ABSTRACT
This chapter considers theories of magnetism based on two mutually exclusive models: the localized moment model and the band or itinerant electron model. The quantum theory of ferromagnetism is derived from the quantum theory of paramagnetism in much the same way as the classical Weiss theory of ferromagnetism was derived from the Langevin theory of paramagnetism. The Curie law of paramagnetic susceptibilities can be derived from the quantum theory of paramagnetism. The field originates from quantum mechanical interactions between the electrons. A discussion of these together with a derivation of the exchange interaction from considering overlap of the electron wave functions has been given in detail by D. H. Martin. W. Heisenberg was the first to include the electron spins in the wave function and then apply the same Heitler-London approximation to obtain the total wave function of a two-electron system.
