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

Abstract

We report the ${}^{59}$Co NMR studies of the in-plane anisotropy of bilayer hydrated Na${}_x\mathrm{Co}{\mathrm{O}}_2\cdot$$y{\mathrm{H}}_2\mathrm{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 $\ddot{I}‡$ 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 $\vec{q}\ne 0$ is also anisotropic and the anisotropy is different from that at $\vec{q}=0$.

4 pages, 5 figures