Understanding magnetic order at the nanoscale, both in its static configuration and dynamic evolution, remains a central challenge in condensed matter physics, particularly for compensated magnetic systems such as antiferromagnets and altermagnets, for which conventional magnetometry and magnetic imaging techniques provide limited sensitivity or spatial resolution.

Among transmission electron microscopy (TEM) techniques, magnetic dichroism in electron energy-loss spectroscopy (EELS) provides access to magnetic order through symmetry-dependent inelastic scattering processes. In this thesis, a unified theoretical and simulation framework for Electron Magnetic Circular and Linear Dichroism (EMCD and EMLD) is developed, enabling vectorial determination of magnetic ordering from EELS spectra. Particular emphasis is placed on the detailed formulation and investigation of the vectorial EMCD/EMLD response, including the influence of experimental geometry, beam conditions, and detector configurations on quantitative magnetic reconstruction. The approach is demonstrated on metamagnetic FeRh, providing access to both antiferromagnetic and ferromagnetic phases within the same crystallographic structure.

Experimentally, preparation procedures for freestanding micrometer-scale ferromagnetic and antiferromagnetic thin films are established and optimized for dichroic TEM investigations. Quantitative EMCD imaging of FeRh with sub-10-nm spatial resolution is achieved across the phase transition, while experimentally viable EMLD configurations are identified and validated across multiple material systems. These results define realistic operational limits for EMLD experiments in TEM. In addition to static magnetic characterization, laser-driven phase transition in FeRh is directly investigated in TEM, revealing the interplay between structural and magnetic order down to the nanometer scale, including defect-mediated phase nucleation pathways.

Overall, this work establishes a unified vectorial electron magnetic dichroism framework combining EMCD and EMLD as a predictive and extensible platform for nanoscale studies of complex magnetic materials in TEM.