Difference in magnetic properties between Co-doped ZnO powder and thin film

This paper reports that the Zn_0.95Co_0.05O polycrystalline powder and thin film were prepared by sol-gel technique under the similar preparation conditions. The former does not show typical ferromagnetic behaviour, while the latter exhibits obvious ferromagnetic properties at 5 K and room temperature. The UV--vis spectra and x-ray absorption spectra show that Co^2 + ions are homogeneously incorporated into ZnO lattice without forming secondary phases. The distinct difference between film and powder sample is the c-axis (002) preferential orientation indicated by the x-ray diffraction pattern and field emission scanning electron microscopy measurement, which may be the reason why Zn_0.95Co_0.05O film shows ferromagnetic behaviour.