Flexoelectric engineering of ordered ferroelectric topologies via intrinsic strain gradients

Ferroelectric topological structures have recently attracted significant attention for their potential in novel nano-electronic devices, driven by unique physical properties like negative capacitance and chirality. In the model PbTiO3/SrTiO3 multilayered system, ordered arrays of flux-closure domains or vortices can be selectively induced by tuning epitaxial strain and layer thickness. However, a stark contrast in their ordering behavior is observed: while the flux-closure domains exhibit strong interlayer coupling and form long-range ordered stacks, the vortex domains are characterized by interlayer decoupling and a random arrangement. The underlying mechanism for this difference remains elusive. In this work, the origin of this disparate interlayer coupling is revealed to be the intrinsic inhomogeneous strain distribution related flexoelectric effect within the topological structures, as elucidated by advanced electron microscopy. It is found that the well-defined 90° and 180° domain walls within flux-closure domains generate strong, localized, and periodic strain gradients, inducing a potent long-range flexoelectric field which drives the strong interlayer coupling and vertical alignment to minimize the system’s total energy. Conversely, the continuous polarization rotation in vortices avoids sharp domain walls, leading to a more homogeneous strain distribution with negligible strain gradients and, thus, a weak flexoelectric coupling. Our findings establish the intrinsic inhomogeneous strain related flexoelectric field as a key parameter for controlling the 3D self-assembly of ferroelectric topologies, providing a blueprint for function-oriented nanodevice design through flexoelectric engineering. Strongly coupled flux-closure domains are identified as ideal for robust 3D memories, while weakly coupled vortices are promising for reconfigurable sensors and logic units.