Observation of inverse magnetoresistance in perovskite oxide tunnel junctions

We have observed a distinct inverse magnetoresistance in a hybrid tunnel junction. Two types of perovskite oxides, La0.67Sr0.33MnO3 and SrRuO3, were used as ferromagnetic electrodes with an SrTiO3 barrier in between them. Micron-scale junctions were fabricated from 90 off-axis sputtered trilayers, using standard optical lithography. Homo-junctions that incorporate either La0.67Sr0.33MnO3 or SrRuO3 as both electrodes showed only normal positive magnetoresistances. In contrast, a clear inverse magnetic switching was found at 10 K for the hybrid junction. The. spin polarization of SrRuO3 is about -9% as calculated based upon Julliere model, which is in quantitative agreement with the value (-10%) determined by the earlier tunneling experiment on SrRuO3/SrTiO3/Al structures. This is not only the manifestation of the existence of a negatively polarized material at the Fermi level, but also demonstrates that the widely-cited Julliere model is applicable even to a negative polarization case. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.