708 



band gives 



BELL SYSTEM TECHNICAL JOURNAL 



= (2/3)xs + Xt = y ( -^ ) iipKEm..y" + Xi- 



(21) 



In Table IV we give theoretical and experimental values for the sus- 

 ceptibilities of the simple metals. The values of Xt are obtained from 

 theory for lithium and by experiment for the other metals. 



TABLE IV 

 Magnetic Susceptibilities * 



* This Table is taken from N. F. Mott and H. Jones, "The Theory of the Proper- 

 ties of Metals and Alloys," Oxford 1936, p. 188. 



t K. Honda, Ann. d. Physik 32, 1027 (1910) and M. Owen, Ann. d. Physik 37, 657 

 (1912). 



Although equation (17) for the spin susceptibility Xs in terms of 

 N{Ei) is generally true, the relationship that Xm = — Xs/3 is true 

 only for the case when N{E) is the free electron distribution. ^^ For 

 some metals N{E) differs greatly from that for free electrons and then 

 larger values of Xm may occur. The high diamagnetism of bismuth is 

 explained in this way. In the next paper, we shall discuss the mean- 

 ing of the freeness of electrons; however, a discussion of electron 

 diamagnetism lies beyond the scope of this paper.^^ 



Ferromagnetism 



The shift of electrons from one band to another for the paramagnetic 

 behavior shown in Fig. 26 persists only so long as the magnetic field 

 is applied. When the magnetic field is removed, the stable arrange- 

 ment is as shown in Fig. 26a, equal numbers of electrons having each 

 spin. The situation is quite different in ferromagnetic materials and, 

 for reasons discussed below, in the stable arrangement there are many 

 more electrons of one spin than of the other. 



Two things are important for the occurrence of ferromagnetism : 

 the exchange effect as illustrated in Figs. 4 and 6 and the structure of 

 the bands arising from the 3d levels. The 3d levels, as is shown in 



^ An even more stringent condition is actually required. 



33 For the diamagnetism of electrons in closed shells the reader is referred to K. K. 

 Darrow's article, "The Theory of Magnetism," Bell System Technical Journal, Vol. 

 XV, 1936, and in particular to Page 247. 



