A major question in the field of femtosecond laser-induced demagnetization is
whereto the angular momentum lost by the electrons is transferred. Recent
ultrafast electron diffraction measurements [Tauchert \textit{et al.}, Nature
{\bf 602}, 73 (2022)] suggest that this angular momentum is transferred to the
rotational motion of atoms on a sub-picosecond timescale, but a theory
confirmation of this proposition has yet to be given. Here we investigate the
coupled electron-nuclear dynamics during ultrafast demagnetization of L1$_0$
FePt, using Ehrenfest nuclear dynamics simulations combined with the
time-dependent density functional theory (TDDFT) framework. We demonstrate that
atomic rotations appear, i.e., the generation of phonons carrying finite
angular momentum following ultrafast demagnetization. We further show that both
ultrafast demagnetization and the generation of phonons with angular momentum
arise from symmetry constraints imposed by the spin-orbit coupling, thus
providing insight in spin-phonon interaction at ultrafast timescales.

Questo articolo esplora i giri e le loro implicazioni.

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