Terahertz Emission from All-Ferromagnet Bilayers: Impact of Spin-Dependent Transport at the Interface

Terahertz emission from spintronic systems has been extensively studied in bilayer structures composed of ferromagnets and heavy metals, where the former functions as the spin source and the latter serves as the spin-to-charge converter through key mechanisms like the inverse spin Hall effect and inverse Rashba-Edelstein effect. Here, we report THz emission from various ferromagnet-only bilayers, including CoFeB/Ni, CoFeB/NiFe, NiFe/Ni, and Fe/Ni, with a particular focus on CoFeB/Ni structures. By systematically varying the pumping power and layer thickness, we were able to isolate the contributions from the individual layers and interfaces. It is found that both anomalous Hall-like conversion at the substrate/CoFeB interface and inverse spin Hall-like conversion in the Ni layer play important roles in terahertz generation. Although the overall emission efficiency is lower than conventional ferromagnet/heavy metal bilayers, the results obtained provide new insights into spin transport across all-ferromagnet interfaces, a topic that has so far been underexplored compared to other types of spintronic systems.