A common behavior of thermoelectric layered cobaltites: incommensurate spin density wave states in [Ca$_2$Co$_{4/3}$Cu$_{2/3}$O$_4$]$_{0.62}$[CoO$_2$] and [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$]

摘要

Magnetism of a misfit layered cobaltite[Ca$_2$Co$_{4/3}$Cu$_{2/3}$O$_4$]$_x^{\rm RS}$[CoO$_2$] ($x \sim$ 0.62, RSdenotes a rocksalt-type block) was investigated by a positive muon spinrotation and relaxation ($\mu^+$SR) experiment. A transition to anincommensurate ({\sf IC}) spin density wave ({\sf SDW}) state was found below180 K (= $T_{\rm C}^{\rm on}$); and a clear oscillation due to a staticinternal magnetic field was observed below 140 K (= $T_{\rm C}$). Furthermore,an anisotropic behavior of the zero-field $\mu^+$SR experiment indicated thatthe {\sf IC-SDW} propagates in the $a$-$b$ plane, with oscillating momentsdirected along the c axis. These results were quite similar to those for therelated compound [Ca$_2$CoO$_3$]$_{0.62}^{\rm RS}$[CoO$_2$], {\sl i.e.},Ca$_3$Co$_4$O$_9$. Since the {\sf IC-SDW} field in[Ca$_2$Co$_{4/3}$Cu$_{2/3}$O$_4$]$_{0.62}^{\rm RS}$[CoO$_2$] was approximatelysame to those in pure and doped [Ca$_2$CoO$_3$]$_{0.62}^{\rm RS}$[CoO$_2$], itwas concluded that the {\sf IC-SDW} exist in the [CoO$_2$] planes.