Unconventional magnetic transition and transport behavior in Na0.75CoO2 -: art. no. 064406

Here we report an unconventional magnetic and transport phenomenon in a layered cobalt oxide NaxCoO2. Only for x=0.75, a magnetic transition of the second order was clearly detected at T(m)similar to22 K where an apparent specific-heat jump, an onset of extremely small spontaneous magnetization, and a kink in resistivity came in. Moreover large positive magnetoresistance effect was observed below T-m. These features of the transition strongly indicate the appearance of an unusual electronic state that may be attributed to the strongly correlated electrons in Na0.75CoO2.