PRESENT STATUS OF FERROMAGNETIC THEORY 



69 



saturation value than pure iron are the iron-cobalt alloys; and com- 

 pounds and alloys of manganese are more magnetic than any others 

 that do not contain iron, cobalt, or nickel. The Heusler alloys, com- 

 posed of manganese, aluminum, and copper, have a saturation almost 

 as high as nickel, and numerous compounds of manganese are ferro- 

 magnetic in a less degree. 



The forces of exchange are purely electrostatic in origin. But they 

 are not electrostatic in the classical sense of the word; they are the 

 result of electric charges distributed in space in a definite way. It does 

 not seem to be possible to describe them easily in words, for it takes a 

 great many mathematical equations to derive the result, which is a 

 consequence of the assumptions of quantum mechanics. These forces 



EXCESS SPINS 

 4 3 



Mn Fe CO 



ELEMENT 



CU 



Cn 



Fe CO 



ELEMENT 



Ni 



Cu 



Fig. 4 — Iron-cobalt alloys ha\e the highest value of saturation magnetization. 



account for the fact that it is easy to align the excess spins of large groups 

 of atoms of some materials. In fact, when the forces of exchange are 

 large as they are in ferromagnetic materials, the stable situation is one 

 in which the spins are parallel, even when no magnetic field is applied. 

 But the parallelism under such circumstances does not extend over 

 the whole of a specimen of ordinary or even of visible size; for some 

 reason not understood it is limited to smaller regions. On the average, 

 those regions are found experimentally to have the volume of a cube 

 about 0.001 inch on an edge. An actual ferromagnetic body is composed 

 of a great many such regions, called ''domains " each domain being 

 magnetized to saturation {i.e., electron spins parallel) in some direction. 

 When the material is said to be unmagnetized, the domains are oriented 

 equally in all directions so that the magnetization of the specimen as 

 a whole is zero. 



