Quantitative Characterization of 3D Vector Fields in Advanced Materials

磁铁和磁性材料在当今的科技社会中扮演着重要的角色。许多商业产品，例如现代汽车，都有数百个永磁体和电磁铁。在微电子领域，磁性元件出现在各种长度尺度上，从电力变压器中厘米大小的磁铁到计算机硬盘驱动器中的微小磁性元件。为了描述这个研究项目的中心目标，想想在高中物理课上磁铁是如何被引入的是很有用的：一块磁铁放在一个水平的表面上，在上面放一张纸。然后，将细铁屑倒在薄片上，它们沿着磁力线排列，从而使磁场可见。在提议的项目中，PI打算进行类似的实验，使磁场线在纳米级磁性物体周围的空间中可见。这需要(1)使用电子显微镜来提供可视化纳米级物体所需的高放大倍率；(2)建立数学模型来解释显微镜下观察到的图像，并将其转化为纳米物体周围磁场的三维表示。这些模型的运作方式类似于医学领域的核磁共振扫描仪；一系列显微镜图像被转换成磁场的三维可视化。反过来，这使我们能够研究这些纳米级磁性物体的功能以及它们如何相互作用。在纳米尺度上量化事物如何工作的能力对我们技术社会的许多方面都至关重要，从长远来看，可能会导致微电子设备和磁记录技术的改进。为了设计和制造下一代纳米级器件，该项目解决了一些需要回答的基本科学问题。该项目将推动纳米磁学领域的发展，并将有助于培养磁学领域的本科生和研究生。第2部分：技术描述提出的研究计划将为现代多相工程材料中的三维矢量场重建创造新方法。将创建能够准确模拟缺陷对比度以及用于层析采集模式的洛伦兹图像的广义前向投影仪。这些投影仪将与基于模型的层析成像重建算法集成，而不是目前使用的传统滤波反投影和同步迭代重建技术方法。我们提出的基于模型的迭代重建（MBIR）方法将能够结合先前的基于物理的模型来提供重建约束和现实的边界条件。这些算法将通过专门的测试样品进行验证，并应用于坡莫合金样品的磁化强度的3D重建，以及Mo线和多相ni基高温合金的缺陷位移场。这项工作的一部分将与俄亥俄州立大学、普渡大学和阿贡国家实验室的同事合作进行。MBIR方法将指导创建一种新颖、高效、模块化、精确、迭代的层析成像重建技术，该技术可用于重建3D矢量场，特别是磁化和缺陷位移场。缺陷对比成像已经在材料界使用了几十年，但是到目前为止，尽管缺陷在工程材料的整体行为中很重要，但还没有任何努力在单个缺陷或缺陷簇的水平上确定位移向量场。所提出的工作将为TEM技术最终常规确定三维矢量场奠定基础。拟议的研究有可能影响定量3D材料表征的广泛领域，并将为3D矢量场重建的有效数据收集提供明确定义的实验方案。所有实验协议和数值算法将以出版物和开源代码的形式提供给更广泛的材料界。拟议项目的教育/推广部分有可能影响匹兹堡附近几所学校的初中和高中科学教育。在当地学校教授高级表征工具的项目为科学教师提供了便携式扫描电子显微镜，使他们能够在课堂上使用这些仪器。此外，PI将与当地卡内基自然历史博物馆的希尔曼矿物和宝石厅合作，为一些本科材料学生提供在结构和化学矿物鉴定领域开展研究的机会。

Non-Technical DescriptionMagnets and magnetic materials play an important role in today's technological society. Many commercial products, e.g., modern cars, have hundreds of both permanent magnets and electromagnets in them. In the micro-electronic world, magnetic components appear at all length scales, from centimeter-sized magnets in power transformers to tiny magnetic components in computer hard drives. To describe the central goal of this research project, it is useful to think about how magnets are often introduced in a high school physics class: a magnet is placed on a level surface, and a sheet of paper is placed on top. Then, fine iron filings are poured on top of the sheet and they arrange themselves along the magnetic field lines, thus making the magnetic field visible. In the proposed project, The PI intends to perform a similar experiment to make magnetic field lines visible in the space surrounding nano-scale magnetic objects. This requires (1) the use of an electron microscope to deliver the high magnification needed to visualize nano-scale objects; and (2) the development of mathematical models to interpret the images observed in the microscope, and turn them into a three-dimensional representation of the magnetic field surrounding the nano-sale objects. These models will operate in a way similar to the use of MRI scanners in the medical world; a series of microscope images is converted into a three-dimensional visualization of the magnetic field. This, in turn, allows us to study how these nano-scale magnetic objects function and how they interact with each other. The ability to quantify how things work at the nano-scale is crucial to many aspects of our technological society and may lead, in the long run, to improved microelectronic devices and magnetic recording techniques. This project addresses some of the fundamental scientific questions that need to be answered in order to design and fabricate the next generation of nano-scale devices. The project will advance the field of nano-magnetics, and will help train both undergraduate and graduate students in the area of magnetism.Part 2: Technical DescriptionThe proposed research program will create novel approaches to the reconstruction of 3D vector fields in modern multi-phase engineering materials. Generalized forward projectors capable of accurately simulating defect contrast as well as Lorentz images for tomographic acquisition modes will be created. These projectors will be integrated with tomographic reconstruction algorithms that are model-based, rather than the conventional filtered back-projection and simultaneous iterative reconstruction technique approaches currently in use. Our proposed model-based iterative reconstruction (MBIR) approach will be capable of incorporating prior physics-based models to provide reconstruction constraints and realistic boundary conditions. The algorithms will be validated using dedicated test samples, and applied to 3D reconstructions of the magnetization in permalloy-based samples, and defect displacement fields in Mo wires and multi-phase Ni-based superalloys. Portions of this work will be carried out in collaboration with colleagues at the Ohio State University, Purdue University, and the Argonne National Laboratory.The MBIR approach will guide the creation of a novel, efficient, modular, accurate, iterative tomographic reconstruction technique, which can be used for the reconstruction of 3D vector fields, in particular magnetization and defect displacement fields. Defect contrast imaging has been used for many decades in the materials community, but thus far, despite the importance of defects in the overall behavior of an engineering material, there have not been any efforts to determine displacement vector fields at the level of individual defects or defect clusters. The proposed work will lay the foundation for the eventual routine determination of 3D vector fields by TEM techniques. The proposed research has the potential to impact the broad area of quantitative 3D materials characterization, and will produce a clearly defined experimental protocol for the efficient collection of data for 3D vector field reconstructions. All experimental protocols and numerical algorithms will be made available to the broader materials community in the form of publications and open source code. The educational/outreach component of the proposed program has the potential to impact middle and high school science education in several schools near Pittsburgh. The TACTILS program (Teaching Advanced Characterization Tools In Local Schools) provides access to portable scanning electron microscopes for science teachers so that they can employ these instruments in their class rooms. In addition, the PI will work with the local Carnegie Museum of Natural History's Hillman Hall of Minerals and Gems to provide a number of undergraduate materials students with opportunities to carry out research in the area of structural and chemical mineral identification.