Direction-selective enhancement of terahertz emission in Py/Ni/Pt via stacking-dependent angular momentum transport

Spintronic terahertz (THz) emitters based on ferromagnet/heavy-metal heterostructures provide a compact platform for broadband THz generation. Here, we demonstrate a direction-selective enhancement of THz emission in Permalloy (Py)/Ni/Pt trilayers associated with spin-orbital transport. We find that the Py/Ni/Pt configuration produces a significantly stronger THz signal than Ni/Py/Pt, far exceeding the sum of bilayer contributions, with an enhancement factor of up to ∼17. Thickness-dependent measurements, interface engineering, and material substitution reveal that this behavior is closely linked to the stacking-sequence-dependent role of the Ni layer. In the Py → Ni → Pt sequence, Ni not only provides an intrinsic orbital contribution but also facilitates spin-to-orbital conversion via spin–orbit coupling, enabling additional transport pathways toward Pt. However, this pathway is largely suppressed in the reversed stacking, resulting in a near-additive response. These results highlight stacking-sequence-dependent angular momentum transport as a key factor governing THz emission and establish ferromagnetic-layer engineering as an effective strategy for optimizing spintronic THz emitters.