Efficient and temperature-independent terahertz emission from CoFeB/NiCu heterostructures

Spintronic terahertz (THz) emission has been investigated in ferromagnetic (FM) metal/nonmagnetic (NM) metal, FM/semiconductor, and FM/topological insulator systems. However, THz emission in the heterojunction of FM and $3d$ transition light metal is rarely reported. Large spin Hall angles have been observed in a light metal alloy of the $3d$ transition metal NiCu, allowing for the emission of high-performance THz waves without the use of heavy metal. Here, we present a comprehensive study of THz emission from a CoFeB/NiCu heterostructure, with peak intensity of emitted THz pulses up to half that of CoFeB/Pt. The THz radiation from CoFeB/NiCu is shown to be generated by the spin-to-charge conversion in NiCu via the inverse spin Hall effect. The evolution of the THz signal of $\mathrm{CoFeB}/{\mathrm{Ni}}_{70}{\mathrm{Cu}}_{30}$ across ${\mathrm{Ni}}_{70}{\mathrm{Cu}}_{30}$ Curie temperature is also explored, and it is discovered that the THz emission signal in this sample is temperature independent and unaffected by the magnetic state of ${\mathrm{Ni}}_{70}{\mathrm{Cu}}_{30}$. Our findings highlight the great potential of light metal alloys containing $3d$ transition elements as spintronic THz emitters. Moreover, it has been demonstrated that THz emission spectroscopy is a helpful tool for investigating inverse spin Hall effect or anomalous Hall effect in ferromagnets.