NMR studies of successive transition in Na0.5CoO2

Abstract To investigate the electric or magnetic characteristics of Na x CoO 2 , the parent system of the superconductor Na 0.3 CoO 2 ·1.3H 2 O, the successive phase transition observed in Na 0.5 CoO 2 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 CoO 2 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 ν Q 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 23 Na-NMR studies of Na 0.5 CoO 2 .