Electric Field Control of Phase Transition and Tunable Resistive Switching in SrFeO_2.5

SrFeO_x (SFO_x) compounds exhibit ionic conduction and oxygen-related phase transformation, having potential applications in solid oxide fuel cells, smart windows, and memristive devices. The phase transformation in SFO_x typically requires a thermal annealing process under various pressure conditions, hindering their practical applications. Here, we have achieved a reversible phase transition from brownmillerite (BM) to perovskite (PV) in SrFeO_2.5 (SFO_2.5) films through ionic liquid (IL) gating. The real-time phase transformation is imaged using in situ high-resolution transmission electron microscopy. The magnetic transition in SFO_2.5 is identified by fabricating an assisted La_0.7Sr_0.3MnO₃ (LSMO) bottom layer. The IL-gating-converted PV phase of a SrFeO_3−δ (SFO_3−δ) layer shows a ferromagnetic-like behavior but applies a huge pinning effect on LSMO magnetic moments, which consequently leads to a prominent exchange bias phenomenon, suggesting an uncompensated helical magnetic structure of SFO_3−δ. On the other hand, the suppression of both magnetic and exchange coupling signals for a BM-phased SFO_2.5 layer elucidates its fully compensated G-type antiferromagnetic nature. We also demonstrated that the phase transition by IL gating is an effective pathway to tune the resistive switching parameters, such as set, reset, and high/low-resistance ratio in SFO_2.5-based resistive random-access memory devices.