Dome-shaped magnetic phase diagram of thermoelectric layered cobaltites -: art. no. 017602

Using muon spin spectroscopy we have found that, for both NaxCoO2 (0.6less than or equal toxless than or equal to0.9) 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.