Permanent manipulation of polar textures in thin ferroelectric films: insights from large scale density functional theory

Thin ferroelectric (FE) films and superlattices are known to be hosts to complex polarization textures such as polar waves, flux-closure domains and polar skyrmion phases. While  technological uses for these exotic morphologies have been proposed, little is known about how they can be deterministically controlled. Presently, it is possible to temporarily manipulate these textures using precisely directed electric fields. Permanent texture "writing", however, is not achieved. Until now, investigations of polar texture control have been limited to experiment and lower levels of theory. The latter limitation arises from the fact that an accurate quantum mechanical treatment using density functional theory (DFT) is prohibited by the computational effort required. That is, exotic polar textures span length scales encompassing thousands of atoms, well beyond the limitations of traditional O(N³) scaling DFT (where N is the number of atoms). To avoid this scaling wall, we deploy two separate large scale DFT methods implemented in CONQUEST able to simulate to thousands of atoms. Using this method, we study two pathways for permanent polar texture manipulation in the PbTiO₃/SrTiO₃ system: (i) built-in bias fields and (ii) engineered surface defects.  For (i), we show that a built-in bias field is present for most polar film-substrate heterostructures. Tuning this built-in field allows one to manipulate chiral order on the nanoscale through the careful choice of substrate, surface termination or use of overlayers. For (ii), we investigate the origin of experimentally observed parallel FE domain wall (DW) surface trench (ST) alignment. After explaining this phenomenon with arguments regarding the restoration of polar continuity, we suggest that STs could be used to engineer other exotic polar textures in a variety of FE nanostructures.