Magnetic Fluctuations in the Metallic State of Na${}_{0.7}$CoO${}_2$ Revealed by ${}^{23}$Na NMR

Abstract

Nuclear-magnetic-resonance (NMR) studies were performed in Na${}_{0.7}$CoO${}_2$ to investigate its magnetic properties from the microscopic point of view. Na${}_{0.7}$CoO${}_2$ is a starting compound of the recently discovered superconductor Na${}_{0.35}$CoO${}_2$$\cdot 1.3$H${}_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 (${}^{23}K$) and ${}^{23}(1/T_{1}T)$ of ${}^{23}$Na continue to increase, suggestive of the vicinity of the magnetic phase. From the relation between ${}^{23}K$ and ${}^{23}(1/T_{1}T)$, 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.

5 pages, 6 figures, submitted to J. Phys. Soc. Jpn