Time-Resolved XUV absorption spectroscopy and magnetic circular dichroism at the Ni $M_{2,3}$-edges

TL;DR

This study introduces a time-resolved XUV absorption and magnetic circular dichroism spectroscopy technique at Ni M-edges, revealing wavelength-dependent ultrafast charge and spin dynamics in magnetic thin films.

Contribution

It develops a novel jitter-free XUV streaking method for simultaneous charge and magnetic measurements, applied here to Ni thin films with detailed transient absorption and dichroism data.

Findings

Charge and magnetic dynamics depend on probing wavelength.

Ultrafast energy shifts of absorption resonances are observed.

Complex electronic and spin interactions require further study.

Abstract

Ultrashort optical pulses can trigger a variety of non-equilibrium processes in magnetic thin films affecting electrons and spins on femtosecond timescales. In order to probe the charge and magnetic degrees of freedom simultaneously, we developed an x-ray streaking technique that has the advantage of providing a jitter-free picture of absorption cross section changes. In this paper, we present an experiment based on this approach which we performed using five photon probing energies at the Ni $M_{2,3}$-edges. This allowed us to retrieve the absorption and magnetic circular dichroism time traces, yielding detailed information on transient modifications of electron and spin populations close to the Fermi level. Our findings suggest that the observed charge and magnetic dynamics both depend on the XUV probing wavelength, and can be described, at least qualitatively, by assuming ultrafast energy shifts of the electronic and magnetic elemental absorption resonances, as reported in recent work. However, our analysis also hints at more complex changes, highlighting the need for further experimental and theoretical analysis in order to gain a thorough understanding of the interplay of electronic and spin degrees of freedom in optically excited magnetic thin films.