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Neuromorphic functionalities in memristive devices are commonly associatedwith the ability to electrically create local conductive pathways by resistiveswitching. The archetypal correlated material, VO2, has been intensivelystudied for its complex electronic and structural phase transition as well asits filament formation under applied voltages. Local structural studies of thefilament behavior are often limited due to time-consuming rastering which makesimpractical many experiments aimed at investigating large spatial areas ortemporal dynamics associated with the electrical triggering of the phasetransition. Utilizing Dark Field X-ray Microscopy (DFXM), a novel full-fieldx-ray imaging technique, we study this complex filament formation processin-operando in VO2 devices from a structural perspective. We show that prior tofilament formation, there is a significant gain of the metallic rutile phasebeneath the metal electrodes that define the device. We observed that thefilament formation follows a preferential path determined by the nucleationsites within the device. These nucleation sites are predisposed to the phasetransition and can persistently maintain the high-temperature rutile phase evenafter returning to room temperature, which can enable a novel training/learningmechanism. Filament formation also appears to follow a preferential pathdetermined by a nucleation site within the device which is predisposed to therutile transition even after returning to room temperature. Finally, we foundthat small isolated low-temperature phase clusters can be present inside thehigh-temperature filaments indicating that the filament structure is notuniform. Our results provide a unique perspective on the electrically inducedfilament formation in metal-insulator transition materials, which further thebasic understanding of this resistive switching.

High-Resolution Full-field Structural Microscopy of the Voltage Induced Filament Formation in Neuromorphic Devices