Nontrivial topological structures offer a rich playground in condensed matters and promise alternative device configurations for post-Moore electronics. While recently a number of polar topologies have been discovered in confined ferroelectric PbTiO₃ within artificially engineered PbTiO₃/SrTiO₃ superlattices, little attention was paid to possible topological polar structures in SrTiO₃. Here we successfully create previously unrealized polar antivortices within the SrTiO₃ of PbTiO₃/SrTiO₃ superlattices, accomplished by carefully engineering their thicknesses guided by phase-field simulation. Field- and thermal-induced Kosterlitz–Thouless-like topological phase transitions have also been demonstrated, and it was discovered that the driving force for antivortex formation is electrostatic instead of elastic. This work completes an important missing link in polar topologies, expands the reaches of topological structures, and offers insight into searching and manipulating polar textures.

Significance: Flux-closures, stable topological polar structures of nanometer size, are considered to be promising candidates as elements of future nanoelectronic and electromechanical devices. Understanding their phase transition pathways under external stimuli is therefore of vital importance for these potential applications. Here, using an atomically resolved in situ electron microscopy technique, we track the evolutions of the polarization of the flux-closure structure under both electric and stress fields with atomic resolution. We find that the flux-closure can be reversibly and controllably manipulated between the topological and ordinary ferroelectric states, enabling potential applications in electromechanical and nanoelectronic devices.

Topologically nontrivial polar structures are not only attractive for high-density data storage, but also for ultralow power microelectronics thanks to their exotic negative capacitance. The vast majority of polar structures emerging naturally in ferroelectrics, however, are topologically trivial, and there are enormous interests in artificially engineered polar structures possessing nontrivial topology. Here we demonstrate reconstruction of topologically trivial strip-like domain architecture into arrays of polar vortex in (PbTiO₃)₁₀/(SrTiO₃)₁₀ superlattice, accomplished by fabricating a cross-sectional lamella from the superlattice film. Using a combination of techniques for polarization mapping, atomic imaging, and three-dimensional structure visualization supported by phase field simulations, we reveal that the reconstruction relieves biaxial epitaxial strain in thin film into a uniaxial one in lamella, changing the subtle electrostatic and elastostatic energetics and providing the driving force for the polar vortex formation. The work establishes a realistic strategy for engineering polar topologies in otherwise ordinary ferroelectric superlattices.

Recently, several captivating topological structures of electric dipole moments (e.g., vortex, flux closure) have been reported in ferroelectrics with reduced size/dimensions. However, accurate polarization distribution of these topological ferroelectric structures has never been experimentally obtained. We precisely measure the polarization distribution of an individual ferroelectric vortex in PbTiO₃/SrTiO₃ superlattices at the subunit cell level by using the atomically resolved integrated differential phase contrast imaging in an aberration-corrected scanning transmission electron microscope. We find, in vortices, that out-of-plane polarization is larger than in-plane polarization, and that downward polarization is larger than upward polarization. The polarization magnitude is closely related to tetragonality. Moreover, the contribution of the Pb─O bond to total polarization is highly inhomogeneous in vortices. Our precise measurement at the subunit cell scale provides a sound foundation for mechanistic understanding of the structure and properties of a ferroelectric vortex and lattice-charge coupling phenomena in these topological ferroelectric structures.

Ferroelectric resistive switching memory is a non-destructive and easy to achieve multilevel storage, which is a breakthrough for further improving the density in the random access memory. However, the application of ferroelectric resistive switching memory is limited by the high operating voltage, the low switching ratio or slow write/read speed. Herein, we show a type of memristor with a thin ferroelectric film, the device can switch its resistance states by controlling the oxygen vacancy migration in the effect of an external electric field. The device still exhibits stable resistive switching phenomena after an endurance test of about 100 cycles and has high switch ratio about 10⁸% and good retention about 1.7 × 10⁵ s. Furthermore, the device has the potential of being a multi-states memory with a low write voltage below 2 V and a fast write/read speed of about 5 μs. Our results suggest new opportunities for the development of high storage density nonvolatile memory.