Localized versus itinerant magnetic moments in Na0.7CoO2

Based on experimental Co-59-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 Co-59-NMR spectra reveal different Co environments below 300 K and their differentiation increases with decreasing temperature. We show that the Na-23- and Co-59-NMR data may consistently be interpreted by assuming that below room temperature the Co 3d electrons are itinerant. We also argue that the Co-59-NMR response is inconsistent with well-defined local magnetic moments on the Co sites. We identify a substantial orbital contribution chi(orb) to the d-electron susceptibility. At low temperatures chi(orb) seems to acquire some temperature dependence, suggesting an increasing influence of spin-orbit coupling. The temperature dependence of the spin-lattice relaxation rate T-1(-1)(T) confirms significant variations in the dynamics of this electronic subsystem between 250 and 300 K, as previously suggested. Below 100 K, Na0.7CoO2 may be viewed as a weak antiferromagnet with T-N below 1 K, but this scenario still leaves a number of open questions.