Atomic‐Scale Visualization of Topotactic Resistive Switching in Epitaxial La 0.5 Sr 0.5 CoO x Neuromorphic Memristors

Resistive switching in oxide memristors is commonly associated with oxygen migration, but the corresponding atomic‐scale structural evolution during electrical operation remains difficult to resolve. Here, we investigate epitaxial Pt/La 0.5 Sr 0. 5CoO 2.5 (LSCO)/La 0.5 Sr 0.5 MnO 3 (LSMO)/Nb‐doped SrTiO 3 memristors exhibiting forming‐free bipolar resistive switching. The high‐resistance state (HRS) and low‐resistance state (LRS) remain distinguishable over 1200 DC switching cycles and for 10 4 s in retention measurements, while pulse measurements demonstrate gradual conductance modulation and representative synaptic functions. Atomic‐resolution scanning transmission electron microscopy reveals that SET transforms the oxygen‐vacancy‐ordered brownmillerite (BM) LSCO into predominantly perovskite (PV)‐like regions with a smaller Grenier (GN)‐like fraction, whereas RESET largely restores the BM‐like structure while leaving residual transformed regions. In situ transmission electron microscopy captures the structural transformation initiating near the Pt/LSCO interface and propagating toward the LSMO electrode under electrical bias. X‐ray photoelectron spectroscopy and electron energy‐loss spectroscopy show changes consistent with oxidation of LSCO upon switching to the LRS. These observations directly correlate the resistance states with topotactic structural evolution in LSCO and support a field‐driven oxygen‐redistribution mechanism.