Coercivities of Sm–Fe–M Sintered Magnets with ThMn₁₂-Type Structure (M = Ti, V)

Abstract

Sm–Fe–M (M = Ti, V) sintered magnets with the ThMn₁₂-type structure which consist of higher Sm content than the stoichiometric composition of ThMn₁₂-type compound, were produced by the powder-metallurgy. Sm–Fe–Ti sintered magnets had a low level of coercivities of about 10 kAm⁻¹. In contrast, Sm–Fe–V anisotropic sintered permanent magnets, particularly, Sm_10.5Fe_74.1V_15.4 magnet exhibited magnetic properties with a coercivity of 648.7 kAm⁻¹, a remanence of 0.73 T and a (BH)_max of 97.6 kJm⁻³. Through microstructure observation of the grain boundary between the adjacent ThMn₁₂-type grains using STEM, no grain boundary phases were observed in Sm_10.0Fe_81.7Ti_8.3 magnet, whereas two types were observed in Sm_10.5Fe_74.1V_15.4 magnet: an amorphous structure with a thickness of about 1 nm and an incomplete periodic structure with a thickness of about 2 nm. EDS analysis indicated that the composition ratios of Sm to the sum of Fe and V were about 1 to 1 and 1 to 1.7, respectively. It suggested that the existence of a grain boundary phase is important to raise coercivity.

Fig. 3 Demagnetization curves of Sm–Fe–V sintered magnets.