Magnetic properties and microstructure of Sm₅Fe₁₇-based composite magnets

We have investigated synthesis, magnetic properties and microstructure of Sm₅Fe₁₇-based hard magnetic phase with a Sm₂₀Fe₇₀Ti₁₀ composition. Ultrahigh coercivities, μ₀H_c=7.18 T at room temperature and μ₀H_c=8.86 T at 10 K, have been achieved. The room-temperature coercivity, determined from high-field pulse measurements, reaches 35% of the anisotropy field μ₀H_a=20.7±0.8 T. Further, it is demonstrated that a coercivity of 2.18 T is maintained even at 500 K. The Curie temperature T_C of Sm₂₀Fe₇₀Ti₁₀ is 577 K and the calculated exchange stiffness parameter A is 7.72 pJ/m. Detailed transmission electron microscopy investigations show a two-phase microstructure consisting of the Sm₅Fe₁₇-based hard magnetic matrix phase with grain size below 200 nm and finer, below 100 nm, Fe₂Ti grains. Majority of the Fe₂Ti phase is located at the grain boundaries with some finer inclusions found also inside the 5:17 grains. Despite the high fraction of the Fe₂Ti grains, nearly single-phase demagnetization loops are observed. In order to enhance M_s, the effect of Ti content on phase constitution, magnetic properties and microstructure was studied in detail. M_s increases and H_c decreases for the Ti-lean compositions.