Observation of anisotropic filamentary conduction pathways in Ca-doped bismuth ferrite thin films

Ionic migration is a key ingredient for applications such as oxide electrolytes and resistive switching memories. We investigate the evolution of the ionic conduction pathways based on optical contrast in an epitaxial Bi_0.7Ca_0.3FeO_3-δ thin film where oxygen vacancies are spontaneously produced.[1] We visualized electroforming processes in the hundreds-micron-scale material channels between coplanar electrodes with a constant electric bias at an elevated temperatures, systemically varying the channel orientation with respect to the crystal axis.[2] At the initial stage of electroforming, conducting filaments are created and propagate nearly along the crystal axes <100>, thereby making orthogonal self-similar networks. We also find that the filament-type ionic conduction is abruptly transformed to the bulk conduction around the time when the filament pathways connect both electrodes. These results offers useful insight into collective ionic migration in crystalline solids.

[1] J. S. Lim et al., NPG Asia Materials 10, 943-955 (2018)
[2] H.-S. Park et al., Applied Physics Letters (in press)