Ultrafast behavior of induced and intrinsic magnetic moments in CoFeB/Pt bilayers probed by element-specific measurements in the extreme ultraviolet spectral range

TL;DR

This study investigates the ultrafast, element-specific magnetic responses in CoFeB/Pt bilayers using extreme ultraviolet spectroscopy, revealing distinct demagnetization dynamics of intrinsic and induced moments and highlighting the role of spin-orbit coupling.

Contribution

It provides the first detailed element-specific analysis of ultrafast magnetic dynamics in CoFeB/Pt bilayers using high harmonic generation spectroscopy.

Findings

Pt exhibits faster demagnetization than CoFeB.

Induced Pt moments show larger demagnetization amplitudes.

Results support the role of spin-orbit coupling and magnon populations in magnetic dynamics.

Abstract

The ultrafast and element-specific response of magnetic systems containing ferromagnetic 3d transition metals and 4d/5d heavy metals is of interest both from a fundamental as well as an applied research perspective. However, to date no consensus about the main microscopic processes describing the interplay between intrinsic 3d and induced 4d/5d magnetic moments upon femtosecond laser excitation exist. In this work, we study the ultrafast response of CoFeB/Pt bilayers by probing element-specific, core-to-valence band transitions in the extreme ultraviolet spectral range using high harmonic radiation. We show that the combination of magnetic scattering simulations and analysis of the energy- and time-dependent magnetic asymmetries allows to accurately disentangle the element-specific response in spite of overlapping Co and Fe M$_{2,3}$ as well as Pt O$_{2,3}$ and N$_7$ resonances. We find a considerably smaller demagnetization time constant as well as much larger demagnetization amplitudes of the induced moment of Pt compared to the intrinsic moment of CoFeB. Our results are in agreement with enhanced spin-flip probabilities due to the high spin-orbit coupling localized at the heavy metal Pt, as well as with the recently formulated hypothesis that a laser generated, incoherent magnon population within the ferromagnetic film leads to an overproportional reduction of the induced magnetic moment of Pt.