Strong asymmetrical bias dependence of magnetoresistance in organic spin valves: the role of ferromagnetic/organic interfaces

We report a highly asymmetric magnetoresistance (MR) bias dependence, with the inverse MR peaking at a negative bias and a sign reversal occurring at a positive bias in prototypical La0.7Sr0.3MnO3(LSMO)/Alq(3)/Co organic spin valve (OSV) with a tunnel barrier between LSMO and Alq(3). This behavior is in strong contrast with the commonly found inverse MR in entire bias range for LSMO/Alq(3)/Co OSVs. The MR bias voltage dependence is independent on the type of the tunnel barrier, either SrTiO3 or Al2O3. Together with first- principle calculations, we demonstrate that the strongly hybridized Co d- states with Alq3 molecules at the interface are responsible for the efficient d- states spin injection and the observed MR bias dependence is originated from the energy dependent density of states of Co d- states. These findings open up new possibilities to engineer interfacial bonding between ferromagnetic materials and a wide variety of molecule selections for the desired spin transport properties.