Unconventional magnetic transition and transport behavior inNa0.75CoO2

Abstract

Here we report an unconventional magnetic and transport phenomenon in a layered cobalt oxide ${\mathrm{Na}}_x{\mathrm{CoO}}_2.$ Only for $x=0.75,$ a magnetic transition of the second order was clearly detected at $T_m\sim 22\mathrm{K}$ 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 ${\mathrm{Na}}_{0.75}{\mathrm{CoO}}_2.$