  Resonant tunneling can lead to inverse tunnel magnetoresistance when impurity
levels rather than direct tunneling dominate the transport process. We
fabricated hybrid magnetic tunnel junctions of CoFe/LiF/EuS/Ti, with an
epitaxial LiF energy barrier joined with a polycrystalline EuS spin-filter
bar-rier. Due to the water solubility of LiF, the devices were fully packaged
in situ. The devices showed sizeable positive TMR up to 16% at low bias
voltages but clearly inverted TMR at higher bias voltages. The TMR inversion
depends sensitively on the thickness of LiF, and the tendency of inversion
disap-pears when LiF gets thick enough and recovers its intrinsic properties.
