Design of Spin Transport Properties in Fullerene-Transition Metal Hybrid Materials

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Systematic studies were conducted on the giant TMR effect found in the fullerene(C_<60>)-cobalt(Co) granular thin films, which are composed of a C_<60>-Co compound matrix and Co nanoparticles precipitated therein. It was revealed that the C_<60>-Co films show TMR ratios as high as several 100%-1000% reflecting the dispersion state of Co nanoparticles which can be control by the sample composition. Synchrotron X-ray absorption and magnetic circular dichroism studies clarified that there exist localized d-spins in the C_<60>-Co compound matrix, and the localized spins have a predominant effect on the temperature dependence of the TMR magnitude. It was clarified from the analysis of the electrical and TMR properties of the devices with a nano-layered structure that the giant TMR effect at low temperatures is due to higher-order cotunnleing process, which is related to the granular structure of the films, and, more importantly, due to nearly-complete spin polarization (higher than 80% at zero temperature and bias) of tunneling electrons generated at the interface of Co nanoparticle and the C_<60>-Co compound. The nearly-complete spin polarization of tunneling electron is indicated to be a characteristic nature of the present hybrid system realized under the condition of well defined granular structure and independent of device structure.