First-order transition between a small-gap semiconductor and a   ferromagnetic metal in the isoelectronic alloys FeSi$_{1-x}$Ge$_x$
  The contrasting groundstates of isoelectronic and isostructural FeSi and FeGe can be explained within an extended local density approximation scheme (LDA+U) by an appropriate choice of the onsite Coulomb repulsion, $U$ on the Fe-sites. A minimal two-band model with interband interactions allows us to obtain a phase diagram for the alloys FeSi$_{1-x}$Ge$_{x}$. Treating the model in a mean field approximation, gives a first order transition between a small-gap semiconductor and a ferromagnetic metal as a function of magnetic field, temperature, and concentration, $x$. Unusually the transition from metal to insulator is driven by broadening, not narrowing, the bands and it is the metallic state that shows magnetic order. 
V. I. Anisimov, R. Hlubina, M. A. Korotin, V. V. Mazurenko, T. M.   Rice, A. O. Shorikov, M. Sigrist