Solid-state electrochemical redox control of the optoelectronic properties for SrFeOx thin films

Abstract

By utilizing redox reactions, the physical properties of several transition metal oxides can be drastically changed, which is useful for developing multifunctional memory devices. Strontium iron oxide (SrFeOx), which exhibits a clear phase transition from antiferromagnetic insulator (x = 2.5) to helimagnetic metal (x = 3), is a good candidate for the active material in multifunctional memory devices. However, practical applications using previous demonstrations of redox reactions in SrFeOx are limited by the use of a liquid electrolyte due to the leakage problem. Here, we demonstrate solid-state electrochemical redox reaction in SrFeOx using a yttria-stabilized zirconia (YSZ) single-crystal substrate as the solid electrolyte. We fabricated the SrFeO2.5 film on the YSZ substrate and the applied electric current using Au electrodes. The phase gradually changed from SrFeO2.5 to SrFeO2.5+x and SrFeO3−x. The color of the film changed from yellowish-transparent to dark brown. Although the as-grown SrFeO2.5 film showed high resistivity (ρ > 101 Ω cm), the ρ dramatically decreased (∼10−2 Ω cm) with increasing the applied charge density. Simultaneously, the thermopower greatly decreased from ∼+200 to ∼−10 μV K−1. The present results would provide a design concept for future SrFeOx-based solid-state multifunctional memory devices.