Spatial Mapping of Ramp Reversal Memory in VO₂

We use optical microscopy to image spatial structure of metal and insulator patches as a thin film of VO₂ is repeatedly driven partway through its temperature-driven insulator-to-metal transition. The location and shape of accumulated memory was tracked after each subloop, revealing for the first time the internal structure of the memory effect. A combination of insulator domains appearing at cluster boundaries, as well as large insulator nucleation sites, was identified. Two large temperature sweeps were subsequently completed to reset the ramp reversal memory. Transition temperature maps reveal that memory is surprisingly also stored deep in the insulating and metallic puddles throughout the entire sample surface. We discuss two possible diffusion models to explain these 2D surface memory maps. These results pave the way to enhancing, controlling and manipulating the memory in this material in the near future.