Spin response in organic spin valves based onLa2∕3Sr1∕3MnO3electrodes

We fabricated spin-valve devices made of organic semiconductor thin films sandwiched between ferromagnetic half-metal ${\mathrm{La}}_{2∕3}{\mathrm{Sr}}_{1∕3}\mathrm{Mn}{\mathrm{O}}_{3}$ (LSMO) and cobalt electrodes, using three different organic molecules. Subsequently, we studied the spin injection and transport properties by measuring the device magnetoresistance (MR) response at various biasing voltages $V$ and temperatures $T$. We found that the spin-valve MR response in all devices monotonically decreases with $V$ and is asymmetric with respect to the voltage polarity. We also found a steep MR decrease with $T$, where it vanishes at $T\ensuremath{\sim}220\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, similar to other MR responses in inorganic tunneling junction devices based on LSMO and Co ferromagnetic electrodes. In contrast, the spin-$\frac{1}{2}$ photoluminescence detected magnetic resonance of the organic interlayer, which directly depends on the spin-lattice relaxation rate of polarons in the organic semiconductor, was found to be temperature independent. We thus conclude that the steep MR dependence on $T$ is due to the temperature dependence of the interfacial spin polarization of the LSMO electrode, which also drastically decreases up to $T\ensuremath{\sim}220\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We thus conclude that (i) the spin-lattice relaxation time in organic semiconductors should not be the limiting factor in fabricating room temperature organic spin valves, and (ii) in order to achieve room temperature spin-valve operation with substantial MR value, spin-injection electrodes other than LSMO need to be involved, having large and less temperature dependent spin polarization.