Weak Magnetic Order in Bilayered-Hydrate Na_xCoO_2・yH_2O Structure Probed by Co Nuclear Quadrupole Resonance : Proposed Phase Diagram in Superconducting Na_xCoO_2・yH_2O(Condensed Matter : Electronic Structure, Electrical, Magnetic and Optical Properties)

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A weak magnetic order was found in a nonsuperconducting bilayered-hydrate Na_xCoO_2・yH_2O sample by Co nuclear quadrupole resonance (NQR) measurement. The nuclear spin-lattice relaxation rate divided by temperature 1/T_1T 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/T_1T in various samples. The higher-ν_Q sample has stronger magnetic fluctuations. A possible phase diagram for Na_xCoO_2・yH_2O is depicted using T_c and ν_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 a magnetic phase, suggesting strongly that magnetic fluctuations play an important role in the occurrence of superconductivity.
一般社団法人日本物理学会の論文
2005-03-15