Weak Magnetic Order in Bilayered-Hydrate NaxCoO2·yH2O Structure Probed by Co Nuclear Quadrupole Resonance –Proposed Phase Diagram in Superconducting NaxCoO2·yH2O–

A weak magnetic order was found in a non-superconducting bilayered-hydrate NaₓCoO₂· yH₂O sample by a Co Nuclear Quadrupole Resonance (NQR) measurement. The nuclear spin-lattice relaxation rate divided by temperature 1/T₁T 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 ~ 300 Oe and is in the $ab$-plane. The magnitude of the internal field suggests that the ordered moment is as small as ~ 0.015 μB using the hyperfine coupling constant reported previously. It is shown that the NQR frequency νQ correlates with magnetic fluctuations from measurements of NQR spectra and 1/T₁T in various samples. The higher-νQ sample has the stronger magnetic fluctuations. A possible phase diagram in NaₓCoO₂· yH₂O is depicted using Tc and νQ, in which the crystal distortion along the c-axis of the tilted CoO₂ octahedron is considered to be a physical parameter. Superconductivity with the highest Tc 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.