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

TL;DR

This paper demonstrates that quantum mechanical selection rules critically affect the interpretation of ultrafast magnetization dynamics in pump-probe experiments, emphasizing the importance of considering matrix elements for accurate analysis.

Contribution

It introduces a fully ab initio approach to account for selection rules in ultrafast magnetization studies, revealing their significant impact on transient dynamics interpretation.

Findings

Selection rules influence the energy-dependent transient dynamics.

Proper consideration of matrix elements clarifies experimental discrepancies.

Magnetization dynamics are affected by optical excitations of the M shell.

Abstract

Ultrafast magnetization dynamics driven by ultrashort pump lasers is typically explained by changes in electronic populations and scattering pathways of excited conduction electrons. This conventional approach overlooks the fundamental role of quantum mechanical selection rules, governing transitions from core states to the conduction band, that forms the key method of the probing step in these experiments. By employing fully ab initio 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, arise from a complex interplay of optical excitations of the M shell. Taking into account the selection rules and conduction electron spin flips, this leads to highly energy-dependent dynamics. These findings address longstanding discrepancies in experimental TMOKE measurements and show that only through meticulous consideration of matrix elements at the probe stage, can one ensure that magnetization dynamics is revealed in its true nature, instead of being muddled by artifacts arising from the choice of probe energy.