  Based on experimental 59Co-NMR data in the temperature range between 0.1 and
300 K, we address the problem of the character of the Co 3d-electron based
magnetism in Na0.7CoO2. Temperature dependent 59Co-NMR spectra reveal different
Co environments below 300 K and their differentiation increases with decreasing
temperature. We show that the 23Na- and 59Co-NMR data may consistently be
interpreted by assuming that below room temperature the Co 3d-electrons are
itinerant. Their magnetic interaction appears to favor an antiferromagnetic
coupling, and we identify a substantial orbital contribution corb to the
d-electron susceptibility. At low temperatures corb seems to acquire some
temperature dependence, suggesting an increasing influence of spin-orbit
coupling. The temperature dependence of the spin-lattice relaxation rate
T1-1(T) confirms significant variations in the dynamics of this electronic
subsystem between 200 and 300K, as previously suggested. Below 200 K, Na0.7CoO2
may be viewed as a weak antiferromagnet with TN below 1 K but this scenario
still leaves a number of open questions.
