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BELL SYSTEM TECHNICAL JOURNAL 



be about 20 instead of many thousands. In the past the process 

 occurring in low fields has been the cause of much speculation, but 

 recently a satisfactory explanation seems to have been found.* The 

 changes that take place here are visualized as displacements of the 

 boundaries of domains (Fig. 12); the transition region of a few atom 



; 

 I 



Fig. 12 — Magnetization in very low fields progresses by slight displacement of domain 



boundaries (Becker). 



diameters (calculated from the forces of exchange to be about 30 

 atom-diameters '') moves so as to enlarge a domain magnetized in the 

 direction of the field at the expense of a domain pointing in a less 

 favorable direction. Such a movement can progress for only a short 

 distance compared with the linear dimensions of a domain, and is 

 limited by the strains present in any actual material. 



Thus in the magnetization of an ordinary well-annealed ferromag- 

 netic material three processes occur, corresponding to the three well 

 known sections of the magnetization curve (Fig. 13) : growth of one 



Fig. 13 — Illustrating the three kinds of change in magnetization. 



