17O NMR Measurements on Superconducting Na0.35CoO2·yH2O

An 17 O NMR measurement was performed on nonoriented polycrystalline Na 0.35 CoO 2 · y H 2 O with superconducting (SC) transition temperature T c = 4.6 K. A weak temperature dependence was observed in the Knight shift at the O site ( 17 K ). The spin part of 17 K ( 17 K spin ) is estimated from the plot of 17 K against bulk susceptibility χ. The 17 K spin decreases in the SC state, indicative of the decrease in the in-plane component of the spin susceptibility. The nuclear spin–lattice relaxation rate 1/ T 1 at the O site 17 (1/ T 1 ) shows a good scaling with 1/ T 1 at the Co site 59 (1/ T 1 ). This indicates that the spin fluctuations at the O site originate from the Co spin dynamics. The relationships between 17 (1/ T 1 T ) and 17 K spin and between 17 (1/ T 1 T ) and 59 (1/ T 1 T ) show the development of incommensurate fluctuations at q ∼ 0 other than q = 0 below 30 K. A clear indication of ferromagnetic correlations at q = 0 was not observed from the present 17 O NMR studies.