Microstructure and extrinsic magnetic properties of anisotropic Sm(Fe,Ti,V)12-based sintered magnets

Anisotropic SmFe ₁₂ -based sintered magnets require an optimum microstructure and alloy composition to have a large coercivity μ ₀ H_c and remanent magnetization μ ₀ M_r. In this study, we investigated the microstructure transformation and extrinsic performance change in the anisotropic Sm(Fe,Ti,V) ₁₂-based sintered magnets via reducing the Ti content and partially substituting Fe with Co. Reduction of Ti content in anisotropic sintered Sm ₈Fe _{73.5+ x}Ti _{8- x}V ₈Ga _0.5Al ₂ magnet eliminated the secondary Fe ₂ (Ti,V) phase, resulting in a remanence of μ ₀ M_r = 0.8 T and energy density of ( BH) _max = 113 kJ/m ³ (14.2 MGOe) in x = 3. A relatively high coercivity μ ₀ H _c of 0.6 T is maintained owing to the formation of thin intergranular phase (IGP). Substitution of Co for Fe in the Sm ₈(Fe _0.95Co _0.05) _73.5Ti ₈V ₈Ga _0.5Al ₂ sintered magnet resulted in a low remanence and low coercivity μ ₀ H_c . The electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) observations indicate the formation of twins inside 1:12 grains which deteriorate the texture and remanent magnetization. Magneto-optical Kerr effect (MOKE) microscopy and micromagnetic simulations revealed that although the magnetization reversal starts at the twinned grains, Sm-rich intergranular phase hinders propagation of reversed domains to the neighboring grains, preventing the deterioration of coercivity. However, the formation of ferromagnetic SmFe ₂-based phase of the MgCu ₂ type in Co-containing magnet is responsible for a further decrease of coercivity to μ ₀ H_c = 0.55 T. This study provides an insight on the optimum microstructure to realize high-performance anisotropic bulk SmFe ₁₂-based sintered magnets, i.e., ones without twins and ferromagnetic SmFe ₂-based phase of the MgCu ₂ type, but with refined grain size, and thin non/weak ferromagnetic IGP isolating the SmFe ₁₂ grains. For SmFe ₁₂ -type phase, reducing the nonmagnetic elements and increasing its saturation magnetization remain crucial to realize the potential to compete with the Nd ₂Fe ₁₄B phase.