The Bell System Technical Journal 



Vol. XIX January, 1940 No. 1 



The Physical Basis of Ferromagnetism 



By R. M. BOZORTH 



After an introductory review of the general nature of the theory 

 of magnetic phenomena and the magnitudes of the atomic forces 

 involved, there is a discussion of Ewing's theory, its results and 

 limitations. The later theory of Weiss is then given briefly in order 

 to fix the concept of the molecular field. In order to elucidate the 

 nature of this field a digression is made to discuss the atomic struc- 

 ture of the ferromagnetic elements and elements having similar 

 structures. With this as a basis the physical nature of the molecular 

 field is discussed at some length. Its relation to the structure of 

 domains, particularly the nature of the boundaries between do- 

 mains, is brought out. 



Finally there is a review of the gyromagnetic effect, its signifi- 

 cance for magnetic theory, the principal expermental method for 

 its determination, and the numerical results supporting the idea 

 that the spin of the electron and not its orbital moment is respon- 

 sible for ferromagnetism. 



Introduction 



IN THE last five or ten years the theory of ferromagnetism has 

 shown indications of maturity. For the first time a plausible story 

 can be told concerning the ultimate magnetic particle, the essential 

 nature of the atom of a ferromagnetic substance, the kind of forces 

 which determine the properties of magnetic crystals, the effect of 

 strain on magnetic materials and the manner in which these various 

 phenomena combine to determine the properties of commercial 

 materials. It is true that the story is largely qualitative, and that 

 there are still many points that are uncertain or missing entirely, but 

 nevertheless it is possible to describe the major features with some 

 confidence. 



The fundamental magnetic particle is the spinning electron. One 

 might think that the orbital motions of the electrons in the atom 

 would also contribute to ferromagnetism, owing to their magnetic 



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