LAUSR

Perovskite and spinel structures are widely found in ferroelectric and magnetic oxides, respectively, making their combination important for multiferroic composites. In this study, the (111) interface between perovskite-type $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ and spinel-type $\mathrm{Ni}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$, has been systematically investigated at the atomic scale combining aberration-corrected scanning transmission electron microscopy and first-principles calculations. The atomic terminations at the interface were determined to be the $\mathrm{Bi}{\mathrm{O}}_{3}$ layer on the $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ side and the tetrahedral Fe layer on the $\mathrm{Ni}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ side. The lattice mismatch between $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ and $\mathrm{Ni}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ is primarily accommodated by the first and second $\mathrm{Bi}{\mathrm{O}}_{3}$ layers inside $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$, indicating a stand-off of misfit dislocations in $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$. A metallic interface is formed between the two insulating phases, with the $\mathrm{Bi}{\mathrm{O}}_{3}$ and tetrahedral Fe layer coupled antiferromagnetically across the interface. The magnetic moment in $\mathrm{Ni}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ and the ferroelectric polarization in $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ drop slightly at the interface and return to the bulk values within two atomic layers from the interface.