Influence of thermal treatment on the ionic valence and the magnetic structure of perovskite manganites La0.95Sr0.05MnO3

Abstract

In traditional views, the magnetic ordering of oxides may be explained using magnetic superexchange (SE) or double exchange (DE) interaction models. Both models are based on an assumption that the valences of all oxygen ions be -2. For example, both La and Mn in LaMnO3 are assumed to be trivalent, in which antiferromagnetic spin structure is explained using the SE interaction between Mn3+ cations mediated by oxygen anions. In La1-xSrxMnO3, there exists a part of Mn4+ cations with the content ratio of Mn4+/Mn3+ being x/(1-x), in which spin structure and electronic transport properties are explained by DE interaction. However, there is a part of monovalent oxygen ions existing in oxides. Cohen [Nature 358 136] has calculated the densities of states for valence electrons in the perovskite oxide BaTiO3 using density functional theory. Results indicate that the average valence of Ba is +2, being the same as that in the traditional one, but the average valences of Ti and O are +2.89 and -1.63 respectively, agreeing with the results obtained using ionicity investigation [Rev. Mod. Phys. 42 317] and X-ray photoelectron spectra (XPS) analysis, but different from the conventional results +4 and -2. In this paper, three samples with the nominal composition La0.95Sr0.05MnO3 are prepared by different thermal-treatments. Likewise, there are only Mn2+ and Mn3+ cations, but no Mn4+ cations in La0.95Sr0.05MnO3, a result obtained by XPS analysis, and the average valence of Mn in La0.95Sr0.05MnO3 samples increases with increaseing thermal-treatment. Although the crystal structures of the samples are the same, the magnetic moments per formula are obviously different. This magnetic structure cannot be explained using the conventional SE and DE interaction models. Using the O 2p itinerant electron model for spinel ferrites proposed recently by our group, we can explain this magnetic structure. The variation trend of the average valences of Mn cations calculated using the magnetic moments per formula of the samples at 10 K, is in accordance with the experiment results of XPS. The O 2p itinerant electron model is based on an assumption that there is a part of monovalent oxygen ions in the oxides, which is the fundamental difference from SE and DE interaction models.