Weak Magnetic Order in Bilayered-Hydrate Na_<i>x</i>CoO₂·<i>y</i>H₂O Structure Probed by Co Nuclear Quadrupole Resonance –Proposed Phase Diagram in Superconducting Na_<i>x</i>CoO₂·<i>y</i>H₂O–

Abstract

A weak magnetic order was found in a non-superconducting bilayered-hydrate\nNa$_{x}$CoO$_{2}\\cdot y$H$_{2}$O sample by a Co Nuclear Quadrupole Resonance\n(NQR) measurement. The nuclear spin-lattice relaxation rate divided by\ntemperature $1/T_1T$ shows a prominent peak at 5.5 K, below which a Co-NQR peak\nsplits due to an internal field at the Co site. From analyses of the Co NQR\nspectrum at 1.5 K, the internal field is evaluated to be $\\sim$ 300 Oe and is\nin the $ab$-plane. The magnitude of the internal field suggests that the\nordered moment is as small as $\\sim 0.015$ $\\mu_B$ using the hyperfine coupling\nconstant reported previously. It is shown that the NQR frequency $\\nu_Q$\ncorrelates with magnetic fluctuations from measurements of NQR spectra and\n$1/T_1T$ in various samples. The higher-$\\nu_Q$ sample has the stronger\nmagnetic fluctuations. A possible phase diagram in Na$_{x}$CoO$_{2}\\cdot\ny$H$_{2}$O is depicted using $T_c$ and $\\nu_Q$, in which the crystal distortion\nalong the c-axis of the tilted CoO$_2$ octahedron is considered to be a\nphysical parameter. Superconductivity with the highest $T_c$ is seemingly\nobserved in the vicinity of the magnetic phase, suggesting strongly that the\nmagnetic fluctuations play an important role for the occurrence of the\nsuperconductivity.\n