Permanent Magnet Assemblies

Modern permanent magnets are ideally suited to generate magnetic fields of magnitudes comparable to their spontaneous polarization, J _s. These fields can be static or variable, uniform or nonuniform. They are generated by permanent magnet structures that demand no continual expenditure of energy for their operation, and dispense with the elaborate cooling systems required by resistive or superconducting electromagnets. Furthermore, the permanent magnet assemblies are compact and can offer the significant advantage of flux confinement in the region of interest. Permanent magnets are now competitive with electromagnets for generating fields of up to about 2 T. More generally, when a high field with a rapid spatial variation is required, they may offer the only practicable solution.

The polarization, J, of a modern hard magnet (Skomski and Cugat 1999) deviates little from its remanent or saturation value, J _s, far into the second quadrant of the hysteresis loop, before switching at the coercive field, H _c (see Rare Earth Magnets: Materials, Magnets: Sintered). The plot of the polarization of the magnet as a function of the magnetic field, H, acting on it is almost rectangular in shape, as indicated in Fig. 1. The intrinsic spontaneous polarization of SrFe ₁₂O ₁₉ is 0.47 T, for example, and that of Nd ₂Fe ₁₄B is 1.61 T. Values of the remanent polarization of typical sintered magnets are given in Table 1. They are typically 10 20% less than the spontaneous polarization of the...