  In order to investigate the relationship between superconductivity and
magnetism in bilayer-hydrate cobaltate Na_x(H_3O)_zCoO_2 \cdot yH_2O, Co
nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR)
measurements were performed on three different samples, which demonstrate
various ground states at low temperatures. The appearance of small internal
fields is observed in the NQR spectra below approximately 6 K on one of the
samples that possesses the largest c-axis length and the highest NQR frequency.
The other two samples exhibit superconducting transition in zero magnetic
field, while these two samples show different ground states in the magnetic
fields greater than 5 T. The comparison of the NMR spectra of these two samples
obtained in high magnetic fields reveals the appearance of static internal
magnetic fields at the Co site below 4 K in the sample that possesses the
intermediate c-axis length and the NQR frequency.
