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.
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