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 NaCoO2, 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 < xc ∼ 0.6 is emphasized. For samples with x ≤ 0.6, signals from two kinds of Co sites with different quadrupole frequencies νQ of ∼4.1 and ∼2.8 MHz were observed, and the former νQ, 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 νQ have larger magnetic moments. They align antiferromagnetically at Tcl∼87K with their direction within the plane, while the Co sites with the smaller νQ 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 νQ at the second transition temperature Tc2∼53K, any other anomaly of the ordered magnetization has not been observed at Tc2 in the T-dependences of neutron and NMR data. As x decreases, the two-dimensional nature of the electrons is enhanced and the antiferroamgnetic correlation increases. On the basis of these results, the superconducting pair state in Na0.3CoO2·yH2O is discussed. (author)