Intrinsic magnetism in Zn1-xCoxO (0.03≤x≤0.10) thin films prepared by ultrasonic spray pyrolysis

The Zn1-xCoxO thin films were prepared on a commercial glass substrate at 400 degrees C by ultrasonic spray pyrolysis. The effect of doping concentration on the structural, optical and magnetic properties of these films has been studied. The structural and optical properties of these films reflect the fact that the Co2+ ions have substituted the Zn2+ ions without changing the wurtzite structure of ZnO. We have not observed any secondary phases even after doping the 10 at.% concentration of Co. All the films were highly oriented with preferred (002) direction. By increasing the Co content, a systematic change in both the c-axis lattice constant and the fundamental band gap energy was observed. Three absorption bands were found in the transmission spectra of the Co doped ZnO films at 657 nm (1.89 eV), 610 nm (2.03 eV) and 567 nm (2.20 eV). The absorption bands were attributed to d-d intraionic transitions of tetrahedrally coordinated high-spin Co2+ ions. Magnetic studies reveal the absence of room temperature ferromagnetic behaviour in these films. The values of coercivity (H-C) similar to 145 and 123 Oe and remanent magnetization (M-r) similar to 4.32 x 10(-5) and 7.28 emu cm(-2) were observed at 5 K for Zn0.95Co0.05O and Zn0.93Co0.07O thin films.