Metamagnetic Transition in Na$_{0.85}$CoO$_2$ Single Crystals

We report the magnetization, specific heat and transport measurements of high quality Na$_{0.85}$CoO$_2$ single crystals in applied magnetic fields up to 14T. In high temperatures, the system is in a paramagnetic phase. It undergoes a magnetic phase transition below about 20K. When the field is applied along the c-axis, the measurement data of magnetization, specific heat and magnetoresistance reveal a metamagnetic transition from an antiferromagnetic state to a quasi-ferromagnetic state at about 8T in low temperatures. However, no transition is observed in the magnetization measurements up to 14T when the field is applied perpendicular to the c-axis. The low temperature magnetic phase diagram of Na$_{0.85}$CoO$_2$ is determined.