Abstract
Thermal diffusivity measurements have been conducted in order to study the origin of ferromagnetism in Zn1−xCoxO. Thin films of Zn1−xCoxO (x = 0.03) were deposited on Si (100) substrates by using ultrasonic spray pyrolysis. Magnetization M(H) measurements at low temperaturs show a hysteresis loop, which indicates the existence of ferromagnetic ordering in Zn0.97Co0.03O. However, the magnetic moment per Co ion is much lower than expected. A comparison of M(T) measured at zero-field-cooled (ZFC) and at field-cooled (FC) conditions shows a superparamagnetic-like behavior, and the blocking temperature is about 130 K. The temperature dependence of the thermal diffusivity of Zn0.97Co0.03O shows a pronounced lambda-shaped minimum at 130 K, which indicates the existence of a second-order phase transition at that temperature. The weak ferromagnetism in Zn0.97Co0.03O with a Curie temperature of 130 K is ascribed to an uncompensated magnetic moment at the surface of CoO nanoclusters with radii of about 1 nm.
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