Relationship between magnetic asymmetry and magnetization in ultrafast transverse magneto-optical Kerr effect spectroscopy in the extreme ultraviolet spectral range

Ultrafast transverse magneto-optical Kerr effect (T-MOKE) spectroscopy in the extreme ultraviolet spectral range provides element-specific information about the magnetization dynamics of complex magnetic structures. However, the relationship between the T-MOKE observable, denoted magnetic asymmetry, and the magnetization of a sample can exhibit significant nonlinearities, even in the case of magnetization changes that are homogeneous along the depth of the sample and without considering any nonequilibrium spin dynamics. Here, we combine static and time-resolved experimental data with simulations based on a wave propagation algorithm for a prototypical magnetic heterostructure that exhibits pronounced deviations from a linear relationship between magnetic asymmetry and magnetization, including increasing values of asymmetry in spite of a reducing magnetization. As an outlook, we describe sample structures and experimental geometries for which a linear response of the T-MOKE observable remains a valid approximation