  Data taken by various kinds of methods including NMR/NQR and neutron
scattering are presented for NaxCoO2, in particular, for Na0.5CoO2. Attentions
have also been paid to the x dependence of the electronic nature. The
pseudo-gap-like behavior observed in the region of x<xc~0.6 is emphasized. For
samples with x<0.6, signals from two kinds of Co sites with different
quadrupole frequencies nQ, ~4.1 MHz and ~2.8 MHz were observed, and the former
value of nQ, which is nearly equal to that of the superconducting system
Na0.3CoO2.yH2O, becomes dominant with decreasing x to 0.3. For Na0.5CoO2, the
Co sites with the larger nQ value have the larger magnetic moments. They align
antiferromagnetically at Tc1~87 K with their direction within the plane, while
the Co sites with the smaller nQ value have the smaller moments. They align
along the direction parallel to the c axis. The ordered moments at the two
distinct sites exhibit the same T dependences, indicating that the existence of
these sites is not due to a macroscopic phase separation. Although we have
observed the splitting of the zero-field NMR signals from the Co sites with the
smaller nQ value at the second transition temperature Tc2~53 K, any other
anomaly of the ordered magnetization has not been observed there in the T
dependences of the neutron and the NMR data. As x decreases, the
two-dimensional nature of the electrons is enhanced and the antiferromagnetic
correlation increases. Based on these results, the superconducting pair state
in Na0.3CoO2.yH2O is discussed.
