68 BELL SYSTEM TECHNICAL JOURNAL 



resultant spins in neighboring atoms be parallel. Calculation of the 

 energies of the electrons indicates that to align the spins in all the 

 atoms in a small region, the diameter of an atom must bear the proper 

 ratio to the diameter of the electron shell in which the electron spins 

 are uncompensated ^^ (Fig. 3). This proper ratio is required because 



I n ferromagnetic 



substances, D/d is 



greater than 1.5 



(Slater) 



Fig. 3 — "Incomplete" shells in neighboring atoms. 



the electron spins and charges influence each other to an amount 

 depending upon the distance between them; and it is only when this 

 influence, which is known technically as the "exchange," has the right 

 value that the spins all can be aligned in the same direction,^^ that is, 

 that the material can become ferromagnetic.^^ 



The forces of "exchange," the existence of which has been realized 

 only in the last few years, act to keep the spins parallel, while thermal 

 agitation tends, obviously, to disturb this alignment. When the 

 temperature is high enough, the temperature agitation prevails and 

 the material ceases to be ferromagnetic. This temperature is the 

 familiar Curie point, or magnetic transformation point, 770 degrees 

 centigrade (or 1,043 degrees absolute) for iron. It is seen then that 

 the height of the Curie point, 6 (on the absolute temperature scale) is 

 an indication of the strength of the forces of exchange, which cannot 

 yet be calculated theoretically except as to order of magnitude. These 

 Curie points are plotted in Fig. 4 for the elements near iron in the 

 periodic table; if a continuous curve be drawn through the points, it 

 has a maximum near cobalt. Now the saturation value of magneti- 

 zation depends both on the exchange and on the number of effective 

 electron spins, that is, upon the number of electrons that can be 

 oriented parallel to the field and the strength of the forces that hold 

 them parallel. In a very rough way, it may be said to depend on the 

 product of the exchange and the number, S, of uncompensated spins 

 in the atom. Adopting as a measure of the exchange forces and 

 forming the product dS, the right-hand curve in Fig. 4 is obtained, 

 which indicates that the highest saturation should be attained in an 

 iron-cobalt alloy, and that under certain appropriate conditions 

 manganese might be ferromagnetic. Both these indications are 

 substantiated by the data: The only known alloys having a higher 



