Progress in permanent-magnet materials

The most widely used permanent-magnet materials are at present the field-treated alloys, cast or sintered, of nickel, aluminium, cobalt and iron. Practice and theory have worked together to obtain, in the best of these alloys, an assembly of aligned single-domain shaped particles in which a very high proportion of the intrinsic magnetization is retained after removal of a magnetizing field and also against high reverse fields. Ceramic-like ferrite magnets have higher coercivity but lower remanence, and the development of these can also be linked with domain theory. Many other alloy materials continue to be used in a limited sphere, but future progress is likely to be linked with the preparation of iron or iron-cobalt particles in elongated single-domain form and the manufacture of permanent magnets from aligned compacts of such particles, as indicated by recent experimental work.