First-Order Transition between a Small Gap Semiconductor and a Ferromagnetic Metal in the Isoelectronic AlloyFeSi1−xGex

The contrasting ground states of isoelectronic, isostructural FeSi and FeGe are explained within an extended local density approximation scheme ($\mathrm{L}\mathrm{D}\mathrm{A}+U$) by an appropriate choice of the Coulomb repulsion $U$ on the Fe sites. A minimal two-band model with interband interactions leads to a phase diagram for the alloys ${\mathrm{F}\mathrm{e}\mathrm{S}\mathrm{i}}_{1-x}{\mathrm{G}\mathrm{e}}_x$. 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.