26 BELL SYSTEM TECHNICAL JOURNAL 



In the iron-cobalt-nickel alloys, high values of maximum permea- 

 bility are also generally obtained by such a heat treatment, provided 

 the magnetic change point is higher than about 500° C. Another 

 feature in connection with this special heat treatment is that the heat- 

 treated specimens have hysteresis loops which tend to have sharp 

 corners and vertical sides. 



An interesting explanation for the effect of heat treatment in a 

 magnetic field is given by Bozorth.^^ According to modern magnetic 

 theory, when a specimen is cooled through the Curie point in the 

 absence of an applied field, small regions or domains are magnetized to 

 saturation in definite directions. The specimen in its entirety exhibits 

 no external magnetic efTects since the vector sum of the" individual 

 effects would be nearly zero and not detectable. The magnetization of 

 the domains produces internal strains in the material due to lengthening 

 in one direction and contraction at right angles (magnetostriction). 

 By the application of an external magnetic field in the temperature 

 region near to, but below, the Curie point, (1) the magnetic domains 

 are oriented nearly in the direction of the applied field, and (2) the 

 internal strains which were introduced by the local magnetization at the 

 Curie point are removed by plastic flow of the surrounding domains. 

 For this to occur, the temperature of Curie point must be higher than 

 that at which plastic flow begins. Subsequent application of an ex- 

 ternal magnetic field at room temperature in the direction of, or op- 

 posite to, the direction of the magnetization at the higher temperature 

 introduces no strains in the material tending to decrease the ease of 

 magnetization. For that reason relatively higher permeabilities are 

 obtained than with material heat treated in the absence of a field, in 

 which case opposing strains are present. 



The theory also offers another explanation for the effects of heat 

 treatment on the permalloys in the region of 75 per cent nickel. This 

 explanation will not be discussed further here except to say that it is 

 concerned with the same magnetostrictive strains as have been 

 mentioned above. 



The fundamental idea of the theory proposed to account for the 

 effects of heat treatment in a magnetic field, and the general effects of 

 heat treatment on certain ferromagnetic materials, is that the magneto- 

 striction associated with the local magnetization in the domains is 

 sufficient to cause plastic flow. Experimental data indicate that the 

 magnetostrictive stress is sufficient to cause such plastic flow at about 

 the temperature which is known to be critical for the heat treatment of 

 the permalloys. 



