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Turning on strong electron correlations in a normal metal often
leads to interesting phases like Mott Insulators, high Tc superconductors,
magnets and non Fermiliquids. But the effect of correlations in band
insulators are less explored. We study effects of an onsite Hubbard
interaction U on a band insulator and find a novel interaction driven
route to a half-metallic (HM) ferrimagnetic state. In the undoped system,
we show that increasing U leads to a first order transition at a finite
value U_AF between a paramagnetic band insulator and an antiferomagnetic
Mott insulator . Upon doping, the system exhibits half-metallic
ferrimagnetism over a wide range of doping and interaction strengths on
either side of U_AF . |