Factors Affecting Magnetic Quality* 



By R. M. BOZORTH 



IN THE preparation of magnetic materials for practical use it is impor- 

 tant to know how to obtain products of the best quahty 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 heat- 

 treatment. 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 hystere- 

 sis loss are highly sensitive and show changes which are extreme among all 

 the physical properties. Properties may thus be divided into slruclure- 

 sensilive and structiire-inseusitive groups. As an example. Fig. 1 shows mag- 

 netization 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 sensi- 

 tive 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. The principal physical and chemical factors which affect these 

 properties are listed in column 3. Their various effects will now be briefly 

 discussed and illustrated. 



Phase Diagram 



Some of the most drastic changes in properties occur when the fabrication 

 or heat treatment has brought about a change in structure of the material. 

 For this reason the phase diagram or constitutional diagram is of the ut- 

 most importance in relation to the preparation and properties of magnetic 

 materials. As an example consider the phase diagram of the binary iron- 

 cobalt alloys of Fig. 2. Here the various areas show the phases, of different 



*This article is the substance of Chapter II of a Iraok entitled "Ferromagnetism" to 

 be published early in 1951 by D. Van Nostrand Company, Inc. 



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