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 La 0.7 Sr 0.3 MnO 3 (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 SrTiO 3 or Al 2 O 3 . Together with first-principle calculations, we demonstrate that the strongly hybridized Co d-states with Alq 3 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.