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. In so doing, 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. 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 $3d$-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. 
Takuya Okabe