Control of Polarization and Polar Helicity in BiFeO ₃ by Epitaxial Strain and Interfacial Chemistry

Abstract The balance between interfacial chemistry, electrostatics, and epitaxial strain plays a crucial role in stabilizing polarization in ferroelectric thin films. Here, these contributions are brought 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, a uniform out‐of‐plane polarization is stabilized in the highly strained BFO films by inserting a Bi 2 O 2 ‐terminated Aurivillius film as a buffer layer. Additionally, signatures of homohelicity in polarization textures of the BFO films are revealed on the level of domain configurations using local polarization switching experiments. The work thus brings new strategies for controlling polarization direction and helicity in oxide ferroelectrics, opening pathways for functional domain‐wall and domain‐based electronics.