Ab Initio Nonadiabatic Molecular Dynamics of Ultrafast Demagnetization in Iron

Ultrafast demagnetization in magnetic metals is the key to spintronics devices. Taking iron as a prototypical system, we investigate the demagnetization mechanism by simulating the charge and spin dynamics using nonadiabatic molecular dynamics in the presence of explicit spin-orbit coupling (SOC). Strong SOC drives ultrafast spin-flip of electrons and holes, which trigger demagnetization and remagnetization, respectively. Their confrontation decreases the demagnetization ratio and finishes the demagnetization within 167 fs, agreeing with the experimental time scale. The joint spin-flip of electrons and holes correlated with the electron-phonon coupling-induced fast electron-hole recombination further decreases the maximum demagnetization ratio, below 5% of experimental value. Although the Elliott-Yafet electron-phonon scattering model can rationalize the ultrafast spin-flip process, it fails to reproduce the experimental maximum demagnetization ratio. The study suggests the key role of SOC on spin dynamics and emphasizes the interplay between SOC and electron-phonon interactions on the ultrafast demagnetization.