Microscopic magnetic nature of layered cobalt dioxides investigated by muon-spin rotation and relaxation

In order to elucidate the nature of layered cobalt dioxides AxCoO2, we have investigated their microscopic magnetism by means of positive muon-spin rotation and relaxation (μ+SR) spectroscopy, in particular for A=Li, Na, and K. The dome-shaped magnetic phase diagram for NaxCoO2 with x⩾0.75 suggests the competition between the spin concentration and geometrical frustration on the two-dimensional triangular lattice of the CoO2 plane. The additional experiment on LixCoO2 and KxCoO2 indicates both a weakly coupled regime for the d electrons in the CoO2 plane and an ignorable weak effect of the inter-plane interaction on their magnetic order at low T.