Department of Physics and Astronomy, Vanderbilt University , Nashville, Tennessee 37235, United States.
Nano Lett. 2014 May 14;14(5):2694-701. doi: 10.1021/nl500601d. Epub 2014 Apr 18.
Complex oxides displaying ferroelectric and/or multiferroic behavior are of high fundamental and applied interest. In this work, we show that it is possible to achieve polar order in a superlattice made up of two nonpolar oxides by means of oxygen vacancy ordering. Using scanning transmission electron microscopy imaging, we show the polar displacement of magnetic Fe ions in a superlattice of (LaFeO3)2/(SrFeO3) grown on a SrTiO3 substrate. Using density functional theory calculations, we systematically study the effect of epitaxial strain, octahedral rotations, and surface terminations in the superlattice and find them to have a negligible effect on the antipolar displacements of the Fe ions lying in between SrO and LaO layers of the superlattice (i.e., within La0.5Sr0.5FeO3 unit cells). The introduction of oxygen vacancies, on the other hand, triggers a polar displacement of the Fe ions. We confirm this important result using electron energy loss spectroscopy, which shows partial oxygen vacancy ordering in the region where polar displacements are observed and an absence of vacancy ordering outside of that area.
具有铁电和/或多铁性的复杂氧化物具有很高的基础和应用价值。在这项工作中，我们通过氧空位有序化证明了由两种非极性氧化物组成的超晶格中可以实现极性有序。通过扫描透射电子显微镜成像，我们展示了在 SrTiO3 衬底上生长的（LaFeO3）2/（SrFeO3）超晶格中，磁性 Fe 离子的极位移。通过密度泛函理论计算，我们系统地研究了超晶格中外延应变、八面体旋转和表面终止的影响，发现它们对位于 SrO 和 LaO 层之间的 Fe 离子（即 La0.5Sr0.5FeO3 单元胞内）的反极位移影响可以忽略不计。另一方面，氧空位的引入会引发 Fe 离子的极位移。我们使用电子能量损失光谱证实了这一重要结果，该光谱显示在观察到极位移的区域存在部分氧空位有序，而在该区域之外不存在空位有序。
