Onset of Multiferroicity in Prototypical Single-Spin Cycloid BiFeO₃ Thin Films

In the room-temperature magnetoelectric multiferroic BiFeO₃, the noncollinear antiferromagnetic state is coupled to the ferroelectric order, opening applications for low-power electric-field-controlled magnetic devices. While several strategies have been explored to simplify the ferroelectric landscape, here we directly stabilize a single-domain ferroelectric and spin cycloid state in epitaxial BiFeO₃ (111) thin films grown on orthorhombic DyScO₃ (011). Comparing them with films grown on SrTiO₃ (111), we identify anisotropic in-plane strain as a powerful handle for tailoring the single antiferromagnetic state. In this single-domain multiferroic state, we establish the thickness limit of the coexisting electric and magnetic orders and directly visualize the suppression of the spin cycloid induced by the magnetoelectric interaction below the ultrathin limit of 1.4 nm. This as-grown single-domain multiferroic configuration in BiFeO₃ thin films opens an avenue both for fundamental investigations and for electrically controlled noncollinear antiferromagnetic spintronics.