Coupled Lattice Polarization and Ferromagnetism in Multiferroic NiTiO₃ Thin Films

Polarization-induced weak ferromagnetism (WFM) was demonstrated a few years back in LiNbO₃-type compounds, MTiO₃ (M = Fe, Mn, Ni). Although the coexistence of ferroelectric polarization and ferromagnetism has been demonstrated in this rare multiferroic family before, first in bulk FeTiO₃, then in thin-film NiTiO₃, the coupling of the two order parameters has not been confirmed. Here, we report the stabilization of polar, ferromagnetic NiTiO₃ by oxide epitaxy on a LiNbO₃ substrate utilizing tensile strain and demonstrate the theoretically predicted coupling between its polarization and ferromagnetism by X-ray magnetic circular dichroism under applied fields. The experimentally observed direction of ferroic ordering in the film is supported by simulations using the phase-field approach. Our work validates symmetry-based criteria and first-principles calculations of the coexistence of ferroelectricity and WFM in MTiO₃ transition metal titanates crystallizing in the LiNbO₃ structure. It also demonstrates the applicability of epitaxial strain as a viable alternative to high-pressure crystal growth to stabilize metastable materials and a valuable tuning parameter to simultaneously control two ferroic order parameters to create a multiferroic. Multiferroic NiTiO₃ has potential applications in spintronics where ferroic switching is used, such as new four-stage memories and electromagnetic switches.