Abstract
Magnetism of layered cobaltites Na x CoO 2 with x = 0.6 and 0.9 has been investigated by a positive muon spin rotation and relaxation ( μ + 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 μ + SR measurements on Na 0.9 CoO 2 indicate a transition to an incommensurate spin density wave state at 19 K ( = T SDW ) . Since Na 0.6 CoO 2 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 2 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 Na 0.6 CoO 2 indicate that Na x CoO 2 is unlikely to be a typical strongly correlated electron system.
Published Version
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