Spin-Orbital Coupling Effects on Magnetoresistance in Organic Materials

We report the studies on magnetoresistance of organic materials based on the light-emitting diode of phosphorescent iridium complex Ir(ppy)$_{3}$ molecules dispersed in fluorescent poly(N-vinylcarbazole) (PVK). The magnetic field-dependent injection current indicates that the PVK of weak spin-orbital coupling exhibits a significant magnetoresistance while the resistance of Ir(ppy)$_{3}$ of super-strong spin-orbital coupling shows an independence of magnetic field up to 3000 Gauss. We find that the magnetoresistance from the Ir(ppy)$_{3}$/PVK composite displays a gradual decrease with increasing the concentration of Ir(ppy)$_{3}$. The magnetic field-dependent electroluminescence confirms that the dispersed Ir(ppy)$_{3}$ molecules account for the change of magnetorsistance in the Ir(ppy)$_{3}$/PVK composite. From the uniform dispersion of Ir(ppy)$_{3}$ molecules observed from transmission electron microscope, we suggest that the spin-orbital coupling is modified by the interface interaction and consequently varies the magnetoresistance in the Ir(ppy)$_{3}$/PVK composite.