  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.
