Generation of high-frequency strain waves during femtosecond demagnetization of Fe/MgO films

TL;DR

This study uses femtosecond x-ray scattering to observe high-frequency strain waves generated during ultrafast laser-induced demagnetization in Fe/MgO films, revealing insights into lattice dynamics at femtosecond timescales.

Contribution

It demonstrates the detection of coherent acoustic phonons and analyzes their wavefronts, highlighting non-thermal lattice dynamics beyond traditional models.

Findings

Strain wavefront width is approximately 100 fs.

High-frequency Fourier components originate from non-thermal lattice dynamics.

Lattice strain propagates during ultrafast demagnetization, correlating with demagnetization timescales.

Abstract

We use femtosecond time-resolved hard x-ray scattering to detect coherent acoustic phonons excited during ultrafast laser demagnetization of bcc Fe films. We determine the lattice strain propagating through the film through analysis of the oscillations in the x-ray scattering signal as a function of momentum transfer. The width of the strain wavefront is ~100 fs, similar to demagnetization timescales. First-principles calculations show that the high-frequency Fourier components of the strain, which give rise to the sharp wavefront, could in part originate from non-thermal dynamics of the lattice not considered in the two-temperature model.