ESR and heat capacity studies of phase transition in Rb1C60

Rb1C60 is studied by ESR as a function of frequency from several tenth MHz to 26 GHz and by heat capacity, to reveal the nature of the phase transition around 50 K. Above the transition temperature, the electronic states are consistent with a metallic state. Below it, the ESR linewidth increases linearly with decreasing T, which is ascribed to a development of short-range order (SRO) based on its frequency dependence. Furthermore, the present analysis of the EPR spectra suggested a distribution of the linewidth that was consistent with broadly distributed transition temperatures of the spin density wave (SDW) or antiferromagnetism. The heat capacity shows a broad anomaly from 50 K down to 30 K The apparent entropy change of such a magnetic anomaly suggests that only one fourth of spins are in the ordered state at 30 K, which is consistent with the reported intensity of AFMR.