The Physical Basis of Ferromagnetism

N T H E 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 1 2 BELL SYSTEM TECHNICAL JOURNAL moments, but it has now been established that when the magnetization is altered all that changes is the direction or "sense" of the spin of certain of the electrons in the atoms--the orbital motions remain practically unchanged. The electrons that are responsible for the magnetic properties of iron, cobalt, nickel and their alloys lie in a definite "shell" in the atom. As shown 'in Fig. 1, there are four shells or regions, more or