A Study of the Origin of Weak Ferromagnetism in Zn1-xCoxO

To study the origin of ferromagnetism in Zn1-x Co (x) O thin films, its thermal diffusivity, in addition to its magnetization measurements, were analyzed. Thin films of Zn1-x Co (x) O (x = 0.03) were deposited on Si (100) substrates through ultrasonic spray pyrolysis. Magnetization M(B) measurements at low temperature showed a hysteresis loop that indicated the existence of ferromagnetic ordering in Zn0.97Co0.03O. However, the magnetic moment per Co ion was much lower than expected. A comparison of M(T) measured at zero-field-cooled and field-cooled conditions showed a superparamagnetic-like behavior, with a blocking temperature of about 130 K. Temperature dependence on the thermal diffusivity of Zn0.97Co0.03O showed a pronounced lambda-shaped minimum at 130 K, which indicated the existence of a second-order phase transition at this temperature. The weak ferromagnetism in the Zn0.97Co0.03O with the Curie temperature of 130 K was ascribed to the uncompensated magnetic moment at the surface of CoO nanoclusters.