Secondary spin current driven efficient THz spintronic emitters

TL;DR

This paper demonstrates that secondary spin currents, driven by hot electrons from heavy metal layers, significantly enhance terahertz emission in ferromagnet/heavy metal heterostructures, revealing a new mechanism for efficient spintronic THz emitters.

Contribution

It uncovers the role of secondary spin currents in boosting THz emission and develops an analytical model for the superdiffusive spin-transport process.

Findings

Secondary spin currents can surpass primary currents in thicker HM layers.

Enhanced THz emission correlates with increased secondary spin current contributions.

An analytical model explains the microscopic superdiffusive spin transport mechanism.

Abstract

Femtosecond laser-induced photoexcitation of ferromagnet (FM)/heavy metal (HM) heterostructures have attracted attention by emitting broadband terahertz frequencies. The phenomenon relies on the formation of ultrafast spin current, which is largely attributed to the direct photoexcitation of the FM layer. However, we reveal that during the process, the FM layer also experiences a secondary excitation led by the hot electrons from the HM layer that travel across the FM/HM interface and transfer additional energy in the FM. Thus, the generated secondary spins enhance the total spin current formation and lead to amplified spintronic terahertz emission. The results also emphasize the significance of the secondary spin current, which even exceeds the primary spin currents when FM/HM heterostructures with thicker HM are used. An analytical model is developed to provide deeper insights into the microscopic processes within the individual layers, underlining the generalized ultrafast superdiffusive spin-transport mechanism.