On the origin of the metallic and anisotropic magnetic properties of NaxCoO2 (x ≈ 0.75)

Nonstoichiometric Na x CoO 2 (0.5 < x < 1) consists of CoO 2 layers made up of edge-sharing CoO 6 octahedra and exhibits strongly anisotropic magnetic susceptibilities as well as metallic properties. A modified Curie-Weiss law was proposed for systems containing anisotropic magnetic ions to analyze the magnetic susceptibilities of Na x -CoO 2 (x ≈ 0.75), and implications of this analysis were explored. Our study shows that the low-spin Co 4+ (S = ½) ions of Na x CoO 2 generated by the Na vacancies cause the anisotropic magnetic properties of Na x CoO 2 and suggests that the six nearest-neighbor Co 3+ ions of each Co 4+ ion adopt the intermediate-spin electron configuration, thereby behaving magnetically like low-spin Co 4+ ions. The Weiss temperature of Na x CoO 2 is more negative along the direction of the lower g factor (i.e., θ || < θ | < 0 and g || < g). The occurrence of intermediate-spin Co 3+ ions surrounding each Co 4+ ion accounts for the apparently puzzling magnetic properties of Na x CoO 2 (x ≈ 0.75), i.e., the large negative Weiss temperature, the three-dimensional antiferromagnetic ordering below ∼22 K, and the metallic properties. The picture of the magnetic structure derived from neutron scattering studies below ∼22 K is in apparent conflict with that deduced from magnetic susceptibility measurements between ∼50 and 300 K. These conflicting pictures are resolved by noting that the spin exchange between Co 3+ ions is more strongly antiferromagnetic than that between Co 4+ and Co 3+ ions.