Dome-shaped magnetic phase diagram of thermoelectric layered cobaltites.

Using muon spin spectroscopy we have found that, for both NaxCoO(2) (0.6<or=x<or=0.9) and three- and four-layer cobaltites, a common low temperature magnetic state (which in some cases is manifest as an incommensurate spin density wave) forms in the CoO(2) planes. Here we summarize those results and report an almost dome-shaped relation between the transition temperature into the low-T magnetic state and the composition x for NaxCoO(2) and/or the high-temperature asymptotic limit of thermopower in the more complex three- and four-layer cobaltites. This behavior is explained using the Hubbard model on a two-dimensional triangular lattice in the CoO(2) plane.