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

To study the origin of ferromagnetism in Zn 1− x Co x O thin films, its thermal diffusivity, in addition to its magnetization measurements, were analyzed. Thin films of Zn 1− 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 Zn 0.97 Co 0.03 O. 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 Zn 0.97 Co 0.03 O 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 Zn 0.97 Co 0.03 O with the Curie temperature of 130 K was ascribed to the uncompensated magnetic moment at the surface of CoO nanoclusters.