Magnetic, structural and magneto-resistance studies of doped LaMnO$_3$ bulk samples prepared by citrate combustion process

We present the structural and magnetic properties of polycrystalline samples of La$_{0.95}$A$_{0.05}$MnO$_{3}$ (where A = Na, Sr, Er, Dy and Ce) prepared by chemical citrate combustion method. Er substituted samples (La$_{1-x}$Er$_{x}$MnO$_{3}$ with x = 5, 10, 20 and 30\%) were also investigated, because their studies lack in the literature. The pervoskite structure was confirmed from X-ray diffraction and Raman spectra in these doped samples, excluding higher Er substituted samples (x $>$ 0.1). The grain sizes were around 2-3 micrometres for all of the sintered samples (at 1300$^{\circ}$C), whereas it was below 100 nm for the powders calcined at 600$^{\circ}$C, determined from the SEM images. Curie transition temperatures in those doped LMO bulk samples were found to be around 250K, which is higher than the ideal value ($\sim$140 K) for undoped samples. The increase in the Curie temperature can be related to non-stoichiometry and cation vacancies created due to higher/lower valence substitutions for trivalent La$^{3+}$ ions. The temperature dependence of resistivity also confirms the MIT transition in some of these samples.