SmFe₁₂-based hard magnetic alloys prepared by reduction-diffusion process

High phase-purity Sm(Fe_0.8Co_0.2)₁₁Ti and Sm_0.8Zr_0.2(Fe_0.8Co_0.2)₁₁Ti powders with the tetragonal ThMn₁₂ crystal structure have been synthesized by reduction-diffusion method. The average particle size is 1.47 ± 0.82 μm for the Zr-free and 2.29 ± 1.22 μm for the Zr-doped samples. The larger particles are multi-grain, without a chemically distinct grain boundary phase as revealed by TEM investigations. The intrinsic magnetic properties are μ₀M_s= 1.42 ± 0.04 T, μ₀H_a= 9.8 ± 0.4 T for Zr-free and μ₀M_s= 1.37 ± 0.04 T, μ₀H_a= 9.7 ± 0.4 T for Zr-doped samples. Temperature-dependent XRD studies show that the ThMn₁₂-type phase begins to decompose around 500 °C which presents difficulties for further development into fully dense bulk magnets. Successful introduction of grain boundary phase with good wettability has been realized by infiltrating low-melting eutectic alloys, resulting in fully dense bulk samples. However, coercivity remains low, μ₀H_c = 0.34 T. The underlying microstructural reasons for not achieving large H_c are discussed.