Spatially Distributed Ramp Reversal Memory in VO₂

We use optical microscopy to image spatial maps of accumulated memory as a thin film of VO₂ is repeatedly driven partway through its temperature-driven insulator-to-metal transition. By mapping for the first time the spatial structure of metal and insulator patches during a temperature ramp reversal sequence, the location and shape of accumulated memory was tracked after each hysteresis subloop, revealing the internal structure of the ramp reversal memory effect in this material. Our measurements demonstrate that new insulating regions appear through front propagation starting at insulator-metal boundaries. Surprisingly, our transition temperature maps reveal that memory is also stored deep inside the insulating and metallic clusters throughout the entire sample surface. We show that the non-volatile memory is globally reset by large temperature sweeps completed afterwards. We have developed a new model based on defect motion that accounts for the observed memory writing and subsequent erasing over the entire sample surface. By spatially mapping the location and character of non-volatile memory encoding in VO₂, our results pave the way toward directly addressing local regions of VO₂ in order to optimize neuromorphic memory elements.