Structural and Magnetic Properties of Pure and Ca-Doped LaCoO3 Nanopowders Obtained by a Sol-Gel Route

Pure and Ca-doped LaCoO3 nanopowders were prepared by a non-alkoxidic sol-gel route using cobalt(II) acetate, lanthanum(III) nitrate and calcium(II) acetate as oxide precursors. The structural evolution and magnetic properties of the samples were studied as a function of thermal treatments in air up to 1273 K. In particular, microstructure and composition of the systems were analyzed by X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS). The perovskitic cobaltite was obtained after annealing at 973 K for both pure and calcium-doped samples. The nanostructured powders (average crystallite size < 30 nm) crystallized in the cubic perovskite phase. This phase was fully retained in the calcium-doped materials even after annealing at higher temperatures, whereas a transition to the rhomboedral polymorph was detected in the pure samples at 1073 K. The magnetic behavior of the nanopowders was investigated as a function of temperature and applied field using both dynamic and static susceptibility measurements. Pure lanthanum cobaltite samples underwent a transition to an ordered state at 88 K, and their magnetic properties changed as a function of thermal treatments. As concerns calcium doped samples, they ordered ferromagnetically at 171 and 185 K, depending on the annealing temperature, and displayed open hysteresis loops with coercive field as large as 1.75 T at low temperatures.