  We report on the use of the LaAlO3 (LAO) high-k dielectric as a tunnel
barrier in magnetic tunnel junctions. From tunnel magnetoresistance (TMR)
measurements on epitaxial La2/3Sr1/3MnO3/LAO/La2/3Sr1/3MnO3 junctions, we
estimate a spin polarization of 77% at low temperature for the
La2/3Sr1/3MnO3/LAO interface. Remarkably, the TMR of La2/3Sr1/3MnO3/LAO/Co
junctions at low bias is negative, evidencing a negative spin polarization of
Co at the interface with LAO, and its bias dependence is very similar to that
of La2/3Sr1/3MnO3/STO/Co junctions. We discuss possible reasons for this
behaviour.
