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, 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. 
S. B. Lee, S. C. Chae, S. H. Chang, J. S. Lee, S. Park, Y. Jo, S. Seo,   B. Kahng, and T. W. Noh