THE PHYSICAL BASIS OF FERROMAGNETISM 17 



It is a problem of theoretical physics to determine the nature of the 

 molecular field. Before discussing what progress has been made in 

 doing this it will be necessary to review some of our knowledge of the 

 structure of the atoms with which we are concerned. 



Atomic Structure of Ferromagnetic Materials 



The structure of an isolated iron atom has already been shown in 

 Fig. 1. The twenty-six electrons are divided into four principal 

 "shells," each shell a more or less well defined region in which the 

 electrons move in their "orbits." The first (innermost) shell contains 

 two electrons, the next shell eight, the next sixteen, and the last two. 

 As the periodic system of the elements is built up from the lightest 

 element, hydrogen, the formation of the innermost shell begins first, 

 and when completed the numbers of electrons in the first four shells 

 are two, eight, eighteen, and thirty-two, but the maximum number in 

 each shell is not always reached before the next shell begins to be 

 formed. For example, when formation of the fourth shell begins, the 

 third shell contains only eight electrons instead of eighteen; it is the 

 subsequent building up of this third shell that is intimately connected 

 with ferromagnetism. In this shell some electrons will be spinning in 

 one direction and others in the opposite, and these two senses of the 

 spins may be conveniently referred to as positive and negative. The 

 numbers on the circles show how many electrons with + and — spins 

 are present in each shell in iron and it will be noticed that all except the 

 third shell contain as many electrons spinning in one direction as in the 

 opposite. The magnetic moments of the electrons in each of these 

 shells mutually compensate one another so that the shell is magnetic- 

 ally neutral and does not have a permanent magnetic moment. In the 

 third shell, however, there are five electrons with a positive spin and 

 one with a negative so that four electron spins are unbalanced or un- 

 compensated and there is a resultant polarization of the atom as a 

 whole. The existence of a permanent magnetic moment for each atom 

 obviously satisfies one of the requirements for ferromagnetism. 



In the free atom the orbital motions of the electrons also contribute 

 to the magnetic moment. When the iron atom becomes part of 

 metallic iron the electron orbits become too firmly fixed in the solid 

 structure to be influenced appreciably by a magnetic field. The 

 corresponding moments do not change when the intensity of magnetiza- 

 tion changes — this is shown by the gyromagnetic experiments discussed 

 later — and it is supposed that the orbital moments of the electrons in 

 various atoms neutralize one another. 



