多铁性薄膜中的氧空位测量及其作用机制研究

In oxide thin films, oxygen vacancies have been approved to be intrinsic defects and are believed to have a critical impact on their functionalities. There have been series of theoretical and experimental works to investigate the oxygen vacancy effects on physical properties. While, there is no clear understanding of the physical processes underlying these effects. This is one of the main drawbacks which limited the application of oxide thin films. On the other hand, X-ray Absorption Fine Structure Spectrometry is well known to be useful to detect the oxygen vacancies in the condensed matter materials. The technique to manipulate the oxygen vacancies and the mechanisms underlying their effects on electric and magnetic properties are challenges that motivate this proposal.. Multiferroic Eu0.5Ba0.5TiO3 (EBTO) has been choosen as the targeted material. The stoichiometric bulk EBTO is antiferromagnetic (AFM) and ferroelectric (FE). We plan to fabricate EBTO thin films by pulsed laser deposition and manipulate the oxygen vacancies during the deposition as well as the post-annealing process. We will investigate the principles of generation of oxygen vacancies using combination of Nuclear Resonance Backscattering Spectrometry and X-ray Absorption Fine Structure Spectrometry. The oxygen vacancies effects on electric and magnetic properties as well as lattice and electronic structure are of main interests here. And how to manipulate electronic and magnetic ordering simultaneously is the central challenge in the present work. We expect that our work will lead to a deeper understanding of the mechanisms that how the oxygen vacancies generates and how their affects the physical properties in oxide thin fims. The main goal is to enhance functionality and practicability of EBTO by manipulating the oxygen vacancies.

氧空位是氧化物薄膜中的固有缺陷，也是调控其功能性的有效手段。虽然现在已有一系列工作研究氧空位对氧化物薄膜物理性质的作用，但对很多基本机制还没有清晰的理解，从而限制了氧化物薄膜的广泛应用。另一方面，X射线吸收精细结构谱已被证明是一种强有力的结构测量手段，且可以有效探测凝聚态物质中包括氧空位在内的各种缺陷。本项目以多铁性Eu0.5Ba0.5TiO3 (EBTO)薄膜为对象，通过核共振背散射谱和X射线吸收精细结构谱的测量，分析氧空位作用于晶格结构和电子结构的机理；通过电、磁性质测量，研究氧空位对多铁性质的作用机制；进而研究如何用氧空位这单一参数，同时调控电、磁两个序参量，并增强磁电耦合作用。同时我们也将研究氧空位的循环特性，通过循环与物理性质的结合，深入理解薄膜中的氧空位效应；并通过氧空位的作用，增强EBTO的功能性。

氧空位是氧化物薄膜中的固有缺陷，也是调控其功能性的有效手段。虽然现在已有一系列工作研究氧空位对氧化物薄膜物理性质的作用，但对很多基本机制还没有清晰的理解，从而限制了氧化物薄膜的广泛应用。本项目以多铁性Eu0.5Ba0.5TiO3 (EBTO)薄膜为对象，分析氧空位作用于晶格结构和电子结构的机理；通过电、磁性质测量，研究氧空位对多铁性质的作用机制。主要进展如下。.1. EBTO薄膜的制备及氧空位调控。用脉冲激光沉积的方法制备了EBTO薄膜，并通过改变沉积氧压，来调控薄膜中氧空位的含量。用卢瑟福背散射(Rutherford backscattering spectrometry, RBS)进行阳离子(Eu, Ba和Ti)之间摩尔比的测试，同时用核共振背散射谱(Nuclear resonance backscattering spectrometry, NRBS)的方法定量测量EBTO薄膜中氧含量。用XPS进行了阳离子价态的测量，发现Ba和Ti的价态基本保持不变，Eu离子的价态随氧空位发生相应变化。.2. EBTO薄膜中氧空位对铁电性质和磁电耦合的作用机制研究。用变温SHG方法测试了EBTO薄膜的铁电居里温度，发现可以通过增加氧空位，使得薄膜的铁电居里温度从250K提高到约400K。通过压电力显微镜(PFM)在室温观测到EBTO薄膜明显的铁电性，证明EBTO成为一种可以在室温应用的铁电性材料。用高分辨STEM的手段发现，氧空位的出现可以使得Ti离子发生明显位移，从而提高材料的铁电居里温度，并用第一性原理方法证明了STEM的结果。.3. EBTO薄膜中氧空位对介电性质的调控。系统研究了氧空位含量对介质材料介电驰豫性质的影响，发现氧空位的存在引起Ti离子位移增强；在电场的作用下，氧空位导致的极化子与Ti离子位移耦合，从而形成EBTO薄膜中的介电驰豫。介电驰豫的激活能随氧空位含量增加而提高。.项目共发表论文23篇，包括PRB 1篇，APL 5篇，ACS Appl. Mater. & Inter. 2篇，应邀综述文章一篇。共培养博士生2名、硕士生2名，其中2名硕士生已经毕业。