Optically driven spin transport is the fastest and most efficient process to manipulate macroscopic magnetization because it is active during the optical excitation and does not rely on secondary mechanisms to dissipate angular momentum . The experimental detection of the optically induced spin transfer (OISTR) is challenging, as it requires access to the element-specific transient density of states around the Fermi energy.
In our joint theoretical and experimental work, we show that OISTR from Pt to Co governs the ultrafast demagnetization dynamics of a CoPt alloy . Furthermore, we demonstrate that the analysis of the transient helicity dependent absorption at the resonant M2,3 transition of Co in the extreme ultraviolet spectral range reveals detailed information on the transient spin-split density of states, which we can directly compare to our theoretical simulations employing time-dependent density functional theory.
The comparison between the theoretical and experimental data allows us to conclude that the laser-driven spin current originates from the available minority states above the Fermi level in conjunction with the electric field of the laser pulse, making this a general phenomenon in all multi-component magnetic systems.
A further all-optical study on different 3d transition metal alloys corroborates that the number of available states above the Fermi level drives OISTR, opening the possibility to control demagnetization on the fastest time scale by the engineering of the density of states.
 J. K. Dewhurst et al., Nano Lett., 18 (2018)
 F. Willems et al., submitted (2019)