Electronic theory for itinerant in-plane magnetic fluctuations in NaxCoO2

Starting from the ab initio band structure for NaxCoO2, we derive the single-electron energies and the effective tight-binding description for the t2g bands using a projection procedure. We find that, due to the presence of the next-nearest-neighbor hoppings, a local minimum in the electronic dispersion close to the Γ point of the first Brillouin zone forms. Therefore, in addition to a large Fermi surface, an electron pocket close to the Γ point emerges at high doping concentrations. The latter yields a new scattering channel resulting in the peak structure of the itinerant magnetic susceptibility at low momenta. This indicates an itinerant in-plane ferromagnetic state above a certain critical concentration xm, which is in agreement with neutron scattering data. Below xm, the magnetic susceptibility shows a tendency towards antiferromagnetic fluctuations. We estimate the value of 0.56 < xm < 0.68 within the rigid band model and within the Hubbard model with infinite on-site Coulomb repulsion consistent with the experimental phase diagram.