Defect-Strain Engineering Stabilized Multiferroic Properties in Non-Stoichiometric SrTiO₃ Epitaxial Thin Films and Freestanding Membranes

The development of perovskite oxide epitaxial films and their freestanding membranes for the next-generation integrated and miniaturized electronics has attracted a lot of attention due to their promising multiferroic properties and low dimensionality. Strontium titanate, (SrTiO₃, STO), as one typical perovskite incipient ferroelectric materials, has been reported to encounter significant challenges in inducing stable ferroelectricity and magnetism at room temperature, especially in the case of freestanding STO membranes. Here we deliberately introduced Ti deficiency/O-vacancy into the targeted STO thin films via defect-strain engineering to prepare the non-stoichiometric epitaxial thin films with large Sr/Ti ratios. The prepared non-stoichiometric STO thin films (Sr/Ti = 0.6/0.4) exhibited stable tetragonal phase with coexistent relaxor ferroelectricity and magnetism, which was thought to be associated with the presence of polar nanoregions. Importantly, owning to the defect-strain engineering, the freestanding non-stoichiometric STO membranes well preserved the initial compressive strain state when releasing from the rigid substrate. The freestanding STO membranes thus exhibited a polar tetragonal phase with significant ferroelectricity and magnetism. This study offers an effective approach for achieving ferroelectricity and magnetism in STO materials, both for epitaxial thin films and freestanding membranes, which gives their great potential in multifunctional semiconductor applications.