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

系统地研究了富勒烯(C&lt；60&gt；)-钴(Co)颗粒薄膜中发现的巨型TMR效应，该薄膜由C&lt；60-Co化合物基质和Co纳米颗粒组成。结果表明，C&lt；60&Gt；-Co薄膜的TMR比高达100%-1000%，反映了Co纳米粒子的分散状态，可以通过样品的组成进行控制。同步辐射X射线吸收和磁性圆二色谱研究表明，在C&lt；60&Gt；-Co化合物基质中存在局域d自旋，并且局域自旋对TMR的温度依赖性起主导作用。对纳米层状结构器件的电学和TMR特性的分析表明，在低温下产生巨大的TMR效应是由于高阶的共隧穿过程，这与薄膜的颗粒结构有关，更重要的是由于在Co纳米颗粒和C_(60)-Co化合物的界面上产生的隧道电子的自旋极化接近完全(在零温和偏压下大于80%)。指出隧穿电子的近乎完全的自旋极化是本杂化系统的一个特征，它是在有明确的颗粒结构且与器件结构无关的条件下实现的。

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.