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

Magnetism of misfit layered cobaltites, [Ca₂CoO₃]_0.62^RS[CoO₂] and [Ca₂Co₄3/Cu₂3/O₄]_0.62^RS[CoO₂] (RS denotes a rocksalt-type block), was investigated by a positive muon spin rotation and relaxation (μ⁺SR) experiment. For the former cobaltite, a transition to an incommensurate (IC) spin density wave (SDW) state was found below 100 K (=T_SDW^on) and a clear oscillation due to a static internal magnetic field was observed below 30 K (=T_SDW). For the latter cobaltite, T_SDW^on=180 K and T_SDW=140 K, respectively. The wide transition width (= 40-70 K) suggested that a short-range IC-SDW order appeared below T_SDW^on and a long-range order completed below T_SDW. Furthermore, an anisotropic behavior of the zero-field μ⁺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 CoO₂ plane.