A common behavior of thermoelectric layered cobaltites: an incommensurate spin density wave state

Abstract

Magnetism of misfit layered cobaltites, [Ca/sub 2/CoO/sub 3/]/sub 0.62//sup RS/[CoO/sub 2/] and [Ca/sub 2/Co/sub 4/3/Cu/sub 2/3/O/sub 4/]/sub 0.62//sup RS/[CoO/sub 2/] (RS denotes a rocksalt-type block), was investigated by a positive muon spin rotation and relaxation (/spl mu//sup +/SR) experiment. For the former cobaltite, a transition to an incommensurate (IC) spin density wave (SDW) state was found below 100 K (=T/sub SDW//sup on/) and a clear oscillation due to a static internal magnetic field was observed below 30 K (=T/sub SDW/). For the latter cobaltite, T/sub SDW//sup on/=180 K and T/sub SDW/=140 K, respectively. The wide transition width (= 40-70 K) suggested that a short-range IC-SDW order appeared below T/sub SDW//sup on/ and a long-range order completed below T/sub SDW/. Furthermore, an anisotropic behavior of the zero-field /spl mu//sup +/SR spectra on the c axis aligned samples indicated that the IC-SDW propagates in the a-b plane, with oscillating moments directed along the c axis. These results suggest that the IC-SDW exist not in the rocksalt-type block but in the CoO/sub 2/ plane.