Presentation of Master Thesis by Hannah Lange: Ultrafast Demagnetization - A Combination of Ab Initio Calculations and Simulation Methods

31.08.2022 at 02:00 

When a thin magnetic film, e.g. a nickel film, is exposed to ultrashort laser pulses, it can loose its magnetic order within the timescale of only few femtoseconds. This phenomenon, which is called ultrafast demagnetization, offers opportunities for faster information processing in magnetic storage devices and ultrafast spintronics at frequencies approaching those of light. Only recently, it was shown that a transfer of angular momentum from the spin system to the lattice occurs on ultrashort time scales during this process [1] and excites circularly polarized phonons that take up most of the angular momentum from the spin system [2]. Hence, combined molecular-spin dynamics simulations using first-principles spin-lattice coupling (SLC) parameters might give access to the central aspects of ultrafast phenomena. In this talk, I will present and compare several schemes to calculate spin-lattice coupling parameters in a fully-relativistic way from first-principles. In particular, a perturbative SLC method with closed expression for the SLC parameters is derived by treating the changes in the spin configuration and atomic positions at the same level (also presented in Ref. [3]). It will be shown that this method gives results in accordance with previously proposed but computationally more demanding schemes. In analogy to that, spin-lattice anisotropy (SLA) parameters are derived. By analyzing the properties of these parameters, in particular their symmetry and their dependence on spin-orbit coupling, we find that even in bcc Fe the leading term for the angular momentum exchange between the spin system and the lattice is a Dzyaloshiskii-Moriya-type interaction, which is due to the symmetry breaking distortion of the lattice. SLC parameters for other materials are presented as well. Furthermore, the investigation of the spin-lattice anisotropy shows that it contains not only the local and two-site anisotropy terms contained in the magnetocrystalline anisotropy without distortions, but also a DMI-like contribution. Lastly, spin-lattice coupling in metal-chromium-oxides and -sulfides and its role in the magneto-volume transition in these materials is investigated. [1] Dornes, Acremann, Savoini, Kubli, Neugebauer, Abreu, Huber, Lantz et al. Nature 565 (2019) [2] Tauchert, Volkov, Ehberger, Kazenwadel, Evers, Lange, Donges, Book et al. Nature 602 (2022) [3] Mankovsky, Polesya, Lange, Weißenhofer, Nowak, Ebert. PRL 129 (2022)