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Theory of superdiffusive spin transport in noncollinear magnetic multilayers

Pavel Bal\'a\v{z}Maciej ZwierzyckiFrancesco CoscoKarel CarvaPablo MaldonadoPeter M. Oppeneer
Dec 2022
Ultrafast demagnetization induced by femtosecond laser pulses in thinmetallic layers is caused by the outflow of spin-polarized hot electroncurrents describable by the superdiffusive transport model. Theselaser-generated spin currents can cross the interface into another magneticlayer and give rise to magnetization dynamics in magnetic spin valves withnoncollinear magnetizations. To describe ultrafast transport and spin dynamicsin such nanostructures we develop here the superdiffusive theory for generalnoncollinear magnetic multilayers. Specifically, we introduce an Al/Ni/Ru/Fe/Rumultilayer system with noncollinear Ni and Fe magnetic moments and analyze howthe ultrafast demagnetization and spin-transfer torque depend on thenoncollinearity. We employ ab initio calculations to compute the spin- andenergy-dependent transmissions of hot electrons at the interfaces of themultilayer. Taking into account multiple electron scattering at interfaces andspin mixing in the spacer layer we find that the laser-induced demagnetizationof the Ni layer and magnetization change of the Fe layer strongly depend on theangle between their magnetizations. Similarly, the spin-transfer torques on theNi and Fe layers and the total spin momentum absorbed in the Ni and Fe layerare found to vary markedly with the amount of noncollinearity. These results suggest that changing the amount of noncollinearity in magneticmultilayers one can efficiently control the hot electron spin transport, whichmay open a way toward achieving fast, laser-driven spintronic devices.