With the goal of pushing spintronic devices towards faster and faster time scales, we have performed ab-initio time-dependent density-functional theory (TDDFT) simulations of antiferromagnetic and ferromagnetic solids, driven by short (few-femto-second) laser pulses. For a variety of compounds (NiMnSb, Co2MnSi, Mn3Ga, Ni2MnGa, Co2FeSi) we demonstrate [1,2] that the local magnetic moment can increase or decrease within a few femto-seconds. The underlying mechanism is an all-optical transfer of spin moment from one magnetic sublattice to another, governed by the availability of suitable states above the Fermi level. During this very fast process the global magnetic moment of the system stays essentially constant. In addition to this process we find a universal mechanism of demagnetization acting on a somewhat larger time-scale of about 30 femto-seconds. We show [3,4] that the demagnetization proceeds in two distinct steps: First, a fraction of the electrons is excited to higher bands without much change in the total spin polarization. In a second step, the spin magnetic moment of the remaining, more localized electrons decreases through spin-flip transitions induced by spin-orbit coupling. In structures of reduced dimensionality, such as systems of a few atomic layers, the demagnetization process tends to be more efficient than in bulk materials. Both processes were first predicted by our ab-initio simulations and later confirmed experimentally. Finally we shall investigate within a model how magnetic skyrmions can be generated by focused vortex laser pulses .
References 1. P. Elliott, T. Müller, J.K. Dewhurst, S. Sharma, E.K.U. Gross, Scientific Reports 6, 38911 (2016). 2. J.K. Dewhurst, P. Elliott, S. Shallcross, E.K.U. Gross, S. Sharma, Nano Lett. 18, 1842-1848 (2018). 3. K. Krieger, J.K. Dewhurst, P. Elliott, S. Sharma, E.K.U. Gross, JCTC 11, 4870 (2015). 4. V. Shokeen, M. Sanchez Piaia, J.-Y. Bigot, T. Mueller, P. Elliott, J.K. Dewhurst, S. Sharma, E.K.U. Gross, Phys. Rev. Lett. 119, 107203 (2017). 5. O. Polyakov, I. Gonoskov, V. Stepanyuk, E.K.U. Gross, Journal of Applied Physics 127, 073904 (2020).