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≅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 ∼22 K