The transfer and control of angular momentum is a key aspect for spintronic applications. When a thin nickel film is subjected to ultrashort laser pulses, it can lose its magnetic order almost completely within merely femtosecond times. This phenomenon, which can also be observed in many other materials, offers opportunities for rapid information processing or ultrafast spintronics at frequencies approaching those of light. Consequently, ultrafast demagnetization is central to modern material research, but a crucial question has remained elusive: If a material loses its magnetization within only femtoseconds, where is the missing angular momentum on such short time scales?

Here we use molecular dynamics simulations to investigate the role of phonons during ultrafast demagnetization in nickel. For this purpose, we transfer angular momentum corresponding to the observed amount of demagnetization into the lattice and calculate the resulting changes in the diffraction pattern. Our results are in line with ultrafast electron diffrac- tion measurements which show an almost instantaneous, long- lasting, non-equilibrium popu- lation of anisotropic high-frequency phonons that appear as quickly as the magnetic order is lost. The anisotropy plane is perpendicular to the direction of the initial magnetization and the atomic oscillation amplitude is 2 pm. Theory and experiment indicate a rotational lattice motion on atomic dimensions after the excitation with the laser pulse that takes up the missing angular momentum 1 before the onset of a macroscopic Einstein-de Haas rotation 2.

Acknowledgements This research was supported by the German Research Foundation (DFG) via SFB 1432.

References
1 S. R. Tauchert et al. Nature 602, 73–77 (2022). 2 C. Dornes et al. Nature 565, 209-212 (2019)