On the theory of ferromagnetism

Abstract

An attempt is made to specify the conditions under which Heisenberg's model of ferromagnetism is correct. It is found that in addition to the exchange term there are other terms in the hamiltonian, describing the effects of the band width and of polar states. The new hamiltonian, which has a simple form, is naturally represented in terms of Fermi creation and absorption operators. The molecular field approximation applied to this new formulation leads essentially to the results of Stoner's collective theory. A better treatment in the low temperature region gives the familiar T3/2-law. A new type of spin wave is found, which should not be difficult to detect experimentally.