Atomic-Scale Phase Transformation in Perovskite LaCoO_x Resistive Switching Memristive Devices

Resistive random-access memory (RRAM) is considered the next-generation nonvolatile memory owing to its simplicity, low power consumption, and high storage density. Resistive switching (RS) occurs in a wide range of materials among the transition metal oxides. Herein, an epitaxial ternary metal oxide layer, LaCoO_x (LCO), grown on Nb-doped SrTiO₃ substrates, is utilized as an RRAM device. When voltage is applied, it exhibits excellent RS behavior. More than 900 cycles are obtained, and the retention time reaches up to 10⁴ s. To investigate the RS behavior, high-resolution transmission electron microscopy and atomic-scale scanning transmission electron microscopy are used to observe the structural evolution and oxygen ion migration in LCO. The structure exhibits a perovskite–brownmillerite topotactic phase transformation from LaCoO_2.5 or LaCoO_2.67 to the LaCoO₃ conductive regions. The reversible transition between the low-resistance states and high-resistance states enables the RS mechanism. Additionally, the valence states are confirmed using high-resolution X-ray photoelectron spectroscopy. This study not only illustrates the oxygen-ion migration mechanism of LCO but also demonstrates its suitability for RRAM applications.