Manipulating ultrafast magnetization dynamics of ferromagnets using the odd–even layer dependence of two-dimensional transition metal di-chalcogenides

Two-dimensional transition metal dichalcogenides (TMDs) have drawn immense interest due to their strong spin-orbit coupling and unique layer number dependence in response to spin-valley coupling. This leads to the possibility of controlling the spin degree of freedom of the ferromagnet (FM) in thin film heterostructures and may prove to be of interest for next-generation spin-based devices. Here, we experimentally demonstrate the odd-even layer dependence of WS₂ nanolayers by measurements of the ultrafast magnetization dynamics in WS₂/Co₃FeB thin film heterostructures by using time-resolved Kerr magnetometry. The fluence (photon energy per unit area) dependent magnetic damping (α) reveals the existence of broken symmetry and the dominance of inter- and intraband scattering for odd and even layers of WS₂, respectively. The higher demagnetization time, τ_m, in 3 and 5 layers of WS₂ is indicative of the interaction between spin-orbit and spin-valley coupling due to the broken symmetry. The lower τ_m in even layers as compared to the bare FM layer suggests the presence of a spin transport. By correlating τ_m and α, we pinpointed the dominant mechanisms of ultrafast demagnetization. The mechanism changes from spin transport to spin-flip scattering for even layers of WS₂ with increasing fluence. A fundamental understanding of the two-dimensional material and its odd-even layer dependence at ultrashort timescales provides valuable information for designing next-generation spin-based devices.