Recent progress in the theory of itinerant electron magnetism

A review is given of recent theoretical investigations toward unified understanding of magnetism in narrow-band electron systems. It is emphasized that the classical controversy between the itinerant and localized models have been resolved into a more general and well-defined problem of spin density fluctuations in a general sense. The local moment picture is a limiting form of general spin fluctuations; and in its opposite limit we have weakly ferro- and antiferromagnetic metals. As an approach from the latter limit, the self-consistent renormalization theory of spin fluctuations is shown to have been quite successful in explaining and predicting a number of qualitatively new physical properties of this class of materials. More recent theoretical studies of spin fluctuations from a general point of view interpolating between the above-mentioned two limits seem to lead to a unified picture of magnetism in narrow-band electron systems including 3d transition metals and magnetic compounds.