Magnetic correlation of NaxCoO2 and successive phase transitions of Na0.5CoO2 -: NMR and neutron diffraction studies

Data taken by various methods including NMR/NQR and neutron scattering are presented for NaxCoO2, particularly Na0.5CoO2. Attention has also been paid to the x dependence of the electronic nature of this system. The pseudo-gap-like behavior observed in the region of x < x(c) similar to 0.6 is emphasized. For samples with x <= 0.6, signals from two kinds of Co sites with different quadrupole frequencies v(Q) of similar to 4.1 and similar to 2.8 MHz were observed, and the former v(Q), which is nearly equal to that of the superconducting system Na(0.3)CoO(2)center dot yH(2)O, becomes dominant with decreasing x to 0.3. For Na0.5CoO2, the Co sites with the larger v(Q) have larger magnetic moments. They align antiferromagnetically at Tc1 similar to 87 K with their direction within the plane, while the Co sites with the smaller vQ have smaller moments. They align in the direction parallel to the c axis. The ordered moments at the two distinct sites exhibit the same T-dependence, indicating that the existence of these sites is not due to macroscopic phase separation. Although we have observed the splitting of the zero-field NMR signals from Co sites with smaller vQ at the second transition temperature T-c2 similar to 53 K, any other anomaly of the ordered magnetization has not been observed at T-c2 in the T-dependences of neutron and NMR data. As x decreases, the two-dimensional nature of the electrons is enhanced and the antiferromagnetic correlation increases. On the basis of these results, the superconducting pair state in Na(0.3)CoO(2)center dot yH(2)O is discussed.