Quantitative Characterization of 3D Vector Fields in Advanced Materials

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

Non-Technical Description Magnets 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 Description The 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.