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

Coercivity temperature coefficient (beta) of the permanent magnet depends on its intrinsic magnetic properties and microstructure. In this paper, the relationship between beta 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 MM13.5Fe79.5B7 (MM-mischmetal: unseparated La-Ce-PrNd alloy) and MMxFe94-xB6 (x = 12, 13, 14, 15, 16, 19) melt-spun ribbons, respectively, are investigated. High beta values near the theoretical limit are obtained either by decreasing grain size or by reducingn MM percentage. In addition, coercivities above room temperature of MM13.5 Fe79.5-yCoyB7 (y = 0, 3, 6, 9, 12, 15) melt-spun ribbons are measured. The detailed influences of Co substitutions on beta are analyzed, and the weak temperature dependence of M-s is proved to the reason for the observed decrease of beta. 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.