Electronic phase transition of the valence-fluctuating fulleride Eu2.75C60

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, ${T}_{\mathrm{V}}$ $=$ 70 K. The magnetic susceptibility shows weak temperature dependence above ${T}_{\mathrm{V}}$, while it changes drastically to Curie-Weiss behavior below ${T}_{\mathrm{V}}$. 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 ${T}_{\mathrm{V}}$. Although ESR signals above ${T}_{\mathrm{V}}$ should be attributed to conduction electrons, the ESR intensity below ${T}_{\mathrm{V}}$ follows the Curie-Weiss law with a distinct increase in the $g$-factor. This should be associated with a strong localization of \ensuremath{\pi} electrons. We also found that, below \ensuremath{\sim}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 N\'eel temperature, ${T}_{\mathrm{N}}$ $=$ 17 K.