Shaohua Zhang

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.

Spintronic terahertz (THz) emission is a powerful probe of ultrafast spin and orbital angular-momentum transport in magnetic heterostructures. Here, we demonstrate interface-selected THz polarity switching in Ni/Permalloy (Py)/Cr trilayers. Ni/Cr and Py/Cr bilayers emit THz pulses with opposite polarity, indicating two distinct Cr-mediated conversion channels. This contrast is attributed to Py-dominated spin-current-induced inverse spin Hall conversion and Ni-mediated orbital-related inverse orbital Hall conversion, which have opposite effective signs in Cr. By constructing Ni/Py/Cr and Py/Ni/Cr trilayers, we show that the THz response cannot be explained by a simple superposition of Ni- and Py-related ferromagnetic (FM) contributions. Instead, the polarity is selected by the local FM/Cr interface. A clear polarity reversal occurs when an ultrathin Ni or Py layer of only 0.5–1 nm is inserted at the Cr interface, revealing the critical role of early-stage interface formation. Our results establish an interface-engineering route for controlling THz polarity through competing spin and orbital conversion channels in Cr-based spintronic emitters.

The utilization of terahertz (THz) emission spectroscopy in femtosecond photoexcited spintronic heterostructures has emerged as a versatile tool for investigating ultrafast spin transport in a noncontact and noninvasive manner. However, the investigation of ultrafast orbital transport is still in the primitive stage. Here, we experimentally demonstrate the orbital-to-charge current conversion in Co/Ru heterostructures and identify signatures that cannot be consistently explained within a conventional spin-current framework. Time-domain measurements reveal delayed and broadened THz waveforms with increasing Ru thickness, suggesting the presence of long-range angular momentum transport associated with orbital contributions. In Co/Pt/Ru trilayers, the THz emission is further enhanced through cooperative effects between the inverse spin Hall effect (ISHE) in Pt and inverse orbital Hall effect (IOHE) in Ru, where collinear charge currents are generated and add constructively. In contrast, reversed stacking configurations exhibit suppressed emission, highlighting the critical role of stacking sequence and interface-controlled transmission. Ferromagnetic resonance (FMR) measurements reveal a strong correlation between damping and THz amplitude, highlighting efficient angular momentum conversion. These results position Co/Ru as a promising orbitronic platform for tunable ultrafast THz emission. Our results not only strengthen the physical mechanism of condensed matter physics but also pave the way for designing promising spin-orbitronic devices and THz emitters.

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.