Local and macroscopic evidence of the existence of ferroelectricity in the multilayer structure. The measurements are carried out on sample 2. (a) Out of plane piezoresponse phase image showing distinct grains. The scan size of the image is 1 μm×1 μm. (b) PFM phase image (4 μm×4 μm) after poling by +6 V and −6 V dc bias, demonstrating the switching of the ferroelectric. The dotted boxes are a schematic of the applied bias (tip bias: +6 V for dark area, −6 V for light area and 0 V for the other area). (c) Single-point PFM phase-voltage hysteresis loop and amplitude-voltage butterfly loop. (d) Polarization as a function of electric field in the multilayer film measured at room temperature at the frequency of 1 kHz.