Modeling ultrafast demagnetization and spin transport: the interplay of spin-polarized electrons and thermal magnons

TL;DR

This paper presents a theoretical model for ultrafast laser-induced spin transport in magnetic heterostructures, emphasizing the roles of electrons and magnons and aligning with recent experimental findings.

Contribution

It introduces an effective spin transport model that treats electrons and thermal magnons equally, including electron-magnon scattering, to analyze ultrafast demagnetization dynamics.

Findings

Interfacial spin current is proportional to the time derivative of magnetization with efficient spin dissipation.

Analytical insights into different relations between spin current and magnetization depending on spin-flip scattering efficiency.

Magnon transport's impact varies with system parameters and cannot be universally neglected.

Abstract

We theoretically investigate laser-induced spin transport in metallic magnetic heterostructures using an effective spin transport description that treats itinerant electrons and thermal magnons on an equal footing. Electron-magnon scattering is included and taken as the driving force for ultrafast demagnetization. We assume that in the low-fluence limit the magnon system remains in a quasi-equilibrium, allowing a transient nonzero magnon chemical potential. In combination with the diffusive transport equations for the itinerant electrons, the description is used to chart the full spin dynamics within the heterostructure. In agreement with recent experiments, we find that in case the spin-current-receiving material includes an efficient spin dissipation channel, the interfacial spin current becomes directly proportional to the temporal derivative of the magnetization. Based on an analytical calculation, we discuss that other relations between the spin current and magnetization may arise in case the spin-current-receiving material displays inefficient spin-flip scattering. Finally, we discuss the role of (interfacial) magnon transport and show that, a priori, it cannot be neglected. However, its significance strongly depends on the system parameters.