Electric field induced relaxor behavior in anisotropically strained SrTiO3 films

Electric fields can modify the dielectric response of ferroelectric and especially relaxor ferroelectric material. Since strained ferroelectric fields represent ideal candidates for relaxor ferroelectrics, we analyzed the impact of ac and dc electric fields and field orientation on the dielectric properties of anisotropically strained epitaxial SrTiO3 films in detail. The tensile strain in the SrTiO3 films causes an increase of the ferroelectric-dielectric phase transition temperature to 258 K and 288 K for small and large tensile strains, respectively. The resulting films represent relaxor-type ferroelectrics with properties that strongly depend on the applied electric field. While a dc bias field significantly suppresses the permittivity in the paraelectric regime ranging from 180 K to 320 K, an ac field leads to an even more pronounced enhancement of the permittivity in an even larger temperature regime (e.g. reduction of up to 50% versus enhancement of up to 380% for 0.5 V/μm dc bias or ac field, respectively). Furthermore the ac field dependence is nonlinear and cannot be explained by the classical Rayleigh law. Frequency dependent measurements show among others that the electric field dependences are strongly related to the relaxor-type behavior. The different dielectric responses are explained in terms of the mobility and dynamic of regimes of uniform polarization, the polar nanoregions, that are generally assumed to be responsible for the relaxor behavior.