Factors Affecting Magnetic Quality

N T H E preparation of magnetic materials for practical use it is important to know how to obtain products of the best quality and uniformity. In the scientific study of magnetism the goal is to understand the relation between the structure and composition on the one hand and the magnetic properties on the other. From both standpoints it is necessary to know the principal factors which influence magnetic behavior. These are briefly reviewed here. The properties depend on chemical composition, fabrication and heattreatment. Some properties, such as saturation magnetization, change only slowly with chemical composition and are usually unaffected by fabrication or heat treatment. On the contrary, permeability, coercive force and hysteresis loss are highly sensitive and show changes which are extreme among all the physical properties. Properties may thus be divided into struclurescnsitive and structure-insensitive groups. As an example, Fig. 1 shows magnetization curves of permalloy after it has been (a) cold rolled, (b) annealed and cooled slowly, and (c) annealed and cooled rapidly. The maximum permeability varies with the treatment over a range of about 20 fold, while the saturation induction is the same within a few per cent. Structure sensitive properties such as permeability depend on small irregularities in atomic spacings, which have little effect on properties such as saturation induction. Some of the more common sensitive and insensitive properties are listed in Table I.