On the nature of the low temperature insulating state of ferromagnetic and charge ordered manganites

Abstract

Based on electroresistance (ER) measurements founded on a current induced resistivity switching (CIRS) phenomena, we establish the presence of a 'colossal' ER in the low temperature ferromagnetic insulating (FMI) phase exhibited by certain hole doped manganites. Notably, concomitant with the build-up of ER, is a sharp drop in the magnetoresistance (MR). This intelligibly demonstrates an effective decoupling of the mechanisms underlying ER and MR in the FMI phase. ER (CIRS) and MR were measured on single crystals of two widely different FMI manganites: La_0.82Ca_0.18MnO₃ and Nd_0.7Pb_0.3MnO₃. The samples have Curie temperatures, T_C≈165 and 150 K, and the FMI state is realized for temperatures, T≳100 and 130 K, respectively. The ER, arising from a strong nonlinear dependence of resistivity (ρ) on current density (j), attains a value ≃100% in the FMI state. The severity of the nonlinear behavior of resistivity at high current densities is progressively enhanced with decreasing temperature. The MR, however, collapses (<20%) even in magnetic field, H=14 T. Comparison with magnetotransport data on charge ordered insulating (COI) manganites reveal discernible differences in response to applied current and magnetic field. This is credible proof that the nature of the insulating state, in the FMI and COI phases, is different.