Quantitative Characterization of Magnetic Domain Structure in Near Eutectoid Co40Pt60 Alloys

Characterization of magnetic domain structure is essential to understand and manipulate the magnetic properties of materials. In this thesis, we have utilized Lorentz Transmission Electron Microscopy (LTEM) in combination with image simulations based on micromagnetic models, to investigate the magnetic domain structure of a unique nano-chessboard structure consisting of L10 and L12 phases in a Co40Pt60 alloy. We have shown high-resolution LTEM images of nano-size magnetic features acquired through spherical aberration correction in Lorentz Fresnel mode. Phase reconstructions based on the transport of intensity equation has been carried out to fully understand the magnetic domain structure and to extract quantitative information, including direction of magnetic induction and magnetic domain wall width, from the Lorentz TEM images. The experimental Fresnel images of the nano-chessboard structure show zig-zag shaped magnetic domain walls at the inter-phase boundaries between L10 and L12 phases. A circular magnetization distribution with vortex and anti-vortex type arrangement is evident in the phase reconstructed magnetic induction maps as well as simulated maps. The magnetic contrast in experimental LTEM images has been properly interpreted with the help of magnetic induction maps simulated for various relative electron beam-sample orientations inside TEM. Apart from the nano-chessboard structure, this alloy shows other interesting microstructural features such as anti-phase boundaries, tweed structure, coarse L10 plates, and macro-twins all of which have been characterized using conventional bright field/dark field TEM imaging and compared with their respective Lorentz TEM images. The magnetic domain wall widths obtained for each microstructure has been compared and the influence of microstructure and the particle size on wall widths has been discussed.