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

TL;DR

This study uses $^{23}$Na NMR to reveal that Na$_{0.7}$CoO$_2$ exhibits dominant ferromagnetic fluctuations below 100 K and additional antiferromagnetic fluctuations below 4 K, indicating complex magnetic behavior.

Contribution

First microscopic NMR investigation revealing coexistence of ferromagnetic and antiferromagnetic fluctuations in Na$_{0.7}$CoO$_2$.

Findings

Ferromagnetic fluctuations dominate below 100 K.

Antiferromagnetic fluctuations develop below 4 K.

No magnetic anomaly observed down to 1.5 K.

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 $^{23}(1/T_1T)$ of $^{23}$Na continue to increase, suggestive of the vicinity of the magnetic phase. From the relation between $^{23}K$ 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.