Linear and nonlinear electronic transport in weakly insulating double layered manganites

Polycrystalline La_1.2Sr_1.8Mn_2-yCr_yO₇ (0≤y≤0.25) is a weakly insulating ferromagnet with low-T thermopower of metalliclike values. This combination of properties indicates electronic transport through percolating paths embedded in an insulating medium. The temperature dependence of the resistivity at low T and the nonlinear conductivity obtained from pulsed I-V measurements over a wide range of currents are consistent with inelastic tunneling through intergrain or intragrain weak links that disrupt the percolating paths. Thermopower and resistivity measurements on two weakly insulating single crystals of La_1.4Sr_1.6Mn_1.85Cr_0.15 O_7-δ and the I-V measurements on the more resistive crystal exhibit features very similar to those found in the polycrystals. In single crystals the nonlinearity of conductivity is pronounced under much lower electric fields than in polycrystals.