Femtosecond X-ray magnetic circular dichroism absorption spectroscopy at an X-ray free electron laser

TL;DR

This paper demonstrates femtosecond X-ray magnetic circular dichroism spectroscopy at an X-ray free electron laser, revealing ultrafast magnetic switching dynamics and transient states in GdFeCo thin films.

Contribution

It introduces a new ultrafast spectroscopy technique using an X-ray free electron laser to study magnetic dynamics at femtosecond timescales.

Findings

Confirmed the existence of a transient ferromagnetic-like state during switching.

Observed longer timescales of magnetic dynamics above the compensation temperature.

Identified secondary demagnetization processes with ~5 ps timescales.

Abstract

X-ray magnetic circular dichroism spectroscopy using an X-ray free electron laser is demonstrated with spectra over the Fe L$_{3,2}$-edges. This new ultrafast time-resolved capability is then applied to a fluence-dependent study of all-optical magnetic switching dynamics of Fe and Gd magnetic sublattices in a GdFeCo thin film above its magnetization compensation temperature. At the magnetic switching fuence, we corroborate the existence of a transient ferromagnetic-like state. The timescales of the dynamics, however, are longer than previously observed below the magnetization compensation temperature. Above and below the switching fluence range, we observe secondary demagnetization with about 5 ps timescales. This indicates that the spin thermalization takes longer than 5 ps.