Magnetic Fluctuations in the Metallic State of Na0.7CoO2 Revealed by 23 Na NMR

Nuclear-magnetic-resonance (NMR) studies were performed in Na0.7CoO2 to investigate its magnetic properties from the microscopic point of view. Na0.7CoO2 is a starting compound of the recently discovered superconductor Na0.35CoO2·1.3H2O, in which Co atoms form a tri- angular structure. Although 23 Na-NMR results show no magnetic anomaly down to 1.5 K, Knight shift ( 23 K) and 23 (1/T1T) of 23 Na continue to increase, suggestive of the vicinity of the magnetic phase. From the relation between 23 K and 23 (1/T1T), it is shown that ferromagnetic fluctuations are dominant below 100 K. In addition to the ferromagnetic correlations, antifer- romagnetic fluctuations are also developed in a low temperature region below 4 K. Magnetic fluctuations suggested by our experiment are discussed. Since the superconductivity in NaxCoO2·yH2O (x � 0.35, y � 1.3) with the transition temperature Tc � 5 K was discovered by Takada et al, 1 understanding of mag- netic properties in the CoO2 plane is one of the most important topics in strongly correlated electron systems. This is because the magnetic properties in the plane might be related to the mechanism of the unconventional superconductivity in the compound. In addition, the Co atoms form a triangular lattice in the plane, which is an- ticipated to possess the geometrical frustrations. Novel types of magnetic transitions and ground states are ex- pected due to a suppression of a long-range magnetic ordering by the geometrical frustrations associated with the triangular structure. In particular, only a few com- pounds have been known to date which possess a metallic property down to the lowest temperature with the geo- metrical frustrations, e.g. (YSc)Mn2 2 and LiV2O4. 3 The NaxCoO2 (0.5 < x < 0.78) compound is considered to be one of the promising metallic compounds with the geometrical frustrations at low temperature. 4 In this paper, we report the magnetic properties in Na0.7CoO2 revealed by our 23 Na-NMR experiments. Powder samples of Na0.7CoO2 were prepared from Na2CO3 (99.99%) and Co3O4(99.9%) by solid-state re- action. Its quality was checked by X-ray powder diffrac- tion. The Na content, which is very sensitive to physical properties, is estimated to be 0.702 by the inductive- coupled plasma atomic emission spectroscopy (ICP- AES) method. Using this well-characterized powder sample, 23 Na- and 59 Co-NMR measurements were per- formed.