Electronic phase transition of the valence-fluctuating fulleride EU_(2.75)C_(60)

摘要

The electronic properties of Eu_(2.75)C_(60) are studied using magnetic susceptibility and electron spin resonance (ESR) from 2 to 300 K. Both the magnetic susceptibility and the ESR parameters clearly show an anomaly around the valence transition temperature, T_v = 70 K. The magnetic susceptibility shows weak temperature dependence above 7V, while it changes drastically to Curie-Weiss behavior below T_v. The low-temperature susceptibility can be reproduced by assuming the moment of free Eu~(2+) ions. This result reveals that Eu_(2.75)C_(60) changes from the intermediate valence state to the divalent state below T_V. Although ESR signals above T_V should be attributed to conduction electrons, the ESR intensity below T_V follows the Curie-Weiss law with a distinct increase in the g-factor. This should be associated with a strong localization of n electrons. We also found that, below ~17 K, the isothermal magnetization exhibits a weak hysteresis and thermoremanent magnetization appears. These results suggest that valence-ordered Eu_(2.75)C_60 undergoes antiferromagnetic ordering with a weak ferromagnetic component at the Neel temperature, T_N = 17 K.