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

Anisotropic SmFe12-based sintered magnets require an optimum microstructure and alloy composition to have a large coercivity μ0Hc and remanent magnetization μ0Mr. 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 Sm8Fe73.5+xTi8-xV8Ga0.5Al2 magnet eliminated the secondary Fe2(Ti,V) phase, resulting in a remanence of μ0Mr = 0.8 T and energy density of (BH)max = 113 kJ/m3 (14.2 MGOe) in x = 3. A relatively high coercivity μ0Hc of 0.6 T is maintained owing to the formation of thin intergranular phase (IGP). Substitution of Co for Fe in the Sm8(Fe0.95Co0.05)73.5Ti8V8Ga0.5Al2 sintered magnet resulted in a low remanence and low coercivity μ0Hc. 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 SmFe2-based phase of the MgCu2 type in Co-containing magnet is responsible for a further decrease of coercivity to μ0Hc = 0.55 T. This study provides an insight on the optimum microstructure to realize high-performance anisotropic bulk SmFe12-based sintered magnets, i.e., ones without twins and ferromagnetic SmFe2-based phase of the MgCu2 type, but with refined grain size, and thin non/weak ferromagnetic IGP isolating the SmFe12 grains. For SmFe12-type phase, reducing the nonmagnetic elements and increasing its saturation magnetization remain crucial to realize the potential to compete with the Nd2Fe14B phase.