氧化物薄膜中的多维应变效应研究

Strain has been approved to be critical to determine physical properties of oxide thin films. There have been series of therotical and experimental works to invesigate the strain effects. Most of them fouced on the strain between thin film and substrate or the strain between different layers on a multilayer structure (called as lateral strain). While such lateral strain control experiments are elegant, the thickness over which substantial strains can be maintained is seriously limited, meaning that use of lateral strain control for any potential applications is restricted. Our recent work presented that self-assembled vertical nanocomposite systems have enormous potential to control strain in much thicker film by creating strain perpendicular to the substrate surface (called as vertical strain). On the other hand, only few papers concerned about the ralatoinsip between the strain and the microstrucre of thin film. It is technologically important and basically interested to control the microstructure of oxide thin film. Strain has been proved to be one of the critical issue. The technique to generate strain in a thicker film and the mechnism to manipulate the architecture in oxide thin films are challenges that motivate this proposal. .The targeted material, EuTiO3 (ETO), are antiferromagnetic (AFM) and paraelectric (PE) in bulk and were predicted to be ferromagnetic (FM) and ferroelectric (FE) in case of strain. We plan to fabricate single phase ETO films on different substrates and ETO based vertical composite thin films to generate lateral and vertical strains, respectively. The lateral and vertical strain effects on a) the physical properties of ETO (especially the FE-FM properties) and b) the microstructures (especially the ordered architectures) are of main interests here. We expect that our work will lead to a deeper understanding of the mechanism that how the strain generates and how it affects the physical properties and the microstructures. Furthermore, the difference between the lateral and vertical strain effects and the spin-optical phonon-strain coupling on the ETO remain active areas for research. The main goal is to enhance functionality and practicability of ETO by manipulating the strain as well as the microstructure.

应变是决定氧化物薄膜丰富物理性质的关键因素，现在已有一系列工作研究应变效应，但这些工作都有一定的局限性。其局限性在于：a)由于应变产生机制的限制，应变效应只能在很薄的薄膜中观测到；b)研究更多的集中于应变对物理性质的影响，对于应变与微结构之间的关系理解不够。本项目将以EuTiO3(ETO)为对象，从多角度研究氧化物薄膜中的应变效应。我们计划通过制备基于ETO的复合薄膜，产生直接作用于垂直方向的应变(垂直应变)，从而改变应变产生的机制。并结合单相ETO薄膜的外延生长，用水平和垂直应变同时调控薄膜的物理性质和微结构,并研究应变-微结构-物理性质之间的关联性。从而结合应变作用方式的三维化和作用对象的多元化，加深对应变产生机制和作用机理这些基本问题的理解。并通过水平和垂直应变效应的比较，深入理解ETO中自旋-声子-应变之间的耦合机制。在此基础上，通过调控应变，增强ETO的功能性和实用性。

应变是决定氧化物薄膜丰富物理性质的关键因素，现在已有一系列工作研究应变效应，但这些工作都有一定的局限性。其局限性在于：a)由于应变产生机制的限制，应变效应只能在很薄的薄膜中观测到；b)研究更多的集中于应变对物理性质的影响，对于应变与微结构之间的关系理解不够。本项目以EuTiO3(ETO)、(Eu,Ba)TiO3 (EBTO)、SrMnO3 (SMO)为对象，从多角度研究氧化物薄膜中的应变效应。主要进展如下。.1. 利用脉冲激光沉积法，在不同基底上制备了EuTiO3:MgO (ETO:MgO)复合薄膜；利用XRD、TEM、STEM等测试手段发现，复合薄膜有清晰的垂直有序结构，其中MgO体现为纳米柱，ETO体现为基体(matrix)；通过这种垂直有序结构可以给ETO施加垂直方向约2.35%的应变。在应变的作用下，ETO:MgO复合薄膜体现出明显的铁磁-铁电性。同时通过测量ETO:MgO复合薄膜中的磁介电效应，发现介电常数在铁磁距离温度（3.5k）附近有明显的反常行为，而这种反常行为随磁场强度的增加而减弱，说明磁电之间存在耦合作用。.2. 研究了EBTO薄膜中的氧空位效应。通过精确调控氧空位含量，使得EBTO薄膜中的磁有序从反铁磁有序变成铁磁有序，同时其铁电居里温度从213 K提高到约450 K。利用SHG的手段，发现薄膜的铁电居里温度随氧空位含量的增加而提高。.3. 制备了Eu0.5Ba0.5TiO3:MgO (EBTO:MgO)复合薄膜。利用XRD、TEM、STEM等测试手段发现，复合薄膜有清晰的垂直有序结构，其中MgO体现为纳米柱，EBTO体现为基体(matrix)；通过这种垂直有序结构可以给EBTO施加垂直方向约2.62%的应变。通过磁性测量发现，复合薄膜中的EBTO由反铁磁序变成了铁磁序，其铁磁居里温度约为2.5K。通过电性质测量发现，复合薄膜中EBTO的铁电居里温度提高到了室温以上，并用PFM在室温观察到了铁电畴的反转，证实了室温的铁电性。. 项目完成后共发表论文25篇。这些文章中包括Advanced Functional Materials 1篇，Physical Review B 1篇，Applied Physics Letters 5篇，在国际会议上作邀请报告3次。项目共培养博士研究生3名、硕士研究生4名，均已毕业。