NMR studies of successive transition in Na0.5CoO2

To investigate the electric or magnetic characteristics of Na x CoO2, the parent system of the superconductor Na0.3CoO2.1.3H2O, the successive phase transition observed in Na0.5CoO2 at 87 K (T c1) and 53 K (T c2) have been studied by the means of NMR/NQR and neutron diffraction. The Co ions at one of the two crystallographically distinct Co sites with the lager ν Q value have the larger magnetic moments. They align antiferromagnetically at T c1 with their direction within the CoO2 plane, while those at the other Co sites with the smaller ν Q value have the smaller moments. They align along the direction parallel to the c-axis. The ordered moments at these two distinct sites exhibit the same T dependences, indicating that the existence of these sites is not due to a macroscopic phase separation. Although we have observed the splitting of the zero-field NMR signals from the Co sites with the smaller νQ value at T c2, any other anomaly of the ordered magnetization has not been observed there in the T dependences of the neutron and NMR data. We also present results of 23Na-NMR studies of Na0.5CoO2