Terahertz spin-to-charge current conversion in stacks of ferromagnets and the transition-metal dichalcogenide NbSe$_2$

TL;DR

This study investigates terahertz emission from NbSe2 in ferromagnet/NbSe2 stacks, revealing insights into spin-to-charge conversion efficiency, spin Hall angle, and ultrafast spin current dynamics in this promising 2D material.

Contribution

It provides the first quantitative analysis of spin-to-charge conversion in NbSe2 using terahertz spectroscopy and models ultrafast spin current injection and relaxation.

Findings

Spin Hall angle of NbSe2 estimated between -0.2% and -1.1%.

Spin-current relaxation length in NbSe2 is a few nanometers.

Bulk NbSe2 significantly contributes to spin-to-charge conversion.

Abstract

Transition-metal dichalcogenides (TMDCs) are an aspiring class of materials with unique electronic and optical properties and potential applications in spin-based electronics. Here, we use terahertz emission spectroscopy to study spin-to-charge current conversion (S2C) in the TMDC NbSe$_2$ in ultra-high-vacuum-grown F|NbSe$_2$ thin-film stacks, where F is a layer of ferromagnetic Fe or Ni. Ultrafast laser excitation triggers an ultrafast spin current that is converted into an in-plane charge current and, thus, a measurable THz electromagnetic pulse. The THz signal amplitude as a function of the NbSe$_2$ thickness shows that the measured signals are fully consistent with an ultrafast optically driven injection of an in-plane-polarized spin current into NbSe$_2$. Modeling of the spin-current dynamics reveals that a sizable fraction of the total S2C originates from the bulk of NbSe$_2$ with the same, negative, sign as the spin Hall angle of pure Nb. By quantitative comparison of the emitted THz radiation from F|NbSe$_2$ to F|Pt reference samples and the results of ab-initio calculations, we estimate that the spin Hall angle of NbSe$_2$ for an in-plane polarized spin current lies between -0.2% and -1.1%, while the THz spin-current relaxation length is of the order of a few nanometers.