Permanent magnets

The review gives an initial survey of magnetic units, the hysteresis loop, domain theory, crystal and shape anisotropy, single-domain particles, self-demagnetising field and demagnetisation factors, the criteria for permanent-magnet materials, and the methods and conditions for magnetisation of magnets. This is followed by an outline of the properties and production methods for those materials now in wide use. Cast or sintered Alnico alloys require field heat treatment to produce a structure of very small elongated single-domain particles in a nonmagnetic matrix. The range of properties obtainable by composition variation is exemplified. Magnets in these alloys are usable from extreme subzero temperatures to about 600° C. The ceramic barium/strontium ferrite magnets depend on the production of an array of single-crystal particles of the size for single domains. Remanence values are lower than for Alinico alloys but coercivity is substantially higher. The widely used flexible-strip magnets are obtained by plastic bonding of the same single-crystal particles. The recently developed cobalt/rare-earth metal magnets give uniquely high properties but with high material cost and difficult production methods. Other permanent magnet materials have limited uses. A short note on comparative costs is given. The factors controlling magnet stability are outlined and the limitations of the various materials discussed. The wide range of permanent-magnet applications are discussed under the classifications of electromechanical (speakers, measuring instruments, eddy-current devices, motors and generators), ion and electron-beam control (magnetrons, travelling-wave tubes) and magnetomechanical (holding and lifting, drives, repulsion devices)