Laser-induced forces on atoms during ultrafast demagnetization

TL;DR

This study uses first-principles calculations to analyze laser-induced atomic forces during ultrafast demagnetization, challenging previous assumptions about the roles of lattice vibrations and angular momentum transfer.

Contribution

First-principles computational approach to evaluate atomic forces during ultrafast laser demagnetization, providing new insights into the mechanisms involved.

Findings

Atomic forces start at -50 fs and peak around 30 fs

Force magnitude is much smaller than empirical estimates

Polarized phonon and Einstein-de Haas effects are unlikely main causes

Abstract

Laser-induced femtosecond demagnetization has attracted a broad attention as a possible candidate for information storage technology. However, whether or not lattice vibration directly participates in demagnetization has been highly controversial over a decade. A recent electron diffraction experiment attributed the demagnetization to the polarized phonon effect, but a similar x-ray diffraction experiment attributed it to the Einstein-de Haas effect. Common to both experiments is that neither the angular momentum of the lattice nor the rotation of the sample was directly probed. Here, we report our first first-principles calculation of forces on atoms induced by an ultrafast laser during ultrafast demagnetization. We employ two complementary methods: (i) the frozen lattice with electronic excitation and (ii) frozen excitation but moving the lattice. We find that the forces on atoms start at -50 fs and peak around 30 fs. The magnitude of the force is far smaller than the empirical estimates. Within the limit of our theory, our results suggest that the polarized phonon effect and the Einstein-de Haas effect are unlikely to be the main course of demagnetization. We expect that our finding has a profound impact on the future direction of laser-induced dynamics in magnetic and quantum materials.