Terahertz Emission from Inverse Spin Hall Effect in β-W

Here we demonstrate THz emission from a magnetic/heavy metal bilayer of CoFe/β-W deposited on a magnetic spinel MgAl₂O₄ substrate. It is believed the mechanism which generates the transient charge currents that are responsible for radiating the THz pulse is the inverse spin hall effect, where a propagating spin current is converted into a transverse charge current. The inverse spin hall effect occurs due to the presence of strong spin-orbit coupling in the heavy metal layer. β-W has been shown to have the largest spin hall angle (a measure of the spin current to charge current conversion efficiency) of all elemental metals. We show the peak THz electric field scales with β-W layer thickness and pump fluence, generating THz at fluences as low as 2mW. We are also able to flip the amplitude of the THz field by switching the magnetization of the CoFe layer, thus switching sign of the majority spin species. From the perspective of applications, these bilayers are attractive as their cost is dominated by the substrate and they are truly capable of generating broadband THz (0.5 - 2 THz) radiation. From a fundamental perspective, THz emission can be a platform for non-destructive probes of materials which exhibit effects due to Berry curvature, such as the spin hall effect.