Hallmarks of terahertz magnon currents in an antiferromagnetic insulator

The efficient transport of spin angular momentum is expected to play a crucial role in future spintronic devices, which potentially operate at frequencies reaching the terahertz range. Antiferromagnetic insulators exhibit significant potential for facilitating ultrafast pure spin currents by terahertz magnons. Consequently, we here use femtosecond laser pulses to trigger ultrafast spin currents across antiferromagnetic NiO thin films in Py|NiO|Pt stacks, where permalloy (Py) and Pt serve as spin-current source and detector respectively. We find that the spin current pulses traversing NiO reach a velocity up to 40 nm/ps and experience increasing delay and broadening as the NiO thickness is increased. We can consistently explain our observations by ballistic transport of incoherent magnon. Our approach has high potential to characterize terahertz magnon transport in magnetic insulators with any kind of magnetic order.