Theory of superdiffusive spin transport in noncollinear magnetic multilayers

TL;DR

This paper develops a superdiffusive theory for ultrafast spin transport in noncollinear magnetic multilayers, showing how demagnetization and spin-transfer torques depend on the angle between magnetic moments, with implications for spintronic device control.

Contribution

The paper introduces a generalized superdiffusive model for noncollinear magnetic multilayers and combines ab initio calculations to analyze ultrafast spin dynamics and transport.

Findings

Demagnetization of Ni and Fe layers depends on magnetization angle.

Spin-transfer torques vary significantly with noncollinearity.

Control of hot electron spin transport via magnetic angle can enable fast spintronic devices.

Abstract

Ultrafast demagnetization induced by femtosecond laser pulses in thin metallic layers is caused by the outflow of spin-polarized hot electron currents describable by the superdiffusive transport model. These laser-generated spin currents can cross the interface into another magnetic layer and give rise to magnetization dynamics in magnetic spin valves with noncollinear magnetizations. To describe ultrafast transport and spin dynamics in such nanostructures we develop here the superdiffusive theory for general noncollinear magnetic multilayers. Specifically, we introduce an Al/Ni/Ru/Fe/Ru multilayer system with noncollinear Ni and Fe magnetic moments and analyze how the ultrafast demagnetization and spin-transfer torque depend on the noncollinearity. We employ ab initio calculations to compute the spin- and energy-dependent transmissions of hot electrons at the interfaces of the multilayer. Taking into account multiple electron scattering at interfaces and spin mixing in the spacer layer we find that the laser-induced demagnetization of the Ni layer and magnetization change of the Fe layer strongly depend on the angle between their magnetizations. Similarly, the spin-transfer torques on the Ni and Fe layers and the total spin momentum absorbed in the Ni and Fe layer are found to vary markedly with the amount of noncollinearity. These results suggest that changing the amount of noncollinearity in magnetic multilayers one can efficiently control the hot electron spin transport, which may open a way toward achieving fast, laser-driven spintronic devices.