Recent Developments in Permanent Magnetism

Abstract

The properties of permanent magnets together with their theoretical background are reviewed. The study of micromagnetics has shown that because of processes such as fanning, buckling, and curling, the high energies once predicted for elongated single domain particle magnets are unlikely to be obtained by shape anisotropy alone. There may be a better chance of obtaining high energies from materials with crystal anisotropy or if displaced loops due to exchange anisotropy could be produced in a practical material at room temperature. The properties of barium ferrite are due to crystal anisotropy, but its magnetization is low. Other materials with high crystal anisotropy at present require expensive materials or manufacturing processes. The properties of Alnico V and related alloys can be explained in terms of shape anisotropy due to two-phase structure. Progress has been made in a group of high coercivity alloys containing more than 4% Ti and 29% Co. A single crystal of one of these alloys has given the highest (BH)max so far obtained, but it is only recently that it has been discovered that small additions of S or Se enable these alloys to be made columnar by simple methods.