How quantum selection rules influence the magneto-optical effects of driven ultrafast magnetization dynamics

Abstract

Ultrafast magnetization dynamics driven by ultrashort pump lasers is typically explained by changes in the electronic populations and scattering pathways of excited conduction electrons. This conventional approach overlooks the fundamental role of quantum mechanical selection rules, governing transitions from the core states to the conduction band, that form the key method of the probing step in these experiments. By employing fully time-dependent density functional theory, we reveal that these selection rules profoundly influence the interpretation of ultrafast spin dynamics at specific probe energies. Our analysis for hcp Co and fcc Ni at the M edge demonstrates that the transient dynamics, as revealed in pump-probe experiments, arises from a complex interplay of optical excitations of the M shell. Taking into account the selection rules and conduction electron spin flips leads to highly energy-dependent dynamics. These findings address long-standing discrepancies in experimental transverse magneto-optical Kerr effect measurements and show that only through meticulous consideration of the matrix elements at the probe stage, can one ensure that the magnetization dynamics is revealed in its true nature, instead of being muddled by artifacts arising from the choice of probe energy. Published by the American Physical Society 2025