ELECTRICAL RESISTIVITY AND MAGNETO-RESISTANCE OF La0.7Sr0.3-xAgxMnO3 PELLETS BETWEEN 10 AND 450 K

Abstract

The temperature dependence of the electrical resistivity and magneto-resistance in silver substituted La 0.7 Sr 1-x Ag x MnO 3 pellets prepared by a simple pyrophoric method has been investigated between 10 and 450 K for H=0 and 0.8T. Ag substitution enhances the conductivity of this system. Curie temperature (T C ) also increases sharply from ~303 K (x=0.05) to ~363 K (x=0.25). Above the metal-insulator transition temperature (T MI ), the electrical resistivity is well represented by usual adiabatic polaronic transport, while in the ferromagnetic region at low temperatures, the resistivity data shows an excellent agreement to an expression containing the residual resistivity, two-magnon scattering term and the term associated with small-polaron metallic conduction, which involves a relaxation time due to a soft optical phonon mode. Based on a scenario that the doped manganites consist of phase separated ferromagnetic metallic and paramagnetic insulating regions, all the features of the temperature variation of the resistivity between ~50 K and 300 K could be described very well. All the Ag doped samples reveal resistivity minimum, which is strongly field dependent (H=0.8 T ). These minimum results have been analyzed and the observed field dependent minimum is found to be in excellent agreement to those calculated from the model based on charge carrier tunneling between antiferro-magnetically coupled grains. Field dependence of magnetoresistance at various temperatures below T c is accounted fairly well by a phenomenological model based on spin polarized tunneling at the grain boundaries. The contributions from the intrinsic part arising from DE mechanism, as well as the part originating from intergranular spin polarized tunneling are also estimated.