Structural, optical and magnetic properties of Zn1−xCoxO prepared by the sol–gel route

Effects of Co doping on the structural, optical and magnetic properties of ZnO samples prepared by the sol–gel method are reported. The X-ray diffraction, X-ray photoelectron spectroscopy and UV–visible spectroscopy confirmed the substitution of Co ions on Zn sites without changing the wurtzite structure. No segregated secondary phases or Co rich clusters were detected. Optical absorption spectra of the samples exhibit a blue shift in the absorption band edge with increasing dopant concentration. The photoluminescence measurements show a blue shift in UV emission peak with the increase in Co concentration and a slight shift in the green emission band at around 509 nm which gets suppressed for higher sintering temperature. The field dependence of magnetization observed at room temperature exhibits clear ferromagnetic behavior. Efforts have been made to fit the experimental M–H data using the magnetic polarons model (BMP) which involves localized carriers and magnetic cations. The calculated concentration of the BMPs is found to be below the typical percolation threshold in ZnO. Thus BMP model alone is not sufficient to explain the room temperature ferromagnetic behavior in ZnO. To know the exact magnetic ordering in the system, we also attempted to fit temperature dependent magnetization curves with the Curie–Weiss Law which shows antiferromagnetic ordering in all samples.