Evidence of non-stoichiometry effects in nanometric manganite perovskites: influence on the magnetic ordering temperature
Résumé
This work tends to evidence that the significant chemical modifications observed in nanometric manganites are not concentrated at the surface of the particles and play a key role on their magnetic properties, especially for the lowest strontium-doping. La1−xSrxMnO3 solid solution with a 27 nm-average crystallite size was prepared via the Glycine Nitrate Process. The evolutions versus x of the Curie temperature (TC) and saturation magnetization of the nanometric solid solution were interpreted taking into account the Goldschmidt tolerance factor, crystallite size, amount of vacancies and the mixed valency of manganese ions. Two distinct populations were distinguished: (i) for x lower than 0.25, the increase of TC with x could be related to the decreasing amount of cationic vacancies that accommodate the substitution at the A-site of the perovskite framework, simultaneously keeping the content of Mn4+% constant; (ii) for x higher than 0.25, the increase of x led to a decrease of the structural distortion, hence favouring orbital overlap and inducing a TC increase. However, when the amount of Mn4+ exceeded the value of ≈35%, competitive superexchange antiferromagnetic interactions were promoted. As a result, the observed behaviour was a compromise between these two competitive tendencies and led to a quasi-constant TC. For x higher than 0.4, the antiferromagnetic interactions became more important, which induced a TC decrease.