Defeating depolarizing fields with artificial flux closure in ultrathin ferroelectrics

Material surfaces encompass structural and chemical discontinuities that often lead to the loss of the property of interest in the so-called dead layers. It is notably problematic in nanoscale oxide electronics, where the integration of strongly correlated materials into devices is obstructed by the thickness threshold required for the emergence of their functionality. Here, we report the stabilization of ultrathin out-of-plane ferroelectricity in oxide heterostructures through the design of an artificial flux-closure architecture. Inserting an in-plane polarized ferroelectric epitaxial buffer provides continuity of polarization at the interface, and despite its insulating nature we observe the emergence of polarization in our out-of-plane-polarized model ferroelectric BaTiO$_{3}$ from the very first unit cell. In BiFeO$_{3}$, the flux-closure approach stabilizes a conceptually novel 251$^{\circ}$ domain wall. Its unusual chirality is likely associated with the ferroelectric analog to the Dzyaloshinskii-Moriya interaction. We thus see that in an adaptively engineered geometry, the depolarizing-field-screening properties of an insulator can even surpass those of a metal and be a source of new functionalities. This should be a useful insight on the road towards the next generation of ferroelectric-based oxide electronics.