Coexistence of Ferromagnetism and Ferroelectric Polarization in Epitaxial NiTiO$_{3}$ thin films with the LiNbO$_{3}$-Type Structure

In a search for new multiferroic materials where the direction of magnetization can be switched by an applied electric field, we have looked for materials in which polarization and magnetization are strongly coupled. Recent theory calculations predicted that the family of compounds MTiO$_{3}$ (M $=$ Mn, Fe, Ni), in a certain polymorphic structure (acentric \textit{R3c}), are promising candidates where a polar lattice distortion can induce weak ferromagnetism (WFM). Guided by these insights, the \textit{R3c} phase of NiTiO$_{3}$ has been prepared in epitaxial thin film form. The synthesis of these NiTiO$_{3}$ films, their full structural characterization, physical property measurements along with first-principles DFT calculations to predict the desired NiTiO$_{3}$ structure, its stability, and the effect electronic structure on the ferroic properties are presented. Optical SHG imaging of the NiTiO$_{3}$ films indicates a polar lattice. Temperature-dependent magnetization measurements suggest a Neel transition consistent with the \textit{R3c} structure. Our field-dependent magnetization results show a residual magnetism below the Neel temperature suggesting the presence of a ferromagnetic moment induced by the polar lattice distortion. These results validate theory predictions about the coexistence of WFM and ferroelectric polarization in MTiO$_{3}$ compounds with the \textit{R3c} structure.