Improvement of room-temperature TCR and MR in polycrystalline La0.67(Ca0.27Sr0.06)MnO3 ceramics by Ag2O doping

Abstract

Polycrystalline La0.67(Ca0.27Sr0.06)MnO3:mol%Agx (LCSMO:Agx, x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) ceramics were prepared by conventional sol-gel method. Effects of Ag-doping (x) on structures, surface morphologies, electrical and magnetic transport properties of the as-prepared LCSMO:Agx samples were evaluated. X-ray diffraction (XRD) showed that all samples crystallized in orthorhombic structures with Pnma space group. As x content increased, main diffraction peak (121) shifted downwards lower angles, giving rise to A site ions (Ca2+ or Sr2+) substitution by Ag+. This, in turn, caused slight expansion of the lattice and improved Mn4+ ion concentration. Meanwhile, grain size gradually increased and grain boundaries (GBs) reduced, the resistivity decreased and metal-insulator transition temperature (TMI) almost kept constant at room temperature. On the other hand, the magnetoresistance (MR) and temperature coefficient of resistance (TCR) increased during first stages and then decreased as x further rose. An optimum doping molar ratio of x = 0.2 was obtained, with room temperature MR and TCR reaching up to 44.5% and 22.5% K−1, respectively. Curve fitting results indicated that in low temperature ferromagnetic metallic (FM) region, conductive behavior of LCSMO:Agx was mainly related to electron-electron, electron-phonon, and electron-magnon scattering. In high temperature paramagnetic insulating (PI) region, data followed the small-polaron-hopping model. Conductive behavior in the entire temperature region was fitted with phenomenological equation under percolation approach, and found to depend on segregation of FM and PI phases. These findings suggest promising applications of LCSMO:Agx materials in photoelectric (uncooled bolometer or infrared detectors) or magnetic devices (uncooled magnetic sensors) at room temperature.