Hidden magnetic transitions in the thermoelectric layered cobaltite [Ca₂CoO₃]_0.62[CoO₂]

A positive muon spin rotation and relaxation (μ⁺SR) experiment on [Ca₂CoO₃]_0.62[CoO₂] (i.e., Ca₃Co₄O₉, a layered thermoelectric cobaltite) indicates the existence of two magnetic transitions at ∼100K and 400–600 K; the former is a transition from a paramagnetic state to an incommensurate (IC) spin-density wave (SDW) state. The anisotropic behavior of zero-field μ⁺SR spectra at 5 K suggests that the IC-SDW propagates in the a-b plane, with oscillating moments directed along the c axis; also the IC-SDW is found to exist not in the [Ca₂CoO₃] subsystem but in the [CoO₂] subsystem. In addition, it is found that the long-range IC-SDW order completes below ∼30K, whereas the short-range order appears below 100 K. The latter transition is interpreted as a gradual change in the spin state of Co ions above 400 K. These two magnetic transitions detected by μ⁺SR are found to correlate closely with the transport properties of [Ca₂CoO₃]_0.62[CoO₂].