Tunable surface acoustic waves on strain-engineered relaxor K0.7Na0.3NbO3 thin films

In this work, we demonstrate the electronic tunability of surface acoustic waves (SAWs) in epitaxially strained relaxor-type ferroelectric thin films. Epitaxial K0.7Na0.3NbO3 thin films of typically 30 nm in thickness are grown via pulsed laser deposition on (110)-oriented TbScO3. A partial plastic lattice relaxation of the epitaxial strain in these samples leads to a relaxor-type ferroelectricity of these films, which strongly affects the SAW properties. Without electronic bias, only tiny SAW signals of ∼0.2 dB can be detected at room temperature, which can be boosted up to ∼4 dB by a static voltage bias added to the high frequency driving current of the SAW transducers. Upon field cooling below the freezing temperature of polar nanoregions (PNRs), this strong SAW signal can be preserved and is even enhanced due to a release of the electronically fixed PNRs if the bias is removed. In contrast, at elevated temperatures, a reversible switching of the SAW signal is possible. The switching shows relaxation dynamics that are typical for relaxor ferroelectrics. The relaxation time τ decreases exponentially from several hours at freezing temperature to a few seconds (<5 s) at room temperature.