Manifestation of intra-atomic 5d6s-4f exchange coupling in photoexcited gadolinium

TL;DR

This study provides direct real-time evidence of intra-atomic exchange coupling between $5d6s$ and $4f$ electrons in gadolinium, revealing how femtosecond laser excitation influences their energy states and spin dynamics.

Contribution

It demonstrates the first direct observation of intra-atomic exchange coupling in gadolinium using femtosecond laser excitation and first-principles calculations, linking electron excitation to spin state shifts.

Findings

$5d6s$ electrons shift toward Fermi level upon excitation

$4f$ states are unaffected despite $5d6s$ excitation

Proposes demagnetization time relates inversely to density of states at Fermi level

Abstract

Intra-atomic exchange couplings (IEC) between $5d6s$ and $4f$ electrons are ubiquitous in rare-earth metals and play a critical role in spin dynamics. However, detecting them in real time domain has been difficult. Here we show the direct evidence of IEC between $5d6s$ and $4f$ electrons in gadolinium. Upon femtosecond laser excitation, $5d6s$ electrons are directly excited, their majority bands shift toward the Fermi level while their minority bands do the opposite. For the first time, our first-principles minority shift now agrees with the experiment quantitatively. Excited $5d6s$ electrons lower the exchange potential barrier for $4f$ electrons, so the $4f$ states are also shifted in energy, a prediction that can be tested experimentally. Although a significant number of $5d6s$ electrons, some several eV below the Fermi level, are excited out of the Fermi sea, there is no change in the $4f$ states, a clear manifestation of intra-atomic exchange coupling. Based on our results, we propose that the demagnetization time of a material be inversely proportional to the density of states at the Fermi level and the excited, not the whole, spin moment. This can be tested experimentally.