Nanoscale piezoresponse studies of ferroelectric domains in epitaxial BiFeO3 nanostructures

We report the dependence of the ferroelectric domain configuration and switching behavior on the shape (square versus round) of epitaxial BiFeO ₃ (BFO) nanostructures. We fabricated (001) oriented BFO (120 nm) / SrRuO₃ (SRO,125 nm) film layers on (001) SrTiO₃ single crystals by rf magnetron sputter deposition, and patterned them to square (500×500 nm²) and round (502 nm in diameter) shaped nanostructures by focused ion-beam lithography. The surface morphology and the crystalline structure of the nanostructures were characterized by scanning electron microscopy and x-ray diffraction, respectively, while the domain configuration was investigated using piezoelectric force microscopy. We found that the square-shaped nanostructures exhibit a single variant domain configuration aligned along the [1̄ 1 1̄] direction, whereas the round-shaped nanostructures exhibit seven variants of domain configuration along the [1̄ 1 1̄], [1 1̄ 1̄], [11 1̄], [111], [1̄ 11], [1 1̄ 1], and [1̄ 1̄ 1] directions. Moreover, local d ₃₃ piezoelectric coefficient measurements showed hysteresis loops with a strong displacement in the voltage axis (strong imprint) for the square-shaped nanostructures, while the round-shaped ones exhibited more symmetric loops. These findings have critical implications for the development of nanocapacitors for gigabyte to terabyte nonvolatile ferroelectric memories.