Domain Walls in Ferromagnetic Shape Memory Alloys

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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.