Coercivity of Pulverized ThMn₁₂ Compounds with a Limited Amount of α-(Fe,Co) Phase

Abstract

We newly prepared the ThMn₁₂ compounds (Sm_0.9Zr_0.1)(Fe_0.8Co_0.2)_11.3Ti_0.7 (A) and (Nd_0.8Zr_0.2)(Fe_0.9Co_0.1)_11.3Ti_0.7N_1.5 (B), which were almost α-(Fe,Co) phase-free. These compounds show magnetic properties superior to those of the Nd₂Fe₁₄B phase in the high-temperature region (473 K). We also prepared a typical α-(Fe,Co) phase-free compound, Nd(Fe_0.8Co_0.2)₁₁MoN_1.3 (C), which has a low saturation polarization, but a high magnetic anisotropy field, and better thermal stability. We examined the effect of pulverization on the coercivity (H_c) of compounds (A)–(C). The critical radii of single-domain particles (R_c) were about 120 nm for (A) and (B), and about 400 nm for (C), and were determined from the measured magnetic domain widths of the most coercive pulverized particles of each sample. As the pulverization time increased, H_c increased to maximum values of 0.10 MAm⁻¹ for (A), 0.14 MAm⁻¹ for nitrogenated (B), and 0.21 MAm⁻¹ for nitrogenated (C). Further pulverization of the samples decreased H_c. The decrease was mainly caused by oxidation in non-nitrogenated (A) and by the accumulation of lattice distortion in (B) and (C).

Fig. 6 Variation of H_c with milling time (0–48 h) (a) and variation of H_c with particle size (b).