We use optical microscopy to image spatial structure of metal and insulator patches as a thin
film of VO 2 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 …