Unconventional electronic transition in NaxCoO2 with a precisely controlled Na nonstoichiometry

An unconventional electronic transition was discovered in a layered cobalt oxide, NaxCoO2, with precisely controlled Na contents. Only for Na-rich samples with x similar or equal to 0.75, a weak-ferromagnetic transition of the second order was clearly detected at 22 K, being accompanied by a clear specific-heat jump, unexpectedly small spontaneous magnetization, kinks in both the resistivity vs. temperature and the thermoelectric power vs. temperature curves. Moreover, large positive magnetoresistance was observed at temperatures below similar to 22 K. (C) 2003 Elsevier Science B.V. All rights reserved.