  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.
