Nonlinear electrical transport through artificial grain-boundary junctions in La_0.7Ca_0.3MnO₃ epitaxial thin films

Abstract

We report investigation of non-linear electronic transport through artificial grain-boundary junctions made on epitaxial films of La_0.7Ca_0.3MnO₃ on bicrystal SrTiO₃ substrates. The experiments carried out over the temperature range 4.2 K-300 K in magnetic field up to 3 T allow us to identify some of the conduction mechanisms that may give rise to nonlinear transport in these grain boundary junctions. The nonlinear transport is associated with multistep inelastic processes in the grain-boundary region, which is moderately affected by the applied magnetic field. However the primary effect of the magnetic field is to enhance the zero-bias conductance [G₀=(dI/dV)_V=0]. The dominant voltage dependent contribution to the dynamic conductance (G=dI/dV) comes from a term of the type V^4/3 at lower temperatures. Other voltage dependent contributions to G, which are of higher order in V, appear only for T≥75K. In addition we found a contribution to G arising from a V^0.5 term, which is likely to arise from the disordered region around the grain boundary (GB). The magnetoresistance in the GB depends on the bias used and it decreases at higher bias. The bias dependence is found to be reduced as temperature is increased. We discuss the physical origins of the various contributions to the nonlinear conduction.