Hierarchical Domain Structure and Extremely Large Wall Current in Epitaxial BiFeO₃ Thin Films

Erasable electrical conductive domain walls in an insulating ferroelectric matrix provide novel functionalities for applications in logic and memory devices. The crux of such success requires sufficiently high wall currents to drive high-speed and high-power nanodevices. This work provides an appealing strategy to increase the current by two orders of magnitude through the careful selection of current flowing paths along the charged walls. The dense walls come into form through the hierarchical evolution of the 71°, 109°, and 180° domains of epitaxial BiFeO₃ films in a planar-geometry ferroelectric resistance-switching memory cell. The engineered films grown on SrTiO₃ and GdScO₃ substrates allow the observation of detailed local configurations and the evolution of the different domain types using vector piezo-force microscopy. The higher local electrical conductivity near the charged domain walls is identified by conductive atomic-force microscopy. It is shown that 180° domain reversal proceeds by three-step 71° rotations of the pristine domains. Surprisingly, a maximum current of ≈300 nA is observed for current paths along charge-uncompensated head-to-head hierarchical domain walls connecting the two electrodes on the film surface. Furthermore, the achievable current level can be conveniently controlled by varying the relative directions of the initial polarization and the applied field.