A common behaviour of thermoelectric layered cobaltites: incommensurate spin density wave states in [Ca2Co4/3Cu2/3O4]0.62[CoO2] and [Ca2CoO3]0.62[CoO2]

Magnetism of a misfit layered cobaltite [Ca2Co4/3Cu2/3O4]xRS[CoO2] (, RS denotes a rocksalt-type block) was investigated by a positive muon spin rotation and relaxation (μ+SR) experiment. A transition to an incommensurate (IC) spin density wave (SDW) state was found below 180 K (= TCon); and a clear oscillation due to a static internal magnetic field was observed below 140 K (= TC). Furthermore, an anisotropic behaviour of the zero-field μ+SR experiment indicated that the IC-SDW lies in the a–b plane, with oscillating moments directed along the c axis. These results were quite similar to those for the related compound [Ca2CoO3]0.62RS[CoO2], i.e., Ca3Co4O9. Since the IC-SDW field in [Ca2Co4/3Cu2/3O4]0.62RS[CoO2] was approximately the same as those in pure and doped [Ca2CoO3]0.62RS[CoO2], it was concluded that the IC-SDW exists in the [CoO2 ] planes.