Magnetic Correlation of NaxCoO2 and Successive Phase Transitions of Na0.5CoO2 -NMR and Neutron Diffraction Studies-

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.