86 



BELL SYSTEM TECHNICAL JOURNAL 



After the chemical strains and the strains resulting from cold working 

 have been removed, there is still another kind of residual strain — that 

 attributable to magnetostriction. These are ordinarily random in 

 direction because they are associated with randomly oriented domains, 

 but by a suitable trick they all can be oriented so as to favor magnet- 

 ization in a single desired direction at the expense of ease of magnet- 

 ization at right angles. This trick is heat treatment in the presence of 

 a magnetic field. Without going into a more detailed explanation, the 

 experimental results obtained ^* about two years ago will be given. 



When an annealed specimen of 65 permalloy is heated for a few 

 minutes at 650 degrees centigrade while it is subjected to a magnetic 

 field of 10 oersteds, the maximum permeability is increased from about 

 20,000 to over 600,000 as shown in Fig. 23. This material holds the 



600,000 



M 



200,000 



100,000 



Fig. 23 — Permeability curves of 65 permalloy after heat treatment in hydrogen and 

 additional heat treatment in a magnetic field (Dillinger and Bozorth). 



records for the highest maximum permeability, the lowest coercive 

 force, and the lowest hysteresis loss at high flux densities. It may be 

 compared with the most permeable material known in 1900, iron with 

 a maximum permeability of less than 3,000. 



