Time Resolved Imaging of Multifunctional Materials in Three Dimensions (TRIMM3D)

多功能铁性材料是指同时表现出铁磁性、铁电性、铁弹性或类铁性等多种铁性的材料。如果我们能更好地理解这一点，表现出一种以上性质的铁性材料是非常有兴趣的，因为不同的性质可能以不同的方式协同工作，并导致令人兴奋的新的潜在应用。例如，磁和铁电有序之间的耦合可用于开发低功率磁电子器件(诸如非易失性磁计算机存储器)，其中电子的自旋极化传输可用于翻转磁存储器位。因此，在理解块体和薄膜材料的潜在作用机制方面，人们做出了积极的努力。我感兴趣的许多多铁性材料都有一种特定的结构，称为钙钛矿晶体结构。在多铁钙钛矿材料能够在器件环境中找到重要用途之前，需要对材料在纳米尺度上的行为有一个清楚的了解。通常，晶体缺陷和其他拓扑结构的作用仍然不清楚，因为(到目前为止)还没有可靠的手段来成像三维图像并实时观察这些影响。此外，如果晶体生长不仔细，它很容易生长在不同的结构中，不显示我们想要研究的性质。为了更好地理解这些材料，我将使用一种名为布拉格相干X射线衍射成像(BCXDI)的技术，而不使用透镜。这是一种X射线显微镜，可以在使用传统光学技术不可行的情况下进行高分辨率成像。BCXDI具有在表面和整体上直接成像材料三维时变结构特性的能力，这可以极大地增加我们对新相如何出现并影响材料性能的理解。BCXDI在多功能材料研究中的应用将使由于对其性质的不完全了解而无法获得的广泛的下一代技术成为可能。一个突出的原型系统是铋铁氧体，它在光激励下表现出大的可逆晶体变形(高达0.5%)。目前尚不清楚铋铁氧体中光学产生的电子-空穴对如何导致巨大的晶格变形，这种变形的传播速度似乎快于材料中的声速。此外，电子-空穴对的确切作用和失真的传播矢量仍不清楚。利用时间分辨的BCXDI将能够在三维中直接可视化晶格扭曲，由此可以推断原子位移，并与模型预测进行对比。本项目将致力于通过使用BCXDI的一种新的确定性形式来获得三维图像来成像广泛的有序多功能矩阵中的时变结构现象。将决定论BCXDI应用于动态结构现象的研究，将提供一种新的、稳健的手段来直接成像经历对称破缺结构相变的材料的三维非平衡动力学。原子平衡位移的知识是以亚埃灵敏度获得的，这将极大地帮助我们理解动力学现象的动力学，这对下一代材料和器件的发展至关重要。这项研究计划将与牛津郡钻石光源的Steve Collins教授和汉堡的欧洲X射线自由电子激光(E-XFEL)设施的Hans Fangohr教授合作进行。

Multifunctional ferroic materials are materials that simultaneously exhibit more than one ferroic property including ferromagnetism, ferroelectricity, ferroelasticity or ferrotoroidicity. Ferroic materials that exhibit more than one property are of great interest because the different properties may work together in different ways and lead to exciting new potential applications, if we could understand this better. For example, the coupling between magnetic and ferroelectric ordering can be utilised to develop low power magnetoelectronic devices (such as non-volatile magnetic computer memory) where the spin polarised transport of electrons can be used to flip magnetic memory bits. As a result there is a vibrant effort to understand the underlying mechanisms at work in bulk and thin film materials. Many of the multiferroic materials that I am interested in have a certain structure called a perovskite crystal structure. Before multiferroic perovskite materials can find significant utility in a device setting, a clear understanding of the materials behaviour at the nanoscale is needed. Often the role of crystal defects and other topological structures remains unclear as (to date) no reliable means exists to image in three-dimensions and observe such effects in real-time. Moreover, if the crystal isn't grown carefully, it can easily grow in a different structure that doesn't display the properties that we want to investigate.To better understand these materials I will use a technique called Bragg coherent X-ray diffractive imaging (BCXDI) without lenses. This is a form of x-ray microscopy that can permit high resolution imaging where the use of conventional optics is not feasible. The ability that BCXDI has to directly image time varying structural properties of materials in three-dimensions at the surface and in the bulk can greatly increase our understanding of how novel phases emerge and influence the material properties. The application of BCXDI to the study of multifunctional materials will enable a wide range of next generation technologies that otherwise are inaccessible due to an incomplete understanding of their properties. A prominent example prototypical system is bismuth ferrite which exhibits a large reversible crystal deformation (up to 0.5%) in response to optical excitation. It remains unclear exactly how optically generated electron-hole pairs in bismuth ferrite can cause a large lattice deformation which appears to propagate faster than the speed of sound in the material. Moreover the exact role of electron-hole pairs and the propagation vector of the distortion remains unclear. Utilising time-resolved BCXDI will enable direct visualisation of the lattice distortion in three-dimensions from which the atomic displacements can be inferred and contrasted with model predictions. This project will focus on imaging time-varying structural phenomena in a wide range of ordered multifunctional matrials by employing a novel deterministic form of BCXDI to obtain three-dimensional images. The application of deterministic BCXDI to the study of dynamic structural phenomena will provide a novel and robust means to directly image in three-dimensions non-equilibrium dynamics of the material undergoing a symmetry breaking structural transformation. Knowledge of the atomic displacements from equilibrium is obtained with sub-angstrom sensitivity and will greatly aid our understanding of the kinetics of dynamic phenomena that are central to the development of next generation materials and devices. This research proposal will be carried out in collaboration with Prof. Steve Collins, Diamond Light Source, Oxfordshire and Prof. Hans Fangohr, European Xray Free Electron Laser (E-XFEL) Facility in Hamburg.