Magnetism and spin entropy in Ru doped NaCoO.

The effect of Ru doping on the magnetic coupling among Co ions and on the Seebeck effect in NaCoO was systematically studied using density functional theory. It was found that the Ru dopant takes the 4+ oxidation state and replaces a Co ion. In addition, the remaining Co ions in NaCoO:Ru were stabilized in a low spin state. Magnetically, the Ru dopants couple in a ferrimagnetic manner with Co ions in the host lattice. Due to the higher electronic degeneracy of Ru dopants, the Seebeck coefficient in NaCoO is predicted to be higher than that of the pristine compound.