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对于磁性氧化物的磁有序, 传统的观点用超交换相互作用(SE)和双交换相互作用(DE) 模型进行解释, 其出发点都建立在全部氧离子是-2价的基础上. 例如, 对于LaMnO3, 认为其中的La和Mn都处于+3价, 用SE模型解释相邻Mn3+离子间的反铁磁序; 当以二价的Sr离子替代一部分La离子后, 认为等量的Mn3+离子变为Mn4+离子, 用DE模型解释相邻Mn3+和Mn4+离子间的铁磁序. 然而, 事实上在氧化物中存在一部分负一价氧离子. Cohen [Nature 358 136]利用密度泛函理论计算了BaTiO3的价电子态密度, 结果得到只有Ba离子的化合价与传统观点相同, 为+2 价; 而Ti和O分别为+2.89价和-1.63价, 不是传统观点的+4价和-2价, 但是与多年来关于氧化物电离度的研究[Rev. Mod. Phys. 42 317] 和X 射线光电子谱(XPS)的研究结果相符合. 本文经过不同热处理条件制备了名义成分为La0.95Sr0.05MnO3 的三个样品, 通过对样品的XPS分析, 发现样品中不存在Mn4+离子, 只存在Mn2+ 和Mn3+离子, 平均价态随热处理程序的增加而升高. 尽管三个样品有相同的晶体结构, 但磁矩明显不同. 对于这样的性能, 不能用SE和DE模型解释其磁结构. 利用本课题组最近在研究尖晶石结构铁氧体磁有序过程中提出的O 2p巡游电子模型解释了这种现象, 利用样品在10 K的磁矩估算出的Mn离子平均价态变化趋势与XPS分析结果一致. O 2p巡游电子模型的出发点建立在氧化物中存在一部分负一价氧离子的基础上, 这是其与SE和DE模型的根本区别.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.
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