  Non-stoichiometric NaxCoO2 (0.5 < x < 1) consists of CoO2 layers made up of
edge-sharing CoO6 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 NaxCoO2 (x ~~ 0.75), and implications of this
analysis were explored. Our study shows that the lowspin Co4+ (S = 1/2) ions of
NaxCoO2 generated by the Na vacancies cause the anisotropic magnetic properties
of NaxCoO2, and suggests that the six nearest-neighbor Co3+ ions of each Co4+
ion adopt the intermediate-spin electron configuration thereby behaving
magnetically like low-spin Co4+ ions. The Weiss temperature of NaxCoO2 is more
negative along the direction of the lower g-factor (i.e., Theta_parallel <
Theta_perp < 0, and g_parallel < g_perp). The occurrence of intermediate-spin
Co3+ ions surrounding each Co4+ ion account for the apparently puzzling
magnetic properties of NaxCoO2 (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 are in apparent conflict with that
deduced from magnetic susceptibility measurements between ~50 - 300 K. These
conflicting pictures are resolved by noting that the spin exchange between Co3+
ions is more strongly antiferromagnetic than that between Co4+ and Co3+ ions.
