Observation of optical properties related to room-temperature ferromagnetism in co-sputtered Zn1−xCoxO thin films

We report on the analysis of optical transmittance spectra and the resulting ferromagnetic characteristics of sputtered Zn1−xCoxO films. Zn1−xCoxO films were prepared on (0001)-oriented Al2O3 substrates by the radio-frequency (rf) magnetron co-sputtering method. The XRD results showed that the crystallinity of films was properly maintained up to x=0.30 and no second phase peaks were detected up to x=0.40. The transmittance spectra showed both the increase of the absorption band intensity and the red shift of the absorption peak as well as the band edge with increasing x. We have proved experimentally that these changes depend on Co concentration. These optical properties suggest that sp–d exchange interactions and typical d–d transitions become activated with increasing x, which leads to the enhancement of ferromagnetic properties in Zn1−xCoxO films as shown in the AGM results. Therefore, it is concluded that the ferromagnetism derives from the substitution of Co2+ for Zn2+ without changing the wurtzite structure.