THEORIES OF MAGNETISM.

The Kondo effect and the theory of spin waves are discussed. The minimum in the resistance of several metals has been traced to transition metal impurities in tiny concentrations. Kondo has shown that the effect follows from the existence of localised spins on the impurity. The scattering of conduction electrons from a centre with an internal degree of freedom has an essential many-body character. When the exchange coupling between the conduction electrons and the localised spin is antiferromagnetic he found as T approaches O a logarithmic divergence in third-order perturbation theory which gave an increase in the resistivity as T decreased and hence a minimum when added to the ordinary term. The interaction between a pair of spins was examined. Several aspects of spin wave theory were considered. A space group theory was set up to describe the form of spin wave spectra, calculate selection rules, etc.