Zn1−xCoxO (x = 0.0 – 0.047) thin films (thickness ∼0.5 – 1 μm) have been prepared on sapphire substrates using metalorganic decomposition (MOD) method. The X-ray diffraction and Raman scattering studies indicate films to be polycrystalline ZnO with wurtzite structure. The optical absorption spectra show an expected bandgap of ∼3.2 eV. The magnetization studies show that the as prepared films lack the room temperature ferromagnetic order, whereas the films when vacuum annealed at a temperature 500 – 600 °C acquire ferromagnetic ordering at room temperature. Further, the observed ferromagnetism (FM) appears only for a limited range of Co concentration, 0.03 < x < 0.10 (after heat treating in vacuum at 550 °C), and it reversibly disappears upon re-annealing in air. The data presented here seem to suggest that the appearance of ferromagnetic order is dictated by both the oxygen defects and the critical concentration of Co, and thus may lend support to a recent model proposed by Coey et al. [Nature Materials
4, 173 (2005)].