  We investigated third harmonic generation in NiO thin films, which exhibit
unipolar resistance switching behavior. We found that the low resistance states
of the films were strongly nonlinear, with variations in the resistance R as
large as 60%. This strong nonlinear behavior was most likely caused by Joule
heating of conducting filaments inside the films. By carefully controlling the
applied dc bias, we obtained several low resistance states, whose values of the
third harmonic coefficient B3f were proportional to R2+w (with w = 2.07). This
suggested that the resistance changes of the NiO films were accompanied by
connectivity changes of the conducting filaments, as observed in classical
percolating systems.
