Separation of emission mechanisms in spintronic terahertz emitters

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 Al2O3 layer between the ferromagnetic and heavy metal layer, the spin-charge conversion is suppressed when the Al2O3 thickness exceeds 1 nm. Our results agree with the theoretical predictions based on a simple spin-diffusion model.