PRESENT STATUS OF FERROMAGNETIC THEORY 



79 



domain at the expense of a neighboring one in the initial portion of the 

 curve, sudden changes of direction of domains (with resulting large 

 energy losses) in the middle portion, and continuous or smooth rotation 

 of the domains in the upper portion. The latter two processes occur 

 during the traversal of a large hysteresis loop with tips at high flux 

 densities; the first process is important only in low fields after de- 

 magnetization. 



Effect of Strain 



This picture of the changes in magnetization has been made for 

 materials that are free from any considerable strain. As a matter of 

 fact, strain can affect magnetization in an important way, and under 

 certain circumstances a tensile stress of 5,000 pounds per square inch 

 may change the flux density B as much as 10,000 gausses ^■' — almost 

 from zero magnetization to saturation (Fig. 14). The effect is il- 



ea PERMALLOY 



65 PERMALLOY 



2 4 6 



H 



10 12 



8 ID 12 



Fig. 14 — Effect of tension on magnetization (Buckley and McKeehan). 



lustrated well by data for 65 and 85 permalloy (iron-nickel alloys 

 containing, respectively, 65 and 85 per cent nickel). For 65 permalloy 

 the effect of tension is to increase the magnetization in all fields; for 

 85 permalloy the effect is the opposite; and in each case the effect of 

 compression is opposite to that of tension. For ordinary iron the 

 effect of tension is to increase the magnetization in small fields, but to 

 decrease it in high fields. 



The effect of strain on magnetization has its counterpart in an effect 

 of magnetization on the length of a piece of ferromagnetic material. 

 When a rod of iron is magnetized its length increases by a small 

 amount. This is but one example of a large class of effects exhibited 



