Origin of coercivity in an anisotropic Sm(Fe,Ti,V)12-based sintered magnet

We have demonstrated an anisotropic bulk SmFe₁₂-based sintered magnet with sufficiently large coercivity of μ₀H_c=1.0 T and a remanence ratio (M_r/M_s) of 0.84 using conventional liquid sintering process of nitrogen jet-milled powders with the nominal composition of Sm₈Fe_73.5Ti₈V₈Ga_0.5Al₂ (at.%). The moderate saturation magnetization of μ₀M_s =0.74 T is due to the dissolution of a large amount of stabilizing elements, Ti, V, and Al, in the 1:12 phase. The anisotropy field of the main 1:12 phase was determined to be µ₀H_A=10.2 T. Detailed multi-scale microstructure characterizations by scanning electron microscope (SEM) and scanning transmission electron microscope (STEM) showed the magnet consists of Sm(Fe,Ti,V,Al)₁₂ grains with the ThMn₁₂-type crystal structure with a size distribution of ~3 − 15 µm that are enveloped by ~3 nm thick Sm-rich amorphous intergranular phase. Secondary phases including metallic (Sm,Ga)-rich, SmO_x, and Fe₂(Ti,V) phases coexist with the 1:12 phase. Measured angular dependence of coercivity follows Kondorsky type magnetization reversal, suggesting the coercivity arises due to the pining of magnetic domain walls. Magneto-optical Kerr effect (MOKE) microscopy revealed magnetization reversal starts at the grain boundaries and interphase interfaces and thin amorphous intergranular phases act as the pinning sites against magnetic domain wall propagation. Small micromagnetic parameter α~0.164 estimated by fitting to the Kronmüllar equation suggest that the reduction of the grain size and engineering of the intergranular phase to an Fe-lean composition are necessary to improve the coercivity toward µ₀H_A/3 = 3.4 T. This work provides guidelines on an optimum microstructure to develop an anisotropic bulk SmFe₁₂-based sintered magnet with a sufficiently large coercivity.