Nuclear-magnetic-resonance (NMR) studies were performed in Na0.7CoO₂ to investigate its magnetic properties from the microscopic point of view. Na0.7CoO₂ is a starting compound of the recently discovered superconductor Na0.35CoO₂· 1.3H_2$O, in which Co atoms form a triangular structure. Although $²³$Na-NMR results show no magnetic anomaly down to 1.5 K, Knight shift ($²³K$) and $²³(1/T_1T)$ of $²³$Na continue to increase, suggestive of the vicinity of the magnetic phase. From the relation between $²³K$ and $²³(1/T_1T)$, it is shown that ferromagnetic fluctuations are dominant below 100 K. In addition to the ferromagnetic correlations, antiferromagnetic fluctuations are also developed in a low temperature region below 4 K. Magnetic fluctuations suggested by our experiment are discussed.
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Ihara et al. (2004) studied this question.
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