La0.7Ca0.3-xNaxMnO3 polycrystalline with high magnetoresistance and temperature coefficient of resistance were prepared via the sol–gel method

Abstract

Doping is a primary method to improve the temperature coefficient of resistance (TCR) and magnetoresistance (MR) of La1-xA′xMnO3 polycrystalline in a low magnetic field. La0.7Ca0.3-xNaxMnO3 (x\u2009=\u20090, 0.02, 0.04, 0.06, 0.08) series of polycrystalline were synthesized by the sol–gel method. X-ray diffraction patterns indicate that the crystal structure changes from orthorhombic to rhombohedral, and the space group changes from Pnma to R3-c\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\text{R}\\stackrel{\\mathrm{-}}{\\text{3}}\\text{c}$$\\end{document} with increasing x. With Na-doping rise, the increase in grain size reduces grain boundaries, weakening the electron scattering at grain boundaries and decreasing the peak resistivity. The excellent MR (59.51%) is observed in La0.7Ca0.24Na0.06MnO3 at the 1 T magnetic field. Meanwhile, TCRmax reaches 12.36% K−1 in the absence of a magnetic field. The resistivity variation with temperature curve fitting illustrates that grain boundary scattering dominates in the low-temperature stage, and the effect of activation energy dominates in the high-temperature stage.