Manipulation of Ferroic Orders via Continuous Biaxial Strain Engineering in Multiferroic Bismuth Ferrite

Continuous strain engineering of multiferroics not only enhances understanding of their properties but also guides the optimization of their performances for use in electronic, optical, and magnetic devices. However, due to technical challenges in real-time monitoring of the ferroic orders, the precise evolution process remains unclear. Here, the evolution of the ferroelectric (FE) and antiferromagnetic (AFM) orders are revealed in multiferroic freestanding BiFeO₃ films under sequential and anisotropic biaxial strain, using rotational anisotropy second harmonic generation (RA-SHG) technology and first-principles calculations. The change and recovery of RA-SHG patterns illustrate the reversible control of the in-plane FE polarization in the films by sequential strain application. The in-plane FE direction can be manipulated within ≈4° by strain along the (100) and (010) directions, while the AFM order is more significantly affected, with ≈8° rotation in RA-SHG patterns. This research unveils the appearance of new SHG peaks in freestanding BFO films under strain and shows that they evolve independently of FE-induced SHG linked to lattice changes, suggesting a spin structure-related variation. This work paves a new way for studying of strain-manipulated 2D multiferroics and highlights the promise of freestanding perovskite films as low-dimensional multifunctional devices.