Superluminal-like magnon propagation in antiferromagnetic NiO at nanoscale distances

Magnon-mediated angular-momentum flow in antiferromagnets may become a design element for energy-efficient, low-dissipation and high-speed spintronic devices(1,2). Owing to their low energy dissipation, antiferromagnetic magnons can propagate over micrometre distances(3). However, direct observation of their high-speed propagation has been elusive due to the lack of sufficiently fast probes(2). Here we measure the antiferromagnetic magnon propagation in the time domain at the nanoscale (<= 50 nm) with optical-driven terahertz emission. In non-magnetic-Bi2Te3/antiferromagnetic-insulator-NiO/ferromagnetic-Co trilayers, we observe a magnon velocity of similar to 650 km s(-1) in the NiO layer. This velocity far exceeds previous estimations of the maximum magnon group velocity of similar to 40 km s(-1), which were based on the magnon dispersion measurements of NiO using inelastic neutron scattering(4,5). Our theory suggests that for magnon propagation at the nanoscale, a finite damping makes the dispersion anomalous for small magnon wavenumbers and yields a superluminal-like magnon velocity. Given the generality of finite dissipation in materials, our results strengthen the prospects of ultrafast nanodevices using antiferromagnetic magnons.

Automated summary

The research demonstrates that magnon-mediated angular-momentum flow in antiferromagnets can be an efficient design element for energy-efficient, low-dissipation, and high-speed spintronic devices. It shows that antiferromagnetic magnons can propagate over micrometre distances with a superluminal-like velocity at the nanoscale. This suggests potential prospects for ultrafast nanodevices using antiferromagnetic magnons due to the generalities of finite dissipation in materials.