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Hidden magnetic transitions in thermoelectric layered cobaltite, [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$]

J. Sugiyama·J. H. Brewer·E. J. Ansaldo·H. Itahara·K. Dohmae·Y. Seno·C. Xia·T. Tani
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Abstract

A positive muon spin rotation and relaxation ($μ^+$SR) experiment on [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$], ({\sl i.e.}, Ca$_3$Co$_4$O$_9$, a layered thermoelectric cobaltite) indicates the existence of two magnetic transitions at $\sim$ 100 K and 400 - 600 K; the former is a transition from a paramagnetic state to an incommensurate ({\sf IC}) spin density wave ({\sf SDW}) state. The anisotropic behavior of zero-field $μ^+$SR spectra at 5 K suggests that the {\sf IC-SDW} propagates in the $a$-$b$ plane, with oscillating moments directed along the c-axis; also the {\sf IC-SDW} is found to exist not in the [Ca$_2$CoO$_3$] subsystem but in the [CoO$_2$] subsystem. In addition, it is found that the long-range {\sf IC-SDW} order completes below $\sim$ 30 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 %% at temperatures above 400 K. These two magnetic transitions detected by $μ^+$SR are found to correlate closely with the transport properties of [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$].

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J. Sugiyama, J. H. Brewer, E. J. Ansaldo, H. Itahara, K. Dohmae, Y. Seno, C. Xia, T. Tani. 2003-08-05. Hidden magnetic transitions in thermoelectric layered cobaltite, [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$]. https://doi.org/10.1103/physrevb.68.134423

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