In situ observations of topotactic phase transitions in a ferrite memristor

Oxide-based memristive devices have recently been proposed for various applications, such as next-generation memory and neuromorphic devices. Microstructural alterations depending on the oxygen ion concentration, such as the formation of conductive filaments and interface reactions, have been posited as the operating mechanism. Accordingly, it is important to explore the role of oxygen ion mobility in the behavior of memristive devices. In this study, memristive devices fabricated with brownmillerite SrFeO_2.5 in the (111) and (001) orientations were studied via high-resolution transmission electron microscopy and in situ current–voltage measurements. The resistance of the devices was changed by a reversible topotactic phase change between the insulating brownmillerite SrFeO_2.5 and the conductive perovskite SrFeO_3−δ. Importantly, the oxygen vacancy channel was designed so that the phase change occurred across the electrodes in (111), where the channel was directly connected to both electrodes, whereas in (001) the channel is ordered along the in-plane direction and a phase change occurs only near the electrode. This work provides direct evidence of a brownmillerite-based mechanism of resistance change and a better understanding of routes to performance improvement.