Theory on the itinerant ferromagnetism in the 3d-transition metal systems

Keeping nickel, cobalt and iron in mind, we investigate the origin of the itinerant ferromagnetism. Recent experiments show that the systems should be in an intermediate coupling regime, where the band width and the interaction energy are of the same order of magnitudes. To treat such a situation, we generalize the Gutzwiller approximation. In that, we take account of the effect of the band degeneracy and the Hund's-rule coupling in addition to the on-site repulsion. In generalizing the Gutzwiller approximation to the bands with degeneracy, we introduce the intuitive way to give the required expressions. After the discussion on nickel, the condition for the incomplete ferromagnetism, observed in cobalt and iron, is argued. Phase diagrams, which show the interplay between the band shape peculiarity and the Hund's-rule coupling, are given. It is found that for the Sd-transition metal systems, both of the Hund's-rule coupling and the special feature of the density of states are necessary to explain the itinerant ferromagnetism.