Electron correlation in the two-dimensional triangular lattice of NaxCoO2 with x>0.6

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 mu(+)SR measurements on Na0.9CoO2 indicate a transition to an incommensurate spin density wave state at 19 K(= T-SDW). 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 explained using the Hubbard model within a mean field approximation on two-dimensional triangle lattice in the Coo, 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. (c) 2005 Elsevier B.V. All rights reserved.