Controlling the structure and ferroic properties of strained epitaxial NiTiO3 thin films on sapphire by post-deposition annealing

Ferroelectrically induced weak ferromagnetism had been predicted earlier in metal titanate compounds MTiO3 (M = Fe, Mn, Ni) with the lithium niobate-type (LiNbO3) structure. In order to stabilize this metastable structure by oxide heteroepitaxy, we attempted to grow epitaxial nickel titanate (NiTiO3) films on the nonpolar, non-magnetic sapphire substrate by pulsed laser deposition. As a follow-up to our earlier work, we investigated the effect of post-synthesis annealing in order to improve physical properties. Our structural data from x-ray diffraction and electron microscopy suggest that the crystalline quality of the NiTiO3 films was greatly improved by annealing the films at 600 and 1000 °C, with new phase formation along the interface at 1000 °C. Our physical property characterization indicates increased ferromagnetism and polarization after 600 °C, but a change in the magnetic state to likely ferrimagnetism, and a loss of polarization after the 1000 °C treatment. Our results suggest that the ferroic domain properties of the NiTiO3 films can be favorably altered by post-synthesis heat treatment at the proper temperature to manipulate interfacial strain and enhance the multiferroic properties.