Rapidly quenched Nd2Fe14B-based permanent magnets owe their high intrinsic coercivity H
to their unique submicron-sized grain structure. While no clear consensus has yet emerged regarding the microscopic origin of coercivity in these fascinating materials, magnetic and structural data suggest pinning of magnetic domain walls at or in an intergranular phase between Nd2Fe14B grains as the most likely mechanism. The small, randomly oriented grains in isotropic melt-spun ribbons produce a magnetic structure wherein each Nd2Fe14B grain is a single magnetic domain, with the domain walls expelled to the grain boundaries. The magnetic interaction between neighboring grains plays an important role in the magnetization reversal of individual grains, as revealed, for example, by detailed examination of the initial magnetization and demagnetization processes. The formation of large domain structures via these interactions is frustrated by the random grain orientation in ribbons, but short range magnetic correlations between grains (“interaction domains”) are observed locally.
The oriented grains in aligned die upset magnets produce a very different magnetic structure having large scale coherent domains which extend through many grains. A variety of experiments, including studies of the temperature dependence of H
, suggest that coercivity arises from pinning of the extended domain walls at Nd2Fe14B grain edges.