Nonlinear transport in nanoscale phase separated colossal magnetoresistive oxide thin films

We report a study of the I-V characteristics of 2.5–5.4 nm epitaxial La_1−xSr_xMnO₃ (x = 0.33 and 0.5) and La_0.7Ca_0.3MnO₃ thin films. While La_0.67Sr_0.33MnO₃ films exhibit linear conduction over the entire temperature and magnetic field ranges investigated, we observe a strong correlation between the linearity of the I-V relation and the metal-insulator transition in highly phase separated La_0.5Sr_0.5MnO₃ and La_0.7Ca_0.3MnO₃ films. Linear I-V behavior has been observed in the high temperature paramagnetic insulating phase, and an additional current term proportional to V^α (α = 1.5–2.8) starts to develop below the metal-insulator transition temperature T_MI, with the onset temperature of the nonlinearity increasing in magnetic field as T_MI increases. The exponent α increases with decreasing temperature and increasing magnetic field and is significantly enhanced in ultrathin films with thicknesses close to that of the electrically dead layer. We attribute the origin of the nonlinearity to transport through the nanoscale coexisting metallic and insulating regions. Our results suggest that phase separation is not fully quenched even at low temperatures and high magnetic fields.