Strong resistance nonlinearity and third harmonic generation in the unipolar resistance switching of NiO thin films

We investigated third harmonic generation in NiO thin films that exhibit unipolar resistance switching. We found that low resistance states (LRSs) were strongly nonlinear with variations in the resistance R as large as 60%, which was most likely caused by the Joule heating of conducting filaments inside the films. By carefully controlling the applied dc bias, we obtained several LRSs, whose values of the third harmonic coefficient B-3f were proportional to R2+w (w=2.07 +/- 0.26). This suggested that the resistance changes in the NiO films were accompanied by connectivity changes of the conducting filaments, as observed in classical percolating systems.