Temperature stability of coercivity in mischmetal-Fe-Co-B melt-spun ribbons

Coercivity temperature coefficient (β) of the permanent magnet depends on its intrinsic magnetic properties and microstructure. In this paper, the relationship between β and the temperature stabilities of magnetocrystalline anisotropy field (H _a) and saturation magnetization (M _s) as well as the microstructure is discussed. Regarding two concerned microstructural factors: grain size and grain boundary, coercivity thermal-stabilities of MM_13.5Fe_79.5B₇ (MM-mischmetal: unseparated La-Ce-Pr-Nd alloy) and MM_xFe_94-xB₆ (x =12, 13, 14, 15, 16, 19) melt-spun ribbons, respectively, are investigated. High β values near the theoretical limit are obtained either by decreasing grain size or by reducing MM percentage. In addition, coercivities above room temperature of MM_13.5Fe_79.5-yCo_yB₇ (y =0, 3, 6, 9, 12, 15) melt-spun ribbons are measured. The detailed influences of Co substitutions on β are analyzed, and the weak temperature dependence of M _s is proved to the reason for the observed decrease of β. These findings suggest that proper strategy to minimize local stray fields is the key to enhance coercivity thermal-stability of 2:14:1 structure magnet.