Magnetic Correlation of Na_<i>x</i>CoO₂and Successive Phase Transitions of Na_0.5CoO₂–NMR and Neutron Diffraction Studies–

Abstract

Data taken by various kinds of methods including NMR/NQR and neutron\nscattering are presented for NaxCoO2, in particular, for Na0.5CoO2. Attentions\nhave also been paid to the x dependence of the electronic nature. The\npseudo-gap-like behavior observed in the region of x&lt;xc~0.6 is emphasized. For\nsamples with x&lt;0.6, signals from two kinds of Co sites with different\nquadrupole frequencies nQ, ~4.1 MHz and ~2.8 MHz were observed, and the former\nvalue of nQ, which is nearly equal to that of the superconducting system\nNa0.3CoO2.yH2O, becomes dominant with decreasing x to 0.3. For Na0.5CoO2, the\nCo sites with the larger nQ value have the larger magnetic moments. They align\nantiferromagnetically at Tc1~87 K with their direction within the plane, while\nthe Co sites with the smaller nQ value have the smaller moments. They align\nalong the direction parallel to the c axis. The ordered moments at the two\ndistinct sites exhibit the same T dependences, indicating that the existence of\nthese sites is not due to a macroscopic phase separation. Although we have\nobserved the splitting of the zero-field NMR signals from the Co sites with the\nsmaller nQ value at the second transition temperature Tc2~53 K, any other\nanomaly of the ordered magnetization has not been observed there in the T\ndependences of the neutron and the NMR data. As x decreases, the\ntwo-dimensional nature of the electrons is enhanced and the antiferromagnetic\ncorrelation increases. Based on these results, the superconducting pair state\nin Na0.3CoO2.yH2O is discussed.\n