Magnetic-field-induced spontaneous superlattice formation via spinodal decomposition in epitaxial strontium titanate thin films

Periodically structured nanomaterials such as superlattices have a wide range of applications. Many electronic devices have been fabricated from these materials. The formation of spontaneous layer structures using epitaxial growth has also been reported for many compound semiconductors but for very few ceramics. We demonstrate that strontium titanate (Sr-Ti-O) thin films having an A-site excess composition in the perovskite structure deposited by pulsed laser deposition under a magnetic field show a spontaneously formed superlattice structure. The spontaneous superlattice formation mechanism has been proven to exhibit spinodal decomposition. Preparation of a part of the phase diagram for Sr-Ti-O thin films is reported for the first time. Although SrTiO₃ bulk is quantum paraelectric, previous reports have described that strained SrTiO₃ thin films show room-temperature ferroelectricity, especially along the in-plane direction. However, induced ferroelectricity along the out-of-plane direction has been reported in films with a limited thickness of less than 10 ml. The results show that the Sr-Ti-O thin films with spontaneously formed superlattice structures exhibit room-temperature ferroelectricity even when 300 nm thick. The induced ferroelectricity is brought about by the strain along the out-of-plane direction and is explained based on thermodynamic considerations.