Quantitative ferroelectric characterization of single submicron grains in Bi-layered perovskite thin films

The local polarization state and the electromechanical properties of ferroe lectric thin films can be probed via the converse piezoelectric effect usin g scanning force microscopy (SFM) combined with a lock-in technique. This m ethod, denominated as piezoresponse SFM, was used to characterize at the na noscale level ferroelectric SrBi2Ta2O9 and Bi4Ti3O12 thin films, grown by p ulsed laser deposition. Two types of samples were studied: polycrystalline films, with grains having random orientations, and epitaxial films, consist ing of (100)(orth)- or (110)(orth)-oriented crystallites, 100 nm to 2 mu m in lateral size, which are embedded into a (001)-oriented matrix. The ferro electric domain structure was imaged and the piezoelectric response under d ifferent external conditions was locally measured for each type of sample. Different investigation procedures are described in order to study the ferr oelectric properties via the electromechanical response. A distinct ferroel ectric behavior was found for single grains of SrBi2Ta2O9 as small as 200 n m in lateral size, as well as for 1.2 mu m x 175 nm crystallites of Bi4Ti3O 12 BY probing separately the crystallites and the matrix the investigations have demonstrated nt the nanoscale level that SrBi2Ta2O9 has no spontaneou s polarization along its crystallographic c-axis, whereas Bi4Ti3O12 exhibit s a piezoelectric behavior along both the a- and c-directions. The electros triction coefficients were estimated to be 3 x 10(-2) m(4)/C-2 for polycrys talline SrBi2Ta2O9 and 7.7 x 10(-3) m(4)/C-2 for c-oriented Bi4Ti3O12. Quantitative measurements at the nanoscale level, within the experimental e rrors give the same values for remanent polarization and coercive field as macroscopic ferroelectric measurements performed on the same samples.