Coupled Lattice Polarization and Ferromagnetism in Multiferroic NiTiO Thin Films.

Polarization-induced weak ferromagnetism (WFM) was demonstrated a few years back in LiNbO-type compounds, MTiO (M = Fe, Mn, Ni). Although the coexistence of ferroelectric polarization and ferromagnetism has been demonstrated in this rare multiferroic family before, first in bulk FeTiO, then in thin-film NiTiO, the coupling of the two order parameters has not been confirmed. Here, we report the stabilization of polar, ferromagnetic NiTiO by oxide epitaxy on a LiNbO 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 MTiO transition metal titanates crystallizing in the LiNbO 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 NiTiO has potential applications in spintronics where ferroic switching is used, such as new four-stage memories and electromagnetic switches.