Transition Metals

Abstract

The theory of magnetism in transition metals has fascinated and intrigued physicists for many years and, despite impressive progress in this area, continues to do so. At the center of present-day interest is still the long-standing controversy about the relative importance of itinerant versus localized features of d electrons, which goes back to Van Vleck [11.1] and Slater [11.2]. In order to avoid misunderstanding, it should be pointed out that without doubt, d electrons are delocalized. Yet the question remains: are correlations so strong that important atomic properties — like Hund’s rule correlations — are also significant in a solid and determine the magnetic properties? A related question arises with respect to d-electron number fluctuations at different sites. To what extent are they suppressed by electron correlations? To answer these questions by means of ab initio calculations has so far proved impossible; instead, one must resort to a multiband model-Hamiltonian for the d electrons. The s and p electrons are included only in renormalized parameters of the d-electron system. A good treatment of electron correlations in the ground state becomes essential for an understanding of the conditions which favor a magnetic ground state. An incomplete treatment of correlations, on the other hand, may result in inaccurate or insufficient criteria for magnetic order given that charge fluctuations diminish automatically in a magnetically ordered ground state. We see this most easily if we consider, for example, a “strong” ferromagnet. It has a filled majority-spin band with the holes situated in the minority-spin band.