A central challenge in spintronics is the efficient generation and control of spin currents for information processing. Here we investigate ultrashort spin transport and the resulting terahertz (THz) emission in a hybrid structure comprised of gold nanoparticles (GNPs), a ferromagnet (FM), and a normal metal (NM), and show that plasmon excitation in the GNPs significantly enhances the electron-magnon scattering rate through heating effects, thereby amplifying spin current generation at the $\mathrm{FM}|\mathrm{NM}$ interface. This enhancement becomes even more pronounced when the FM is an ultrathin insulator with a thickness much smaller than the GNP size. In this case, the GNPs and NM substrate form a plasmonic nanocavity with the FM acting as a dielectric spacer, trapping plasmons inside the gap. The resulting spin current can be boosted by up to two orders of magnitude compared to the structure without plasmon excitations. Our findings reveal a pathway to design efficient spintronic THz devices and further open the door to the interdisciplinary field of spintronics and nanophotonics.

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• Plasmons
• Spin current
• Spin dynamics
• Spin generation
• Boltzmann theory