Evolution and Suppression of Spin Cycloid in Epitaxial BiFeO₃ Thin Films

A systematic study of the effect of film thickness on the stability of the spin cycloid in BiFeO₃ grown epitaxially on TbScO₃ (110) substrates reveals a complex evolution of both the crystal and ferroelectric domain structures as well as the magnetic order. For films thicker than ∼5 nm, the structure remains rhombohedral, but the lattice mismatch is accommodated by the formation of 71° ferroelastic-closure domains, rather than misfit dislocations, followed by the formation of 109° domains. For films ≲ 5 nm, a mixed-phase coexistence of a polar, rhombohedral-like (R3c) phase and an antipolar (Pnma) phase is observed. Scanning nitrogen-vacancy magnetometry reveals a change in the propagation vector of the spin cycloid with thickness. It evolves from parallel to the ferroelectric domains for 50 nm thick samples and thicker and reorients to perpendicular to the ferroelectric domains for intermediate thicknesses, and vanishes for films ≲ 5 nm, which is reflected in macroscopic spin transport measurements and supported by the simulations. Ultimately, this work provides a deep understanding of the role of film thickness and electrostatic boundary conditions on the ferroelectric domain configuration and, therefore on the spin cycloid to design the device with electric field control antiferromagnetism.