Enhancement of magnetic properties and microstructural changes in TbCu₇-type Sm-Fe-Co-Nb-B melt-spun ribbons

The effects of annealing conditions on magnetic properties, phase, and microstructural changes in Sm-Fe-Co-Nb-B melt-spun ribbons were investigated, and the dependence of magnetic properties on Sm and B composition was also investigated. Sm_xFe_80.9−(x+y)Co_16.4Nb_2.7B_y amorphous ribbons were prepared by melt-spinning and then annealing. After annealing at 625 °C, the Sm_6.2Fe_66.4Co_16.4Nb_2.7B_8.3 ribbons exhibited crystallization of hard-magnetic TbCu₇-type phase, and the coercivity increased as the annealing time was increased to 9 h. The grain size of the TbCu₇-type phase as estimated from transmission electron microscopy (TEM) images was 20–50 nm for ribbons annealed for 9 h. Three-dimensional atom probe (3DAP) analysis revealed that Nb and B became enriched at grain boundaries and tended to condense at the grain boundaries with increasing annealing time. Non-magnetic Nb and B enrichment at the grain boundaries in fine TbCu₇-type grains thus promotes magnetic decoupling between TbCu₇-type phases and suppresses domain wall propagation, resulting in high coercivity. Composition dependence on magnetic properties was investigated. Optimum annealing time tended to increase with increasing Sm content. The Sm_6.7Fe_65.9Co_16.4Nb_2.7B_8.3 ribbons exhibited a maximum coercivity of 695 kA⋅m⁻¹ by annealing at 625 °C for 21 h and subsequent slow cooling at a cooling rate of 4 °C⋅h⁻¹. As B content decreased, coercivity decreased and remanence increased. The Sm_6.2Fe_68.7Co_16.4Nb_2.7B_6.0 ribbons exhibited a high σ_r of 100 A⋅m²⋅kg⁻¹ and moderate H_cJ of 540 kA⋅m⁻¹ after optimal annealing. An isotropic bonded magnet was fabricated from this ribbon. The bonded magnet exhibited high magnetic properties of B_r = 0.82 T, H_cJ = 538 kA⋅m⁻¹, and (BH)_max = 98 kJ⋅m⁻³, and small temperature coefficients of α(B_r) = −0.06%⋅°C⁻¹ and β(H_cJ) = −0.27%⋅°C⁻¹.