  Fe$_3$O$_4$ is a ferrimagnetic spinel ferrite that exhibits electric
conductivity at room temperature (RT). Although the material has been predicted
to be a half metal according to ab-initio calculations, magnetic tunnel
junctions (MTJs) with Fe$_3$O$_4$ electrodes have demonstrated a small tunnel
magnetoresistance effect. Not even the sign of the TMR ratio has been
experimentally established. Here, we report on the magnetic properties of
epitaxial Fe$_3$O$_4$ films with various crystal orientations. The films
exhibited apparent crystal orientation dependence on hysteresis curves. In
particular, Fe$_3$O$_4$(110) films exhibited in-plane uniaxial magnetic
anisotropy. With respect to the squareness of hysteresis, Fe$_3$O$_4$ (111)
demonstrated the largest squareness. Furthermore, we fabricated MTJs with
Fe$_3$O$_4$(110) electrodes, and obtained an TMR effect of -12\% at RT. The
negative TMR ratio corresponded to the negative spin polarization of
Fe$_3$O$_4$ predicted from band calculations.
