Ferroelectric properties of Na0.5K0.5NbO3 films at low temperatures

The temperature dependence of polarization and dielectric behaviour of Na0.5K0.5NbO3 thin films were studied from 25 K to room temperature. NKN films were deposited on Pt80Ir20 substrates by RF-magnetron sputtering. The freezing of domain walls with decreasing temperature makes domain walls switching difficult and suppresses the domain walls vibration. Both saturation remanent polarization and coercive field increased with decreasing the temperature. A pronounced low-temperature dielectric relaxation process was observed below 100 K; the relaxation rate fellows the Arrhenius law. Low temperature dielectric behaviour has been studied for the NKN films at various frequencies and bias electrical fields. Constant phase element model is introduced and can interpret the dielectric behaviour well. Dielectric constant, ac conductivity, loss tangent and universal law dielectric equation can be easily deduced from our model. The measured loss tangent and fitted result at various temperature and bias electrical fields agreed very well for our NKN films.