Dome-Shaped Magnetic Phase Diagram of Thermoelectric Layered Cobaltites

Using muon spin spectroscopy we have found that, for both NaₓCoO₂ (0.6≤x≤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 CoO₂ 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 NaₓCoO₂ 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 CoO₂ plane.