In Situ Transmission Electron Microscopy Observation of Redox-Structure-Coupled Resistive Switching in High-Entropy Oxide Memristors.

High-entropy oxides (HEOs) have emerged as potential candidates for resistive random-access memory (RRAM) owing to their structural robustness and highly tunable electronic configurations. In this study, a transition-metal HEO thin film composed of Ca, Ti, Sr, Ta, Nb, and O was integrated into Au/HEO-based RRAM devices that exhibited a low SET voltage, fast switching speed (30 ns), and long data retention time (104 s). The underlying resistive switching mechanism was uncovered by employing in situ transmission electron microscopy to observe the real-time structural evolution under an electrical bias. A localized monoclinic-to-cubic symmetry transition was observed during the SET process. Energy-dispersive X-ray spectroscopy and electron-energy-loss spectroscopy analyses revealed cationic rearrangements and Ti/Nb redox interactions accompanied by the generation of oxygen vacancies. These results establish the redox-structure-coupled mechanism for understanding resistive switching in complex oxides. In addition, the HEO-based memristors exhibit stable switching characteristics and synaptic plasticity, highlighting their feasibility for next-generation memory and neuromorphic applications.