Stacking-direction-dependent terahertz emission in Ni/W-based magnetic heterostructures

Ultrafast terahertz (THz) emission from magnetic heterostructures provides a sensitive probe of spin-orbit-coupled angular-momentum transport. Here, we report stacking-direction-controlled THz emission in Ni/W-based heterostructures. Fe/W and W/Fe reference bilayers show opposite THz polarities, consistent with spin-current-driven inverse spin Hall conversion. In contrast, Ni/W and W/Ni exhibit the same effective polarity but strongly different amplitudes. Additional W-based and Ni-based control bilayers further show that this non-reversing response is material selective rather than a generic consequence of either Ni or W alone. W-thickness-dependent measurements reveal a nearly fixed delay in Ni/W but a systematic delay increase in W/Ni, suggesting different angular-momentum propagation and conversion mechanisms. By introducing Fe or Permalloy as an additional ferromagnetic (FM) layer, we show that the THz emission can be selectively enhanced, suppressed, or reversed depending on the FM/Ni coupling. Rather than simply adding an independent FM emission channel, the added layer amplifies or reshapes the intrinsic transport asymmetry between Ni/W and W/Ni. Our results demonstrate that stacking direction provides an effective route for modifying ultrafast THz emission through interfacial angular-momentum transport.