Covalent bonds against magnetism in transition metal compounds
Covalent bonds against magnetism in transition metal compounds
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DOI：
10.1073/pnas.1606367113
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发表时间：
2016-02
期刊：
影响因子：
--
通讯作者：
S. Streltsov;D. Khomskii
中科院分区：
文献类型：
--
作者：
S. Streltsov;D. Khomskii
重要性提出了一种轨道自由度可以强烈影响具有关联电子的系统的磁性的机制。通过分析处理和数值模拟，我们证明了部分轨道上共价键的形成可以强烈地降低磁矩，并解释了不同的4d和5d过渡金属氧化物的磁性。特别是，轨道选择效应可能导致抑制双交换--过渡金属和化合物中铁磁性的主要机制之一，例如巨磁电阻锰氧化物或CrO2--许多器件中使用的材料。过渡金属化合物中的磁性通常被认为是从对孤立离子的描述开始的，尽可能准确，并在稍后阶段处理它们(交换)的相互作用。我们表明，这种标准方法在许多情况下可能会失效，特别是在4d和5d化合物中。我们认为存在一个重要的位间效应--共价金属-金属键的轨道选择性形成，导致相应的电子从磁子系统中被“排除”，从而强烈地影响系统的磁性。这种效应对于非整数电子数尤其显著，当它导致著名的双交换被抑制时，双交换是过渡金属化合物铁磁性的主要机制。我们从解析和数值上研究了这种机制，表明它不仅可以解释几种4d-5d材料的磁性，包括Nb2O2F3和Ba5AlIr2O11，而且还可以用于3D过渡金属氧化物，例如加压下的CrO2中。我们还讨论了自旋-轨道耦合在共价性和磁性竞争中的作用。我们的结果表明，强烈的格点间耦合可能会使考虑磁性的标准单格点起点失效，并可能导致一种定性的新行为。
Significance A mechanism by which orbital degrees of freedom can strongly affect magnetic properties of systems with correlated electrons is proposed. Using analytical treatment and numerical simulations, both in general and for some particular substances, we show that the formation of covalent bonds on part of the orbitals may strongly reduce magnetic moments and explain magnetic properties of different 4d and 5d transition metal oxides. In particular, orbital-selective effects may result in suppression of double exchange—one of the main mechanisms of ferromagnetism in transition metals and compounds, such as colossal magnetoresistance manganites or CrO2—the materials used in many devices. Magnetism in transition metal compounds is usually considered starting from a description of isolated ions, as exact as possible, and treating their (exchange) interaction at a later stage. We show that this standard approach may break down in many cases, especially in 4d and 5d compounds. We argue that there is an important intersite effect—an orbital-selective formation of covalent metal–metal bonds that leads to an “exclusion” of corresponding electrons from the magnetic subsystem, and thus strongly affects magnetic properties of the system. This effect is especially prominent for noninteger electron number, when it results in suppression of the famous double exchange, the main mechanism of ferromagnetism in transition metal compounds. We study this mechanism analytically and numerically and show that it explains magnetic properties of not only several 4d–5d materials, including Nb2O2F3 and Ba5AlIr2O11, but can also be operative in 3d transition metal oxides, e.g., in CrO2 under pressure. We also discuss the role of spin–orbit coupling on the competition between covalency and magnetism. Our results demonstrate that strong intersite coupling may invalidate the standard single-site starting point for considering magnetism, and can lead to a qualitatively new behavior.