Magnetic properties of superconducting cobalt oxides NaxCoO2.yH2O
  Studies of the NMR Knight shift K of Na0.3CoO2.yH2O have been carried out in detail. The suppression of K by the occurrence of the superconductivity reported previously by the present authors in both magnetic field directions perpendicular and parallel to the c axis has been confirmed, indicating that the Cooper pairs are in the singlet state. The anisotropy of the suppression amplitudes is consistent with the anisotropy of the hyperfine coupling constant Aspin estimated by the K-kai plot. It has also been found that even samples, which do not exhibit a significant amount of the Curie-Weiss-like increase of the uniform magnetic susceptibility kai with decreasing temperature T, exhibit the superconducting transition, which indicates that the superconducting Na0.3CoO2.1.3H2O is not necessarily be in the proximity region of the ferromagnetic phase. It has also been confirmed that the superconducting transition temperature Tc of the samples prepared by mixing Na0.7CoO2, H2O (or D2O) and bromine for 4 h, and separating the resultant powder from the solution by filtration, depends on the elapsed time t after filtration. For these samples, the Curie constant of kai estimated at low temperatures increases with t, supporting the idea that the number of the lattice imperfection possibly due to the oxygen-vacancy-formation increases with t. Even for such samples, the superconductivity appears, which seems to exclude the possibility of the anisotropic superconducting order parameter. These results are favorable for the full-gapped superconductivity. 
Yoshiaki Kobayashi, Taketo Moyoshi, Hidekazu Watanabe, Mai Yokoi and   Masatoshi Sato