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

多功能铁性材料是同时表现出一种以上铁性质的材料，包括铁磁性、铁电性、铁弹性或铁环性。如果我们能更好地理解这一点，那么表现出多种特性的铁材料就会引起人们的极大兴趣，因为不同的特性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，磁性和铁电有序之间的耦合可用于开发低功率磁电子器件（如非易失性磁性计算机存储器），其中电子的自旋极化输运可用于翻转磁性存储器位。因此，有一个充满活力的努力，以了解在散装和薄膜材料工作的潜在机制。我感兴趣的许多多铁性材料都有一种叫做钙钛矿晶体结构的结构。在多铁钙钛矿材料在器件设置中发挥重要作用之前，需要清楚地了解材料在纳米尺度上的行为。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）没有可靠的方法来三维成像和实时观察这些影响。此外，如果晶体生长不小心，它很容易以不同的结构生长，而不显示我们想要研究的性质。为了更好地理解这些材料，我将使用一种叫做Bragg相干x射线衍射成像（BCXDI）的技术。这是x射线显微镜的一种形式，可以在使用传统光学技术不可行的地方进行高分辨率成像。BCXDI能够直接对材料表面和体中的三维随时间变化的结构特性进行成像，这可以大大增加我们对新相如何出现和影响材料特性的理解。将BCXDI应用于多功能材料的研究将使下一代技术成为可能，否则由于对其性质的不完全了解而无法获得这些技术。一个突出的例子原型系统是铋铁氧体，它在光激发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-空穴对是如何引起大的晶格变形的，这种变形似乎比材料中的声速传播得更快。此外，电子-空穴对的确切作用和畸变的传播矢量仍不清楚。利用时间分辨的BCXDI将能够在三维空间中直接可视化晶格畸变，从中可以推断原子位移并与模型预测进行对比。该项目将通过采用一种新的确定性形式的BCXDI来获得三维图像，重点研究在广泛的有序多功能材料中成像时变结构现象。将确定性BCXDI应用于动态结构现象的研究，将为材料发生对称破缺结构转变的三维非平衡动力学直接成像提供一种新颖而稳健的手段。原子从平衡位移的知识是通过亚埃灵敏度获得的，这将极大地帮助我们理解对下一代材料和器件发展至关重要的动态现象的动力学。该研究计划将与牛津郡钻石光源的Steve Collins教授和汉堡欧洲x射线自由电子激光器（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.