Electronic phase transition of the valence-fluctuating fulleride Eu _2.75C₆₀

The electronic properties of Eu_2.75C₆₀ 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 T_V, while it changes drastically to Curie-Weiss behavior below T_V. The low-temperature susceptibility can be reproduced by assuming the moment of free Eu2⁺ ions. This result reveals that Eu_2.75C₆₀ 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 π 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_2.75C₆₀ undergoes antiferromagnetic ordering with a weak ferromagnetic component at the Néel temperature, T_N = 17 K.