Coercivity and Microstructure Dependence on B Content in ThMn₁₂-Type Sm-Zr-Fe-Co-Ti-Cu-B Alloys

Abstract

The effect of B content was studied on the crystalline structures, magnetic properties, and microstructures of (Sm_0.80Zr_0.20)_8.6(Fe_0.71Co_0.20Ti_0.08Cu_0.01)_91.4−xB_x (x = 0.8, 2.0, 5.0, and 8.0) melt-spun alloys with a ThMn₁₂-type structure (1–12 phase) to achieve high saturation magnetization (μ₀M_s) and large intrinsic coercivity (H_cj). With increasing x, the amorphous forming ability improved in the as-quenched alloys, and the average crystallite size decreased in the annealed alloys. The x = 5.0 alloy annealed at 1173 K for 60 min exhibited the largest H_cj of 454 kA/m and the μ₀M_s was estimated to be 1.29 T. In addition, the x = 5.0 alloy exhibited demagnetization curve without a two-step shape, which indicates a low volume fraction of soft magnetic phases. The scanning transmission electron microscopy revealed that 1–12 phase grains have sizes in the range of 30–100 nm in the x = 5.0 alloy after annealing at 1173 K. Fine Ti-B precipitates were also observed. The reduction in the 1–12 phase grain size is attributed to the suppression of crystal nucleation and the crystal growth caused by the amorphous formation in the as-quenched alloy, and the grain boundary pinning by the Ti-B inhibiting the grain growth. The grain sizes in the x = 5.0 alloy were smaller than the estimated critical single-domain diameter of 190 nm, which contributes to the increase in H_cj.