Electron Correlation In The Two-Dimensional Triangle Lattice Of Naxcoo2

Magnetism of layered cobaltites NaxCoO2 with x=0.6 and 0.9 has been investigated by a positive muon spin rotation and relaxation (mu(+)SR) spectroscopy together with magnetic susceptibility and specific-heat measurements, using single-crystal samples in the temperature range between 250 and 1.8 K. Zero-field-(ZF-) mu(+)SR measurements on Na0.9CoO2 indicate a transition from a paramagnetic to an incommensurate spin-density wave state (IC-SDW) at 19 K(=T-SDW). The anisotropic ZF-mu(+)SR spectra suggest that the oscillating moments of the IC-SDW direct along the c axis. Since Na0.6CoO2 is paramagnetic down to 1.8 K, the magnitude of T-SDW is found to strongly depend on x. This behavior is well explained using the Hubbard model within a mean-field approximation on two-dimensional triangular lattice in the CoO2 plane. Also, both the appearance of the IC-SDW state by the change in x and the magnitude of the electronic specific-heat parameter of Na0.6CoO2 indicate that NaxCoO2 is unlikely to be a typical strongly correlated electron system.