Regensburg 2022 – scientific programme
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MA: Fachverband Magnetismus
MA 8: Ultrafast Magnetization Effects 1
MA 8.7: Talk
Monday, September 5, 2022, 16:30–16:45, H37
Polarized phonons carry the missing angular momentum in femtosecond demagnetization — •Hannah Lange1, Martin Evers1, Andreas Donges1, Sonja Tauchert1,2, Mikhail Volkov1,2, Peter Baum1,2, and Ulrich Nowak1 — 1University of Konstanz, Fachbereich Physik, 78464 Konstanz — 2LMU Munich, Am Coulombwall 1, 85748 Garching
When a thin nickel film is subjected to ultrashort laser pulses, it can lose its magnetic order almost completely on within hundreds of femtoseconds. This phenomenon offers opportunities for rapid information processing or ultrafast spintronics. However, a crucial question has remained elusive: Where is the angular momentum transferred to in such short time? Here we use molecular dynamics simulations to investigate the role of phonon angular momentum. When exciting a rotational lattice motion which carries the amount of angular momentum corresponding to the demagnetization, we observe an anisotropy of the atoms' velocities which violates the equipartition theorem. When calculating the corresponding electron diffraction pattern, this results in an anisotropy of normally equivalent Bragg spots. This is in line with ultrafast electron diffraction which show an almost instantaneous, long-lasting, non-equilibrium population of phonons. Theory and experiment hence indicate the formation of polarized phonons that take up the missing angular momentum [1] before the onset of a macroscopic Einstein-de Haas rotation [2].
[1] Tauchert et al. Nature 602, 73-77 (2022).
[2] Dornes et al. Nature 565, 209-212 (2019).