Unconventional magnetic transition and transport behavior in Na_0.75CoO₂

Here we report an unconventional magnetic and transport phenomenon in a layered cobalt oxide Na_xCoO₂. Only for x = 0.75, a magnetic transition of the second order was clearly detected at T_m ∼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 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 Na_0.75CoO₂.