Separation of emission mechanisms in spintronic terahertz emitters

Abstract

Spintronic thin films were shown to be promising high-performance broadband terahertz (THz) emitters. For spintronic ferromagnet/heavy metal heterostructures, THz emission arises from three mechanisms: the magnetic dipolar radiation due to the ultrafast demagnetization, the transient current radiation due to spin-charge conversion, and anomalous Hall effect. Here, we demonstrate that the three mechanisms show distinct dependences on the thickness of the ferromagnetic layer. With the insertion of an ultrathin ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ layer between the ferromagnetic and heavy metal layer, the spin-charge conversion is suppressed when the ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ thickness exceeds 1 nm. Our results agree with the theoretical predictions based on a simple spin-diffusion model.