Observing and Understanding Complex Magnetic Spin Textures with Lorentz Microscopy.

다국어 초록 (Multilingual Abstract)

There is a wide interest in the fundamental physical nature of topological spin textures, such as magnetic skyrmions, due to their many unique properties that result from topological protection. Individual skyrmions have been extensively studied, but the collective behavior of skyrmions in dense lattices is still poorly understood. In the work presented in this thesis, lattices of Neel skyrmions and Bloch-type magnetic bubbles were investigated in the van der Waals ferromagnets Fe3GeTe2 (FGT) and Cr2Ge2Te6 (CGT). The response of spin textures to varied temperature and applied magnetic fields were observed using in situ Lorentz transmission electron microscopy (LTEM) imaging, which yielded insights into the magnetic energy landscape of these materials. Skyrmion lattices in FGT displayed a temperature-dependent hysteresis effect in the lattice ordering, which is understood through a quantitative, statistical analysis of skyrmion sizes and through the application of an analytic domain wall model. CGT was shown to support both skyrmion-like homochiral lattices of magnetic bubbles as well as mixed-chirality bubble lattices, and magnetoelastic coupling between strain and magnetic domains was observed. Computational tools were also developed to analyze LTEM image data and extract quantitative information. These include a new method for calculating the electron phase shift imparted by magnetic samples, which enables a more accurate simulation of LTEM images for three-dimensional magnetic samples. Machine learning was also utilized, both to extract quantitative information from LTEM images using neural networks, and to solve inverse problems by applying automatic differentiation to physics-based forward models. This enables the reconstruction and isolation of the magnetic phase shift from a single defocused LTEM image and reconstruction of the sample magnetization configuration from a tilt-tableau of LTEM images. The work presented in this thesis demonstrates that combining in situ LTEM with advanced analysis methods enables quantitative studies that provide new insights into the physical properties of complex spin textures.