Domain Walls in Ferromagnetic Shape Memory Alloys

技术摘要：像差校正的洛伦兹透射电子显微镜将被用来研究许多多铁性材料的磁畴壁行为，这些多铁性材料表现出多个相变，每个相变都会产生精细的磁畴微结构。这些磁区之间的相互作用(例如，晶体孪晶界与磁区壁之间，或反相界与磁区壁之间)是拟议研究的主要焦点。磁性材料是一类重要的工程应用材料，了解它们在变化的磁场下的行为是很重要的。当外加磁场反转时，材料中的磁化强度将尝试跟随磁场的方向，但这种反转过程强烈地受到其他晶体实体的存在的影响，如界面和线缺陷。位相重构的LTEM是确定局域磁化态和磁化场壁与晶格缺陷之间相互作用的首选技术。选择的合金是成分接近化学计量比Ni2MnGa化合物的铁磁形状记忆合金和Pd含量约为30at%的Fe-Pd合金。这项拟议的研究将包括静态和动态条件下对磁场壁的实验观察，以及基于微磁模型的图像模拟。此外，磁力显微镜(MFM)和电子通道对比成像(ECCI，在扫描电子显微镜中)将被用来成像一些不同长度尺度的微结构特征以及表面穿透缺陷。经像差校正的LTEM提供了更高的空间分辨率，将使这种静态和动态磁化壁过程的定量研究成为可能，并将使人们有可能通过进行不同观察方向的观察来获得关于材料磁化状态的三维信息。非技术摘要：拟议的研究计划将研究当施加缓慢变化的磁场时，材料的磁化状态如何变化。一种特殊的实验技术，利用透射电子显微镜，将被用来将磁化图案可视化，要么是静态图案，要么是加场时动态变化的图案。磁性材料构成了先进工程应用的一类重要材料；例如，在现代汽车中，有数百个磁铁用于各种部件(电动马达、门锁和各种传感器)。在能源效率方面，了解这些材料在变化的磁场中使用时的行为至关重要。所提出的研究还可能在新型磁记录介质领域产生影响。在这项研究中使用的实验方案以及直接观测将向更广泛的科学界提供。拟议计划的教育部分可能会影响大匹兹堡地区几所学校的初中和高中科学教育。该计划将向当地初中和高中的学生提供几台扫描电子显微镜，并将教育一批科学教师，使他们能够在课堂上使用这些显微镜。此外，拟议的计划将为一些本科生提供一个机会，通过与卡内基·梅隆大学旁边的卡内基自然历史博物馆的希尔曼矿物和宝石大厅合作，在矿物鉴定领域开展研究。

TECHNICAL SUMMARY:Aberration-corrected Lorentz Transmission Electron Microscopy (LTEM) will be used to study magnetic domain wall behavior in a number of multi-ferroic materials which exhibit multiple phase transitions, each giving rise to a fine-scale domain microstructure. The interactions between these domains (for instance, between crystallographic twin boundaries and magnetic domain walls, or between anti-phase boundaries and magnetic domain walls) is the main focus of the proposed research. Magnetic materials form an important class of materials for a wide variety of engineering applications and it is important to understand how they behave under a changing magnetic field. When an applied field is reversed, the magnetization in the material will attempt to follow the direction of the field, but this reversal process is strongly influenced by the presence of other crystallographic entities, such as interfaces and line defects. Phase reconstructed LTEM is the technique of choice to determine the local magnetization state and the potential interactions between domain walls and lattice defects. The alloys of choice are the ferromagnetic shape memory alloys with compositions near the stoichiometric Ni2MnGa compound and Fe-Pd alloys with around 30 at% Pd. The proposed research will consist of experimental observations of domain walls in static and dynamic conditions, supported by image simulations based on micro-magnetic models. In addition, Magnetic Force Microscopy (MFM) and Electron Channeling Contrast Imaging (ECCI, in a scanning electron microscope) will be employed to image micro-structural features as well as surface-penetrating defects at a number of different length scales. The improved spatial resolution offered by aberration-corrected LTEM will enable this quantitative study of static and dynamic domain wall processes, and will make it possible to obtain 3-D information about the magnetization state of the material by carrying out observations with different viewing directions.NON-TECHNICAL SUMMARY:The proposed research program will study how the magnetization state of a material changes when a slowly changing magnetic field is applied. A specialized experimental technique, making use of a transmission electron microscope, will be used to visualize the magnetization pattern, either as a static pattern, or as a dynamic changing pattern when a field is applied. Magnetic materials form an important class of materials for advanced engineering applications; for instance, in a modern car, there are several hundred magnets for a variety of components (electric motors, door locks, and various sensors). Understanding the behavior of these materials while they are used in a changing magnetic field is crucial in the context of energy efficiency. The proposed research may also have an impact in the area of novel magnetic recording media. The experimental protocols used during this research as well as direct observations will be made available to the broader scientific community. The educational component of the proposed program has the potential to impact middle and high school science education in several schools in the greater Pittsburgh area. The program will make several scanning electron microscopes available to students in local middle and high schools, and will educate a number of science teachers so that they can use these microscopes in their class rooms. In addition, the proposed program will provide an opportunity to a number of undergraduate material students to carry out research in the area of mineral identification through collaboration with the Hillman Hall of Minerals and Gems in the Carnegie Museum of Natural History located right next to Carnegie Mellon University.