10 BELL SYSTEM TECHNICAL JOURNAL 



Summarizing the general properties of soft and of hard magnetic 

 materials, in the former, high maximum permeability is associated with 

 low coercive force and a hysteresis loop of small area. In the latter, a 

 low permeability, is associated with a high coercive force and a large 

 hysteresis loop. The range of properties which can be obtained in 

 magnetic materials by alloying and other metallurgical control is most 

 remarkable. Permeabilities from 1 to over 600,000 and coercive 

 forces from 0.012 to 600 oersteds, are attainable. 



In manufacturing soft magnetic materials, the metallurgist strives 

 constantly to eliminate those chemical elements (impurities) which 

 broaden the hysteresis loop, while in the case of the permanent magnet 

 materials he intentionally adds certain of the same, or other elements, 

 and resorts to heat treatments which broaden the loop. Some details 

 of these procedures with well known materials, and also with some 

 more recently developed, are described in the following pages. 



Soft Magnetic Materials 

 Magnetic Iron 



Of the ferromagnetic elements iron, nickel, and cobalt, iron, at the 

 present time, is the only one of industrial importance as a soft magnetic 

 material in the unalloyed condition. Throughout the last half century, 

 a continual improvement in the magnetic quality of iron has been 

 effected until at present, laboratory samples have been prepared with 

 permeabilities ^' " of over 200,000. Careful analysis of the data shows 

 that this improvement has paralleled closely increases in the purity of 

 the iron. 



Since the production of iron of high magnetic quality is of consider- 

 able interest at this time, it seems worth while to examine, in some de- 

 tail, the methods which have been used for producing high purity iron. 

 Methods which have been developed for the purification of iron, have, 

 in each instance, removed from the iron, elements (impurities) which 

 are harmful to easy magnetization. 



The elements which enter into iron as impurities may be grouped, 

 structurally, into two classes: substitutional elements and interstitial 

 elements. The substitutional elements exist in the iron structure at 

 lattice points; that is, they substitute for iron atoms in the lattice 

 shown in Fig. 2. These elements include most of the metals, for ex- 

 ample : nickel, cobalt, manganese and silicon. The interstitial elements 

 enter into the iron lattice at intermediate points; that is, they take 

 positions in the structure between the iron atoms. Elements included 

 in this classification are carbon, oxygen, and nitrogen. It is these latter 

 elements which, although present in small quantities, cause severe 



