In this work, RF- magnetron sputtering technique has been employed to deposit nanocrystalline ZnFe2O4 thin films at room temperature. The as grown films were ex-situ annealed in air for 2 h at temperatures from 150°C to 650°C. X-ray diffraction, vibrating sample magnetometer and ferromagnetic resonance were used to analyze the phase formation, magnetic properties and microwave properties respectively. From the hysteresis loops and ferromagnetic resonance spectra taken at room temperature, a systematic study on the effect of O2 plasma on microwave properties with respect to processing temperature has been carried out. © 2014 AIP Publishing LLC.