Laser pulse induced efficient terahertz emission from Co/Al heterostructures

Investigation on terahertz (THz) emission is important not only for fundamental research but also for practical application. A rapidly developing approach to generate THz emission is using a heterostructure composed of ferromagnetic metal (FM) and nonmagnetic metal (NM) which is pumped by femtosecond laser pulse. Previous works in this regard mainly focused on bilayer with heavy NM (such as Pt) with a large spin Hall angle. Here we present a comprehensive investigation on THz emission from Co/Al heterostructures stemming from the inverse spin Hall effect. It is surprising to find that although the spin Hall angle of Al is two orders of magnitude smaller than that of Pt, the measured THz signals are close to one-third of that of the typical Co/Pt heterostructures. To explore the underlying physics, theoretical models are employed to investigate the spin-related properties of Al. We obtain that the upper limit of the spin Hall angle of Al is 0.55% and the spin-diffusion length is 2.2 +/- 0.2 nm; the diffusion length is comparable to that of Pt (2.3 +/- 0.1 nm). This work shows that it is worth revisiting light metals for the spintronic THz emitter.