  Magnonics is a research field complementary to spintronics, in which the
quanta of spin waves (magnons) replace electrons as information carriers,
promising less energy dissipation. The development of ultrafast nanoscale
magnonic logic circuits calls for new tools and materials to generate coherent
spin waves with frequencies as high, and wavelengths as short, as possible.
Antiferromagnets can host spin waves at THz frequencies and are therefore seen
as a future platform for the fastest and the least dissipative transfer of
information. However, the generation of short-wavelength coherent propagating
magnons in antiferromagnets has so far remained elusive. Here we report the
efficient emission and detection of a nanometer-scale wavepacket of coherent
propagating magnons in antiferromagnetic DyFeO3 using ultrashort pulses of
light. The subwavelength nanoscale confinement of the laser field due to large
absorption creates a strongly non-uniform spin excitation profile, thereby
enabling the propagation of a broadband continuum of coherent THz spin waves.
The wavepacket features magnons with detected wavelengths down to 125 nm and
supersonic velocities up to 13 km/s that propagate over macroscopic distances.
The long-sought source of coherent short-wavelength spin carriers demonstrated
here opens up new prospects for THz antiferromagnetic magnonics and coherence
mediated logic devices at THz frequencies.
