Superparamagnetic behavior of MOCVD grown ZnO Co films

Temperature and field dependences of magnetization have been measured for Co-doped ZnO films with different Co content grown by MOCVD on sapphire substrates. Measured field dependences of magnetization show ferromagnetic-like hysteresis loops and paramagnetic contribution. Coercive field decreases with an increase of Co content and temperature. The difference between temperature dependences of magnetization measured in zero field cooling and field cooling conditions was observed in the temperature range from 2 K to 300 K indicating superparamagnetic behaviour of the films. The distribution of blocking temperatures extracted from the temperature dependences of magnetization is broad and shifts to the low blocking temperatures with an increase of Co conten