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

Abstract

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.

Magnetic-field-induced phase separation via spinodal decomposition in epitaxial manganese ferrite thin films

“…On the other hand, when the growth rate is higher, the composition variation occurs in the direction normal to the film plane. This phenomenon, called surface-directed spinodal decomposition (SDSD), generates the superlattice structure in the film after phase separation [17,29]. In our previous study, we reported the superlattice structure formation through SDSD in SrTiO 3 films grown by Dynamic Aurora PLD [17].…”

“…This phenomenon, called surface-directed spinodal decomposition (SDSD), generates the superlattice structure in the film after phase separation [17,29]. In our previous study, we reported the superlattice structure formation through SDSD in SrTiO 3 films grown by Dynamic Aurora PLD [17]. Here, the composition wave propagates in the vertical direction [17,28,29].…”

“…The design and details of Dynamic Aurora PLD have been described in some of our previous reports [17–21]. The direction of the in situ magnetic field is parallel to the direction from the target to the substrate.…”

“…On the other hand, we have developed a novel pulsed laser deposition (PLD) system by introducing an electromagnet in a PLD vacuum chamber. We can apply the magnetic field during thin film growth in this sophisticated PLD system which is named as Dynamic Aurora PLD [17–21]. In Dynamic Aurora PLD, ion-impingement to the film can be enhanced through suppression of the recombination of ions and electrons in the plume by application of in situ magnetic field during deposition.…”

“…In Dynamic Aurora PLD, ion-impingement to the film can be enhanced through suppression of the recombination of ions and electrons in the plume by application of in situ magnetic field during deposition. We have reported about the spontaneous superlattice formation in Sr 1+ y TiO 3+ y films deposited under in situ magnetic field using Dynamic Aurora PLD, which can be explained by SD along the out-of-plane direction [17]. We have also reported that SD can be enhanced in Co ferrite thin films by applying in situ magnetic field during the film growth without post-deposition annealing [20].…”