Characterization of superconductivity in NaxCoO2 · yH2O

Five samples of NaxCoO2 · yH2O were prepared and characterized carefully by X-ray diffraction, inductive-coupled plasma atomic emission spectroscopy, and redox titration, and their magnetic properties were elucidated. Anomalous behavior caused by a certain magnetic order was observed in magnetic susceptibilities of two samples instead of superconductivity. This result strongly suggests that a magnetically ordered phase exists adjacent to the superconducting phase and that superconductivity of the system is caused by the magnetic fluctuation. The presence of the magnetic phase is not consistent with a c/a-TC phase diagram proposed previously, suggesting that other parameters should be considered to draw the phase diagram. The phase diagram and the magnetic state of the non-superconducting phase are discussed.