  Understanding the effects of electron-electron interactions in half metallic
magnets (HMs), which have band structures with one gapped spin channel and one
metallic channel, poses fundamental theoretical issues as well as having
importance for their potential applications. Here we use determinant quantum
Monte Carlo to study the impacts of an on-site Hubbard interaction $U$, finite
temperature, and an external (Zeeman) magnetic field on a bilayer tight-binding
model which is a half-metal in the absence of interactions, by calculating the
spectral density, conductivity, spin polarization of carriers, and local
magnetic properties. We quantify the effect of $U$ on the degree of thermal
depolarization, and follow relative band shifts and monitor when significant
gap states appear, each of which can degrade the HM character. For this model,
Zeeman coupling induces, at fixed particle number, two successive transitions:
compensated half-metal with spin-down band gap $\rightarrow$ metallic
ferromagnet $\rightarrow$ saturated ferromagnetic insulator. However, over much
of the more relevant parameter regime, the half-metallic properties are rather
robust to $U$.
