A Survey of Magnetic Materials in Relation to Structure* 



By W. C. ELLIS and EARLE E. SCHUMACHER 



The structure dependence of magnetic characteristics of the more im- 

 portant magnetic materials is discussed. The natural grouping into soft 

 magnetic materials and hard or permanent magnet materials is used. In 

 the first group are: magnetic iron, silicon steel and the alloys of iron, nickel 

 and cobalt. The latter group includes the well-known carbon and alloy 

 steels, and also the newly developed precipitation hardening a-solid solution 

 types. Examples of the latter are the iron-nickel-aluminum and the iron- 

 cobalt-molybdenum alloys. 



In discussing the properties, the relationship to structure is emphasized. 

 With the soft magnetic materials purification and the development of proper 

 structural configuration are important. The special metallurgical control 

 required to produce certain magnetic materials in dust form is described. 

 The generalization is made that permanent magnet characteristics are due to 

 precipitation effects resulting from decomposition of supersaturated solid 

 solutions. 



Introduction 



MAGNETIC materials may be classified according to properties 

 into two groups: (1) soft magnetic materials, and (2) hard or 

 permanent magnet materials. The differences in properties of the two 

 groups are illustrated in Fig. 1, which shows typical magnetization 

 cycles for the two types. The dashed line is a normal magnetization 

 curve while the closed full line is known as the hysteresis loop. 



A soft magnetic material is, in general, characterized by a steeply 

 ascending magnetization curve; that is, large values of flux density are 

 produced by small magnetizing forces. For certain applications where 

 the flux density is low, the initial portion of the curve is important. 

 For intermediate flux density applications the steeply ascending portion 

 is of paramount interest while for higher densities, the upper portion is 

 of prime importance. 



Another term which is widely used in discussing soft magnetic ma- 

 terials is permeability. The permeability at a given flux density is the 

 slope of the straight line joining that point on the magnetization curve 

 with the origin. Algebraically, it is the ratio of the flux density, B, to 

 the magnetizing force, //. It varies with the flux density, and for soft 

 magnetic materials at low magnetizing forces, is a large quantity. The 

 permeabilities usually reported in discussing magnetic properties are 

 the initial permeability, mo, and the maximum permeability, Mmkix.. The 

 initial permeability is the initial slope of the magnetization curve. The 



* Published in Metals and Alloys, first part in December 1934 issue, second and 

 concluding [)art in January 1935 issue. 



