Interface-selected terahertz polarity switching in Cr-based spintronic heterostructures

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.