Defect Motion as a Driver of Ramp Reversal Memory in VO₂

Recently a new ramp reversal memory effect was observed in VO₂, in which the resistivity was observed to increase by 20% upon applying a particular temperature sequence. One possible microscopic mechanism for the phenomenon is that "scars" accumulate wherever the metal-insulator domain walls reverse direction. [Vardi et al., Adv. Mater., 2017] By using optical microscopy to track the evolution of metal and insulator domains in VO₂ during the ramp reversal temperature protocol, we find that memory also accumulates deep inside the metal/insulator patches rather than only at phase boundaries, which was not foreseen by the "scar" model. We develop a new model based on defect motion during the ramp reversal protocol, using the diffusion-segregation equation. Our calculations show that memory can happen deep inside the metallic and insulating regions and that certain regions in the sample can have a lowered transition temperature rather than a raised one, in agreement with our optical microscopy data. Our results pave the way toward using this "non-volatile" memory effect to mimic synaptic behavior for neuromorphic computing.