Implementing Real Ab Initio in Out-of-Equilibrium Dynamics
Final Report Abstract
During my Forschungsstipendium, I used advanced ab initio methods combined with atomistic and microscopic simulations to study ultrafast dynamics in laser excited materials focussing on ultrafast relaxation processes between phonons, electrons and spin excitations. The aim of this research project is to develop a new level of understanding of microscopic mechanisms to excite spin systems on ultrafast timescales, which enables new spin-based devices for high-speed data storage or data processing applications. In a first project, I have studied electron-phonon relaxation in metallic materials. We have developped a microscopic out-of-equilibrium dynamics model, which includes phonon-mode dependent equations of motion considering phonon-mode dependent electron-phonon and phonon-phonon coupling. Our results have reveal a complex dynamical behaviour of the phonon system and strong out-of-equilibirum states at the picosecond timescale after laser excitation in all studied materials. This underlines the importance of the phonon-mode dependence of electron-phonon coupling in non-equilibrium dynamics in metals and will have a large impact also for light-induced manipulations of the magnetic system. Furthermore, I have explored light-induced magnetisation dynamics and spin currents by integrating super-diffusive spin-dependent electron transport calculations as input into atomistic spin dynamics simulations. I have simulated the excitation of high frequency magnons in the THz regime due to laser-induced hot electron spin currents. These results are in good agreement to experimental studies and show the formation of standing waves in the picosecond regime. In addition, I have studied light-induced helicity dependent switching phenomena and implemented induced magnetic moments and the coupling with the initial magnetic moments and studied helicity-dependent switching of ferromagnetic FePt and antiferromagnetic CrPt and MnPt and our results demonstrate helicity dependent switching probabilities due to induced magnetic moments via the inverse Faraday effect. Moreover, I explored new microscopic mechanisms derived from relativ equations of motion for angular momentum. The results show that in antiferromagnetic oxides, magnetic field pulses can excite antiferromagnetic resonance in the THz regime, but in addition a new torque term appears, which can even enhance the excitation. This torque is given by the time-derivative of the magnetic field pulse and can be relevant especially for chirped magnetic field pulses at high frequencies. In another project, I studied the dynamical behavior of magnetic skyrmions and antiskyrmions under spin-orbit torques in ultrathin magnetic films Pd/Fe/Ir(111). While skyrmions move rectilinear, I discovered new dynamical regimes for antiskyrmions: trochoidal motion and skyrmion-antiskyrmion pair generation. Both can appear due to deformations of the metastable antiskyrmion under largespin-orbit torques. These results offer new pathways to skyrmions and antiskyrmion dynamics in ultrathin magnetic films through DMI engineering and can be relevant for future spintronic devices. In several news outlets, such as Physics World and phys.org, these results have been presented.
Publications
- Trochoidal motion and pair generation in skyrmion and antiskyrmion dynamics under spin-orbit torques, Nat. Electron. 1, 451 (2018)
U. Ritzmann, S. v. Malottki, J. Kim, S. Heinze, J. Sinova, B. Dupé
(See online at https://doi.org/10.1038/s41928-018-0114-0) - Domain wall dynamics due to femtosecond laser-induced superdiffusive spin transport
P. Baláž, K. Carva, U. Ritzmann, P. Maldonado, P. M. Oppeneer
(See online at https://doi.org/10.1103/PhysRevB.101.174418) - Terahertz spin dynamics driven by field-derivative torque
R. Mondal, A. Donges, U. Ritzmann, P. M. Oppeneer, and U. Nowak
(See online at https://doi.org/10.1103/PhysRevB.100.060409)
