Weak Magnetic Order in Bilayered-Hydrate Na_xCoO₂·yH₂O Structure Probed by Co Nuclear Quadrupole Resonance —Proposed Phase Diagram in Superconducting Na_xCoO₂·yH₂O—

Abstract

A weak magnetic order was found in a nonsuperconducting bilayered-hydrate Na_xCoO₂·yH₂O sample by 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 the analyses of the Co NQR spectrum at 1.5 K, the internal field is evaluated to be ∼300 Oe and found to be in the ab-plane. The magnitude of the internal field suggests that the ordered moment is as small as ∼0.015μ_B using the hyperfine coupling constant reported previously. It is shown that the NQR frequency ν_Q correlates with magnetic fluctuations from measurements of NQR spectra and 1⁄T₁T in various samples. The higher-ν_Q sample has stronger magnetic fluctuations. A possible phase diagram for Na_xCoO₂·yH₂O is depicted using T_c and ν_Q, 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 T_c is seemingly observed in the vicinity of a magnetic phase, suggesting strongly that magnetic fluctuations play an important role in the occurrence of superconductivity.