THE QUANTUM PHYSICS OF SOLIDS 



709 



Fig. 6, are only partially filled for free atoms of the ferromagnetic 

 elements iron, cobalt, and nickel. We shall show first how the par- 

 tially filled 2)d bands together with exchange forces can produce ferro- 

 magnetism and later discuss the theory of why only the last three of 

 the eight transition elements are ferromagnetic. 



The splitting of the atomic energy levels into bands is shown in 



NUMBER OF QUANTUM STATES PER UNIT ENERGY 



Fig. 27 — Distribution of states in energy for copper. The distribution is prob- 

 ably quite similar for iron, cobalt, and nickel and in the absence of calculations for 

 these other metals, this figure will be used for them. The total number of quantum 

 states per atom in the 4s and 2>d bands having energies less than the ordinates of the 

 dashed lines are given by the corresponding integers. 



