of (BH)_max for the compression-molded isotropic bonded magnet prepared from {Sm₁₀(Fe_0.95Co_0.05)_89.4W_0.3Nb_0.3}_{82 .7}N_17.3 powder was 102.1 kJ/m³ (12.8 MGOe) when the density of the bonded magnet was 6.1 Mg/m³. The reversible temperature coefficient of J_r was α(J_r)=-0.06%/°C, and the temperature coefficient of H_cJ in the range from 25°C to 125°C obtained by a linear extrapolation was α(H_cJ)=-0.40%/°C.