Abstract
Ferroelectric vortices have received much attention in the past decade due to their functionalities and potential applications in nanoelectronics. However, most of the vortices fabricated in the nanoislands have a size of more than a hundred nanometers, which limits the miniaturization of electronic devices. Here, we show the realization of a high-density vortex lattice with ~10- to 20-nanometer size in ultrathin bismuth ferrite/barium titanate bilayer by strain engineering. The obtained vortex shows a nested structure, e.g., an outer center-type structure with a nested inner ring included, which is well supported by piezoelectric force microscopy, transmission electron microscopy, and phase-field simulations. The nested vortex structure is not only more stable than the center-type vortex structure but also exhibits controllable multistate conduction with low energy consumption (<100 zeptojoules, e.g., 10(-19) joules), good stability (>1 year), and high endurance. This is of great interest for high-density [~2 terabits per square inch (3100 terabits per square meter)] ferroelectric memristor conception for neuromorphic computing.