Weak Magnetic Order in the Bilayered-hydrate Na${}_x$CoO${}_2\cdot y$H${}_2$O Structure Probed by Co Nuclear Quadrupole Resonance - Proposed Phase Diagram in Superconducting Na${}_x$CoO${}_2\cdot$ $y$H${}_2$O

Abstract

A weak magnetic order was found in a non-superconducting bilayered-hydrate Na${}_x$CoO${}_2\cdot y$H${}_2$O sample by a Co Nuclear Quadrupole Resonance (NQR) measurement. The nuclear spin-lattice relaxation rate divided by temperature $1/T_{1}T$ shows a prominent peak at 5.5 K, below which a Co-NQR peak splits due to an internal field at the Co site. From analyses of the Co NQR spectrum at 1.5 K, the internal field is evaluated to be $\sim$ 300 Oe and is in the $ab$-plane. The magnitude of the internal field suggests that the ordered moment is as small as $\sim 0.015$ $\hat{I}{¼}_B$ using the hyperfine coupling constant reported previously. It is shown that the NQR frequency $\hat{I}{½}_Q$ correlates with magnetic fluctuations from measurements of NQR spectra and $1/T_{1}T$ in various samples. The higher-$\hat{I}{½}_Q$ sample has the stronger magnetic fluctuations. A possible phase diagram in Na${}_x$CoO${}_2\cdot y$H${}_2$O is depicted using $T_c$ and $\hat{I}{½}_Q$, in which the crystal distortion along the c-axis of the tilted CoO${}_2$ octahedron is considered to be a physical parameter. Superconductivity with the highest $T_c$ is seemingly observed in the vicinity of the magnetic phase, suggesting strongly that the magnetic fluctuations play an important role for the occurrence of the superconductivity.

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