Fig. 1: Real-time visualization of the electroforming process in BCFO film.

a Schematic of a BCFO channel protected by a capping layer between a pair of coplanar electrodes with a 400 μm gap. Real-time current through the channel and optical microscope video are simultaneously recorded while an electric bias +25 V is applied. b This schematic describes homogeneously distributed oxygen vacancies in the as-grown state that migrate toward the negatively biased electrode, thereby creating a dark-colored formed phase. The front of the phase propagates to the left at a positive bias. c Optical microscope images at elapsed time t = 0 s (as-grown state), 70 s (intermediate state), 180 s (expansion of the formed phase) and 240 s (completion of a forward-formed state) after the bias is turned on at 310 °C. d Real-time visualization of the optical contrast along the centerline of the channel during electrical forward forming. The as-grown yellow phase (tetragonal: T₀) turns into an intermediate dark-yellow phase (tetragonal: T) and finally converts into a dark formed phase (monoclinic: M_C). Color lines and marks correspond to the centerlines of still images in c. e In-situ electrical current monitoring during forming. Curvature of the logarithmic current changes with the T₀ to T transition (green-dashed line). The current is saturated after completion of the electrical forming process (red-dashed line)