Phase separation, thermal history and magnetic behaviour of Sr doped LaCoO3

High-resolution transmission electron microscopy observations and magnetic ac susceptibility measurements have been carried out to study the changes i n the magnetic properties of La1-xSrxCoO3 samples (x less than or equal to 0.3) processed at different annealing temperatures, ranging from 1273 to 15 73 K. Although neutron diffraction excludes the coexistence of distinct pha ses with different chemical composition, room temperature micrographs taken along the [0, -1, 1] rhombohedral zone-axis show bright fringes of differe nt widths, extending over regions with sizes ranging from 8 to 40 nm. These regions are detected in all the samples, whatever the Sr content or the an nealing temperature T-a. However, their number depends strongly on the proc essing conditions and becomes smaller and smaller as T-a increases. The per iodicity of the bright fringes cannot be accounted for, either by variation s in the sample thickness or by crystallographic defects, but it can be sim ulated if the presence of aperiodically alternate Sr-rich and La-rich (0, 1 , 1) planes is assumed. On the other hand, the susceptibility measurements show that the amount of ferromagnetic phase present in each specimen depend s on its thermal history. This suggests that the segregation of a second el ectronic phase takes place, the nucleation of which is influenced by the sp atial homogeneity of the dopant distribution.