  Neuromorphic functionalities in memristive devices are commonly associated
with the ability to electrically create local conductive pathways by resistive
switching. The archetypal correlated material, VO2, has been intensively
studied for its complex electronic and structural phase transition as well as
its filament formation under applied voltages. Local structural studies of the
filament behavior are often limited due to time-consuming rastering which makes
impractical many experiments aimed at investigating large spatial areas or
temporal dynamics associated with the electrical triggering of the phase
transition. Utilizing Dark Field X-ray Microscopy (DFXM), a novel full-field
x-ray imaging technique, we study this complex filament formation process
in-operando in VO2 devices from a structural perspective. We show that prior to
filament formation, there is a significant gain of the metallic rutile phase
beneath the metal electrodes that define the device. We observed that the
filament formation follows a preferential path determined by the nucleation
sites within the device. These nucleation sites are predisposed to the phase
transition and can persistently maintain the high-temperature rutile phase even
after returning to room temperature, which can enable a novel training/learning
mechanism. Filament formation also appears to follow a preferential path
determined by a nucleation site within the device which is predisposed to the
rutile transition even after returning to room temperature. Finally, we found
that small isolated low-temperature phase clusters can be present inside the
high-temperature filaments indicating that the filament structure is not
uniform. Our results provide a unique perspective on the electrically induced
filament formation in metal-insulator transition materials, which further the
basic understanding of this resistive switching.
