Novel Transitions in Doped Mott-Insulator K_xCoO₂

By measuring the temperature dependence of the magnetic susceptibility, two distinct transitions have been observed in potassium doped cobalt oxide K_xCoO₂ (x = 0.2-1.0). The transition around 125 K corresponds to a phase transition between a weak ferromagnetic plus a strong superparamagnetic phase (below 125 K) and a superparamagnetic phase (above 125 K). The susceptibility above the Curie point T_c cannot be described by the Curie-Weise law, being consistent with the superparamagnetism. Magnetization-hysteresis-loops have been measured in different temperature regions to uncover the underlying physics. The second transition near 30.8 K shows a strong novelty.