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The pressure of requirements concerning mass, size, temperature coefficient, resistance to demagnetization, price, machinability, etc., has given rise to three trends in magnet development: a) high energy-density magnets, notably RE-Co alloys; b) ductile magnets, notably Fe-Cr-Co alloys; c) low-cost magnets, notably Mn-Al attoys. For each of these materials we consider how the present state relates to what can be obtained theoretically, how further progress can be made and what the constraints are concerning availability of raw materials and commercial feasibility.

The critical field for unpinning or nucleation of a domain wall in a ferromagnetic crystal at an antiphase boundary (APB) is calculated as a function of the magnetic coupling constant η across the APB. It turns out that for positive values of η pinning occurs and that for negative values there is very strong pinning and also nucleation.

The critical field strength at which a classical domain wall moves through a perfect ferromagnetic crystal of simplified structure is calculated as a function of the ratio between anisotropy energy K and ferromagnetic coupling energy C , taking into account the discrete nature of the spins. When K approaches C intrinsic wall pinning becomes appreciable with critical fields approaching the anisotropy field when K \gg C . For K/C \geq 2/3 , the wall has a thickness of one atomic distance and thus resembles a ferroelectric wall.

The defect populations in various MnAl permanent magnet materials have been studied by transmission electron microscopy. It is suggested by the observations that the good hard magnetic properties in the extruded materials are connected with the low density of antiphase boundaries and the relatively high density of defects such as grain boundaries. The relation between treatment, defect population and magnetic properties will be discussed.

The process of magnetization reversal in ferromagnets with uniaxial crystal anisotropy is discussed. It is shown that the "critical-size concept" has no relevance to coercivity. Critical fields are distinguished as nucleation fields and propagation, or pinning, fields. The pinning of walls is discussed and illustrated by experiments on various substances. Intrinsic pinning in highly anisotropic materials is predicted.