  Sustaining exotic quantum mechanical phases at high temperatures is a
long-standing goal of condensed matter physics. Among them, half-metals are
spin-polarized conductors that are essential for realizing room-temperature
spin current sources. However, typical half-metals are low-temperature phases
whose spin polarization rapidly deteriorates with temperature increase. Here,
we first show that a low-temperature insulator with an unequal charge gap for
the two spin channels can arise from competing Mott and band insulating
tendencies. We establish that thermal fluctuations can drive this insulator to
a half-metal through a first-order phase transition by closing the charge gap
for one spin channel. This half-metal has 100% spin polarization at the onset
temperature of metallization. Further, varying the strength of electron
repulsion can enhance the onset temperature while preserving spin polarization.
We outline experimental scenarios for realizing this tunable finite temperature
half-metal.
