Nonlinear transport in nanoscale phase separated colossal magnetoresistive oxide thin films

摘要

We report a study of the Ⅰ-Ⅴ characteristics of 2.5-5.4nm epitaxial La_(1-x)Sr_xMnO_3 (x = 0.33 and 0.5) and La_(0.7)Ca_(0.3)MnO_3 thin films. While La_(0.67)Sr_(0.33)MnO_3 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_(0.5)Sr_(0.5)MnO_3 and La_(0.7)Ca_(0.3)MnO_3 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 a 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.