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

• Published in: NPG Asia materials 2016-06, Vol.8 (6), p.e279-e279
• Main Authors: Wakiya, Naoki, Sakamoto, Naonori, Koda, Shota, Kumasaka, Wataru, Debnath, Nipa, Kawaguchi, Takahiko, Kiguchi, Takanori, Shinozaki, Kazuo, Suzuki, Hisao
• Format: Article
• Language: English
• Subjects: 639/301/357/551; 639/301/357/995; Biomaterials; Chemistry and Materials Science; Energy Systems; Ferroelectricity; Formations; Materials Science; Optical and Electronic Materials; original-article; Semiconductors; Spontaneous; Strontium; Strontium titanates; Structural Materials; Superlattices; Surface and Interface Science; Thin Films
• ISSN: 1884-4049; 1884-4057; 1884-4057
• DOI: 10.1038/am.2016.76

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 SrTiO 3 bulk is quantum paraelectric, previous reports have described that strained SrTiO 3 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. Superlattices: crafting magnets using irresistible attractions Applying magnetic fields during growth of ceramic thin films creates a tipping point that spontaneously produces stress-based ferromagnets. While atoms in a normal crystal lattice are spaced closely together, researchers can now make ‘superlattices’ consisting of nanometer-wide strips of different crystals periodically stacked on top of each other. In semiconductors, thin films can be induced to form superlattices by tweaking the deposition conditions. Now, Naoki Wakiya from Shizuoka University in Japan and co-workers have replicated this feat for thin films of strontium titanate, an insulating oxide with magnetic and superconducting capabilities. The team used a pulsed laser to deposit strontium titanate onto a substrate and a strong magnetic field to stretch the usual crystal framework. The compressive stress in the new thin film initiates a transformation to a superlattice state and room-temperature ferroelectric behavior. We demonstrate that strontium titanate (Sr-Ti-O) thin films having A-site excess composition in perovskite