Abstract
The magnetic property of K xC 70 upon K doping process was investigated by electron spin resonance (ESR) and low-magnetic field microwave absorption (low-field signal: LFS). In the early stage of K doping (doping time t d < 40 h), ESR signal with a Curie-like behavior was observed, which means an insulating state of a doped phase in this stage. In t d > 40 h, ESR spectra broadened suddenly and the almost temperature-independent behavior of ESR intensity appeared in the high temperature region. This suggests a change from an insulating K xC 70 phase to a metallic K xC 70 phase. The chemical analysis has presented that the insulating phase corresponds to K 3C 70 and the metallic phase corresponds to K 4C 70. A very weak LFS below ca. 9 K was observed in the doping stage of t d ≥ 10 h, indicating the coexistence of a small volume of some superconducting phase. A possible origin of the superconducting phase is described.
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