  We show that doped cubic iron pyrite, which is a diamagnetic semiconductor,
becomes ferromagnetic when $p$-type doped. We furthermore find that this
material can exhibit high spin polarization both for tunneling and transport
devices. These results are based on first principles electronic structure and
transport calculations. This illustrates the use of $p$-type doping without
magnetic impurities as a strategy for obtaining ferromagnetic semiconducting
behavior, with implications for spintronic applications that require both
magnetic ordering and good mobility. This is a combination that has been
difficult to achieve by doping semiconductors with magnetic impurities. We show
that phosphorus and arsenic may be effective dopants for achieving this
behavior.
