Influence of laser-excited electron distributions on the x-ray magnetic circular dichroism spectra: Implications for femtosecond demagnetization in Ni

TL;DR

This study theoretically investigates how non-equilibrium electron distributions caused by laser pulses affect the XMCD spectra of Ni, revealing that many ultrafast spin transfer signatures may be due to electron dynamics rather than actual spin transfer.

Contribution

It is the first to analyze the impact of laser-excited non-equilibrium electron distributions on XMCD spectra and their implications for interpreting femtosecond demagnetization in Ni.

Findings

Non-equilibrium electron distributions significantly alter XMCD signals.

Most features attributed to ultrafast spin transfer can be explained by electron distributions.

XMCD sum rules remain valid despite non-equilibrium conditions.

Abstract

In pump-probe experiments an intensive laser pulse creates non-equilibrium excited electron distributions in the first few hundred femtoseconds after the pulse. The influence of non-equilibrium electron distributions caused by a pump laser on the apparent X-ray magnetic circular dichroism (XMCD) signal of Ni is investigated theoretically here for the first time, considering electron distributions immediately after the pulse as well as thermalized ones, that are not in equilibrium with the lattice or spin systems. The XMCD signal is shown not to be simply proportional to the spin momentum in these situations. The computed spectra are compared to recent pump-probe XMCD experiments on Ni. We find that the majority of experimentally observed features considered to be a proof of ultrafast spin momentum transfer to the lattice can alternatively be attributed to non-equilibrium electron distributions. Furthermore, we find the XMCD sum rules for the atomic spin and orbital magnetic moment to remain valid, even for the laser induced non-equilibrium electron distributions.