Magnetic phase transition at 88 K in Na0.5CoO2 revealed by Na-23 NMR investigations

[摘要] Na0.5CoO2 exhibits a metal-insulator transition at T-MI=53 K upon cooling. The nature of another transition at T-X=88 K has not been fully clarified yet. We report the results of measurements of the electrical conductivity sigma, the magnetic susceptibility chi and Na-23 NMR on a powder sample of Na0.5CoO2, including the mapping of NMR spectra, as well as probing the spin-lattice relaxation rate T (-1)(1)(T) and the spin-spin relaxation rate T (-1)(2)(T), in the temperature range between 30 and 305 K. The NMR data reflect the transition at T-X very well but provide less evidence for the metal-insulator transition at T-MI. The temperature evolution of the shape of the spectra implies the formation of a staggered internal field below T-X, not accompained by a rearrangement of the electric charge distribution. Our results thus indicate that in Na0.5CoO2, an unusual type of magnetic ordering in the metallic phase precedes the onset of charge ordering, which finally induces an insulating ground state.

[发布日期] 2005-12-01

[关键词] CU-63,CU-65;CHARGE