PRESENT STATUS OF FERROMAGNETIC THFZORY 



83 



This relation between internal strain and permeability is illustrated 

 by the data ^* shown in Fig. 19. The permeabilities of a series of 

 specimens of 70 permalloy tape, originally cold rolled, increase as the 

 annealing temperature is raised. X-ray data (the angular width of the 

 reflected X-ray beam) on these same specimens indicate the magnitude 

 of the internal strains existing, and show that they become progres- 

 sively less as the annealing temperature is increased, the most rapid 

 change taking place in each case between 400 and 600 degrees centi- 



20,000 



100 200 300 400 500 600 700 800 900 1000 1100 1200 



TEMPERATURE OF ANNEAL IN DEGREES CENTIGRADE 



Fig. 19 — Magnetic permeability rises as internal strain is diminished by annealing 



(Dillinger and Haworth). 



grade, in which region the microscope shows recrystallization has 

 occurred. 



Following out this same idea, it may be surmised that to make good 

 material for a permanent magnet something with very intense internal 

 strains is required. The direct determination by X-rays of internal 

 strain in a good permanent magnet, confirms this view (Fig. 20). 

 Here the widths of the reflected X-ray beams directly measure the 

 internal strains. For comparison with the permanent magnet ma- 

 terial, curves are shown for other materials with less internal strain. 

 The magnet material in this case was an iron-nickel-aluminum alloy 

 that was precipitation-hardened, a method used more and more 

 extensively during the last three or four years for such materials. 

 This method is often applicable when the alloy ^^ in the stable condition 

 consists of two phases at room temperature (Fig. 21), but when at a 



