Nonlinear current-voltage characteristics of oxygen-deficient La$_{0.67}$Ca$_{0.33}$MnO$_{3-y}$ films

Two different types of nonlinear current-voltage characteristics are observed in oxygen-deficient La$_{0.67}$Ca$_{0.33}$MnO$_{3-y}$ (LCMO) films at temperatures below insulator-metal transition. The parabolic-like dynamic conductance $G(V)$, defined as d$I$/d$V$, curves near zero bias observed in highly oxygen-deficient LCMO films implies the contribution from the spin-dependent tunneling transport between ferromagnetic clusters with magnetic-disordered regions serving as tunneling barriers. On the other hand, for the slightly oxygen-deficient LCMO films, dips around zero bias were observed in nonlinear $G(V)$ curves and have been attributed to spin-flip scattering with oxygen vacancies serving as scattering centers.