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 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${}_{2.75}$C${}_{60}$ undergoes antiferromagnetic ordering with a weak ferromagnetic component at the Néel temperature, $T_{\mathrm{N}}$ $=$ 17 K.

• Received 17 February 2010

DOI:https://doi.org/10.1103/PhysRevB.83.245103