Na0.75CoO2-an itinerant magnet?

NaxCoO2 for x ∼ 0.75 is a rather unusual metal. It is an ...ABAB... stack of CoO2 sheets intercalated with Na atoms. It has a large Curie-Weiss magnetic susceptibility. Neutron scattering sees spin correlations which are ferromagnetic in plane and antiferromagnetic in stack direction. Below about 22 K it orders in this way with an unusual small moment of about 0.2 μBohr per Co atom. Its Fermi surface measured by photoemission is a cylinder around the stack axis. Density functional calculations in local spin density approximation are presented which treat the partial Na occupation in a well justified virtual crystal approximation. They provide all those findings and yield an understanding as itinerant magnetism. In addition, a ferromagnetic order also in stack direction, which is a very clear half-metal, is nearly energetically degenerate with the antiferromagnetic ground state, with dramatic Fermi surface reconstruction. This could yield a clue to the anomalous thermopower and its strong field dependence