Separation of emission mechanisms in spintronic terahertz emitters
Yongshan Liu, Houyi Cheng, Yong Xu, Pierre Vallobra, Sylvain Eimer, Xiaoqiang Zhang, Xiaojun Wu, Tianxiao Nie, Weisheng Zhao
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.