Complex oxide heterointerfaces, which play host to an incredible variety of interface physical phenomena, are of great current interest in introducing new functionalities to systems. Here, coherent super‐tetragonal BiFeO3/LaAlO3 and rhombohedral BiFeO3/LaAlO3 heterointerfaces are investigated by using a combination of high‐angle annular dark‐field (HAADF) imaging and annular bright‐field (ABF) imaging in a spherical aberration (Cs) corrected scanning transmission electron microscope (STEM), and first‐principles calculations. The complicated ferroelectric polarization pinning and relaxation that occurs at both interfaces is revealed with atomic resolution, with a dramatic change in structure of BiFeO3, from cubic to super‐tetragonal‐like. The results enable a detailed explanation to be given of how non‐bulk phase structures are stabilized in thin films of this material.

The control of magnetism via an electric field has attracted substantial attention because of potential applications in magnetoelectronics, spintronics and high-frequency devices. In this study, we demonstrate a new approach to enhance and control the magnetization of multiferroic thin film by an electric stimulus. First, to reduce the strength of the antiferromagnetic superexchange interaction in BiFeO 3 , we applied strain engineering to stabilize a highly strained phase. Second, the direction of the ferroelectric polarization was controlled by an electric field to enhance the Dzyaloshinskii-Moriya interaction in the highly strained BiFeO 3 phase. Because of the magnetoelectric coupling in BiFeO 3 , a strong correlation between the modulated ferroelectricity and enhanced magnetization was observed. The tunability of this strong correlation by an electric field provides an intriguing route to control ferromagnetism in a single-phase multiferroic.