Hidden magnetic transitions in the thermoelectric layered cobaltite [ Ca 2 CoO 3 ] 0.62 [ CoO 2 ]

A positive muon spin rotation and relaxation $({\ensuremath{\mu}}^{+}\mathrm{SR})$ experiment on $[{\mathrm{Ca}}_{2}{\mathrm{CoO}}_{3}{]}_{0.62}[{\mathrm{CoO}}_{2}]$ (i.e., ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{9},$ a layered thermoelectric cobaltite) indicates the existence of two magnetic transitions at $\ensuremath{\sim}100\mathrm{K}$ 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 ${\ensuremath{\mu}}^{+}\mathrm{SR}$ spectra at 5 K suggests that the IC-SDW propagates in the $a\ensuremath{-}b$ plane, with oscillating moments directed along the c axis; also the IC-SDW is found to exist not in the $[{\mathrm{Ca}}_{2}{\mathrm{CoO}}_{3}]$ subsystem but in the $[{\mathrm{CoO}}_{2}]$ subsystem. In addition, it is found that the long-range IC-SDW order completes below $\ensuremath{\sim}30\mathrm{K},$ 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 ${\ensuremath{\mu}}^{+}\mathrm{SR}$ are found to correlate closely with the transport properties of $[{\mathrm{Ca}}_{2}{\mathrm{CoO}}_{3}{]}_{0.62}[{\mathrm{CoO}}_{2}].$