Ultrahigh-density nested polar nanovortices with independently controllable conduction for ferroelectric memristor

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.