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.
@article{arxiv.cond-mat/0512561,
title = {Localized versus itinerant magnetic moments in Na0.72CoO2},
author = {J. L. Gavilano and B. Pedrini and K. Magishi and J. Hinderer and M. Weller and H. R. Ott and S. M. Kazakov and J. Karpinski},
journal= {arXiv preprint arXiv:cond-mat/0512561},
year = {2009}
}