  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 Tm ~ 22 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 Tm. 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.
