Structural and magnetic properties of Co-doped ZnO thin films grown by ultrasonic spray pyrolysis method

Cobalt-doped ZnO thin films with several different percentage of Co from 0 up to 15 at% were synthesized via a cheap, simple and versatile method i.e. ultrasonic spray pyrolysis at atmospheric pressure and a substrate temperature of 350 °C. The structure of the as-prepared samples was characterized by X-ray diffraction (XRD), Raman spectroscopy and FTIR. The Co-doping effect is revealed by the presence of three additional peaks around 235, 470 and 538 cm−1 respect to the Raman spectra of the unsubstituted film. Fourier transform infrared spectroscopy (FTIR) put in evidence the decrease of the bond force constant f with increasing Co-doping. By ultra-violet visible near infrared (UV–Vis–NIR) spectroscopy on Co-doped samples it was possible to show the presence of additional absorption bands at approximately 570, 620 and 660 nm suggesting that Co2+ ions do not change their oxidation when substituted to zinc and the ZnO lattice does not change its wurtzite structure as well. Finally, all our samples exhibit a paramagnetic behavior without any trace of intrinsic room temperature ferromagnetism. •The bond length ZnO (L) in the unit cell diminishes with increasing the rate of doping of the cobalt.•FTIR results show the decreasing of the bond force constant f with increasing Cobalt doping.•All our samples exhibit paramagnetic behavior because of the 3d electron of Co2+.•The observed paramagnetism might also originate from oxygen vacancies and/or by zinc-related defects in the ZnO lattice.