# Control of polarization and polar chiral textures in BiFeO$_3$ by epitaxial strain and interfacial chemistry

## TL;DR

This study demonstrates how epitaxial strain and interfacial chemistry can be used to control polarization and chiral textures in BiFeO₃ thin films, enabling new functionalities in ferroelectric devices.

## Contribution

It introduces a method to stabilize and manipulate polarization and chiral textures in BiFeO₃ through epitaxial strain and interfacial engineering, including buffer layers and surface chemistry.

## Key findings

- Nanoscale domain architecture stabilized by Bi₂O₂ surface layer.
- Uniform out-of-plane polarization achieved via buffer layer insertion.
- Detection of homochiral polarization textures in the films.

## Abstract

The balance between interfacial chemistry, electrostatics, and epitaxial strain plays a crucial role in stabilizing polarization in ferroelectric thin films. Here, we bring these contributions into competition in BiFeO$_3$ (BFO) thin films grown on the charged-surface-terminated La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO)-buffered NdGaO$_3$ (001) substrates. The large anisotropic compressive strain from the substrate promotes the formation of ferroelectric domains despite the expected stabilization of a uniform out-of-plane polarization by the (La,Sr)O$^{0.7+}$ termination of the metallic buffer. Piezoresponse force microscopy and scanning transmission electron microscopy reveal that the resulting nanoscale domain architecture is stabilized by the deterministic formation of a fluorite-like Bi$_2$O$_2$ surface layer on regions polarized oppositely to the LSMO-imposed polarization orientation. Leveraging this polarization compensation mechanism, we stabilize a uniform out-of-plane polarization in our highly strained BFO films by inserting a Bi$_2$O$_2$-terminated Aurivillius film as a buffer layer. Additionally, we reveal signatures of homochiral polarization textures in our BFO films on the level of domain configurations using local polarization switching experiments. Our work thus brings new strategies for controlling polarization direction and chiral textures in oxide ferroelectrics, opening pathways for functional domain-wall and domain-based electronics.