Ferromagnetism in doped excitonic insulators

The theory of doped excitonic insulators is reinvestigated in light of recent experiments on hexaborides. For the appropriate valley-degenerate X-3,X-3' band structure, ``intravalley{''} condensation is energetically favored. Ferromagnetism occurs upon doping due to the quenching of kinetic energy at the otherwise direct first-order excitonic insulator-metal transition. The phase diagram includes states of spatially inhomogeneous density and magnetization at low temperatures.