Coercivity Mechanism in Nanocrystalline and Bonded Magnets

Abstract

The interpretation of the coercive field of hard magnetic materials has been a highly active field since the early times of commercial use of permanent magnets (pms). Whereas the early pms were based on the pinning of domain walls by grain boundaries, precipitations and dislocations, for AlNiCo and hard ferrites the nucleation mechanism became relevant. This latter mechanism also prevails in numerous rare-earth-transition metal compounds of sintered and nanocrystalline (nc) pms. In spite of worldwide activities on models of coercivity mechanisms the discussions have not led to a final general agreement concerning the dominant coercivity mechanisms in nc systems of bulk pms and bonded pms. It is the aim of this contribution to show the dominance of the nucleation mechanism in nc and bonded pms. Experimental results on the temperature dependence of the coercivity and its dependence on the special microstructures of different types of pms and the role of grain boundaries will be discussed within the framework of the nucleation coercivity model.