In-plane Anisotropy of the Magnetic Fluctuations in NaxCoO2-yH2O

TL;DR

This study reveals the in-plane anisotropy of magnetic fluctuations in hydrated Na$_x$CoO$_2$ using $^{59}$Co NMR, showing temperature-dependent changes and anisotropic behavior at finite wave vectors.

Contribution

First observation of in-plane anisotropy of $^{59}$Co NMR parameters in hydrated Na$_x$CoO$_2$ using oriented powder samples, highlighting temperature-dependent electronic and magnetic state changes.

Findings

Large in-plane anisotropy of $K$, $1/T_1$, and $1/T_2$ below 75 K.

Hyperfine coupling constants change around 150 K.

Magnetic fluctuations at finite wave vectors are anisotropic and differ from uniform susceptibility.

Abstract

We report the $^{59}$Co NMR studies of the in-plane anisotropy of bilayer hydrated Na$_{x}\mathrm{CoO_{2}} \cdot$$y\mathrm{H_{2}O}$ using a oriented powder sample by a magnetic field in Fluorinert FC70. We found for the first time the $ab$-plane anisotropy of the $^{59}$Co NMR Knight shift $K$, the nuclear spin-lattice relaxation rate 1/$T_{1}$ and the nuclear spin-spin relaxation rate 1/$T_{2}$ at a magnetic field $H \sim$ 7.5 T up to 200K. Below 75 K, the anisotropy of $K$ is large compared with that at high temperatures. The hyperfine coupling constants seem to change around the temperature 150 K, in which the bulk susceptibility $\chi$ shows broad minimum, suggesting a change of the electronic state of CoO$_{2}$ plane. 1/$T_{1}$ also shows a significant anisotropy, which cannot be explained only by the anisotropy of the hyperfine coupling constants nor the anisotropic uniform spin susceptibility. The difference in the in-plane anisotropy of $T_{1}$ from that of $K$ indicates that the magnetic fluctuation at a finite wave vector $\vec{q} \neq 0$ is also anisotropic and the anisotropy is different from that at $\vec{q} = 0$.