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Data taken by various kinds of methods including NMR/NQR and neutronscattering are presented for NaxCoO2, in particular, for Na0.5CoO2. Attentionshave also been paid to the x dependence of the electronic nature. Thepseudo-gap-like behavior observed in the region of x<xc~0.6 is emphasized. Forsamples with x<0.6, signals from two kinds of Co sites with differentquadrupole frequencies nQ, ~4.1 MHz and ~2.8 MHz were observed, and the formervalue of nQ, which is nearly equal to that of the superconducting systemNa0.3CoO2.yH2O, becomes dominant with decreasing x to 0.3. For Na0.5CoO2, theCo sites with the larger nQ value have the larger magnetic moments. They alignantiferromagnetically at Tc1~87 K with their direction within the plane, whilethe Co sites with the smaller nQ value have the smaller moments. They alignalong the direction parallel to the c axis. The ordered moments at the twodistinct sites exhibit the same T dependences, indicating that the existence ofthese sites is not due to a macroscopic phase separation. Although we haveobserved the splitting of the zero-field NMR signals from the Co sites with thesmaller nQ value at the second transition temperature Tc2~53 K, any otheranomaly of the ordered magnetization has not been observed there in the Tdependences of the neutron and the NMR data. As x decreases, thetwo-dimensional nature of the electrons is enhanced and the antiferromagneticcorrelation increases. Based on these results, the superconducting pair statein Na0.3CoO2.yH2O is discussed.Comment: 12 pages, submitted to J. Phys. Soc. Jp

journal of the physical society of japan

Magnetic Correlation of NaxCoO2and Successive Phase Transitions of Na0.5CoO2–NMR and Neutron Diffraction Studies–