Effect of La0.67Sr0.33MnO3 electrodes on organic spin valves

We report the effect of La0.7Sr0.3MnO3 (LSMO) electrodes on the temperature dependence of the magnetoresistance (MR) of LSMO/polymer/cobalt spin valves (SVs). LSMO films have been prepared by pulsed laser deposition on three different single crystal substrates using different deposition parameters. The films were characterized for their surface morphologies, structural, magnetic, and magnetotransport properties. Low deposition rate is found to be detrimental for growth of good quality films and polycrystalline films with grain boundary effects are observed in thicker films. The films on MGO (100) substrate show a broad paramagnetic to ferromagnetic transition, accompanied with a metal-insulator transition below room temperature. This indicates growth of some strained structures due to large lattice mismatch (9%) between the substrate and the film and presence of polycrystalline grain boundaries. The deposited films on STO (100) and NGO (001) show much sharper magnetic transition and metallic behavior indicating higher spin polarization (SP) of LSMO on these substrates at room temperature. SVs made on STO (100) show improvement in switching behavior and better MR response compared to the devices made on MGO (100) at low temperatures. No difference in MR response was found at room temperature in either case. We conclude that the bulk spin polarization of LSMO films is not very important in the SV operation. The loss of most of the SP carriers at the LSMO/organic semiconductor interface at room temperature is a more dominant effect and drastically reduces the MR signal.