MAGNETIC MATERIALS IN RELATION TO STRUCTURE 15 



high temperatures, the impurities continually diffuse to the surface of 

 the sample where the reaction with hydrogen occurs. Another 

 possibility is that the reactions occur within the body of the sample, in 

 which case the hydrogen must diffuse in and the reaction products out. 

 The first possibility appears the more promising in that the diffusion 

 of reaction products through the iron samples would be difficult, since, 

 in general, the products are rather large molecules. 



Another type of high purity iron which is being used for magnetic 

 purposes is carbonyl iron '^ manufactured by the I. G. Farbenindus- 

 trie in Germany. It is prepared by first forming iron carbonyl under 

 suitable conditions of temperature and pressure, and subsequently de- 

 composing the carbonyl. For the best quality, the material is further 

 purified by a low temperature hydrogen treatment. This iron is in the 

 form of spherical particles, a few microns in diameter. 



One use for this material is in cores of high frequency inductance 

 coils. The cores of such coils are prepared by pressing the insulated 

 magnetic particles into the desired core shape. 



The material is also used in preparing iron sheet ^^ and as an alloying 

 constituent for the manufacture of iron-nickel alloy sheet." The 

 process of alloying consists of pressing the powders and sintering at a 

 high temperature, followed by working and annealing in hydrogen or 

 vacuum. Typical permeability values for iron sheet prepared in this 

 manner are: mo = 2000-3000; /Xmax. = 15,000. The values are de- 

 cidedly lower than those for a laboratory sample of hydrogen-treated 

 iron shown in Fig. 4. 



Iron Nickel Alloys 



No alloy system has been more fruitful in yielding interesting and 

 useful magnetic alloys than that of iron and nickel. In this system, the 

 three regions marked with arrows on the constitutional diagram in Fig. 

 5 are of principal importance. 



In the proximity of 25 per cent of nickel, which is the region of re- 

 tarded phase change, alloys can be obtained at room temperature in the 

 non-ferromagnetic state. How this is accomplished is evident from a 

 consideration of the constitutional diagram. The y solid solution 

 above the magnetic transformation is non-ferromagnetic while the a 

 solid solution is ferromagnetic. When alloys in the region of 25 per 

 cent nickel are cooled, the transformation to the ferromagnetic state is 

 not completed until temperatures below room temperature are reached. 

 If the alloy is cooled until the ferromagnetic structure is obtained, the 

 transformation to the non-ferromagnetic state does not occur on heat- 

 ing until the temperature has reached approximately 600° C. 



