Remarks on Theories of Ferromagnetism

No attempt will be made here to present the complex and not too well understood theoretical background of ferromagnetic phenomena. Treatments on these topics will be found elsewhere. (See Refs. 15, 43, 44.) 

Two or three points will be mentioned because they bear directly on some of the experimental results to be presented below. 

The phenomenon of the Curfe point is understandable on the basis of Heisenberg s theory of exchange interaction, with a series of modifications by later investigators. The relation foimd for the Curie point is that already presented above in another connection, namely, 

For our present purposes it is important merely to note that as a ferromagnetic substance becomes diluted with a diamagnetic substance, either / or z or both may diminish, with a consequent lowering of the Curie point. This effect has useful practical and theoretical implications as will be shown below. 

The phenomenon of hysteresis has received fairly adequate theoretical treatment on the two conceptions of crystal anisotropy and magnetostriction. (A cubic crystal may exhibit ferromagnetic anisotropy, owing presumably, to interaction between electron spins and lattice, as a result of which certain preferred directions are found for spontaneous magnetization.) 

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