Superluminal propagation of antiferromagnetic magnons in nanometer-scale NiO

Magnons, the quasiparticles of spin waves, are the elementary low-energy collective excitations in magnetic materials. Antiferromagnetic insulators (AFMIs) can host terahertz (THz)-frequency magnons to carry angular momentums without moving charges, enabling high-speed and low-dissipation operation of spin-devices. In addition, compact AFMI devices at nanoscales are desirable due to the absence of stray field. However, the magnon propagation speed, which is a key parameter to determine data operation time, remains elusive in AFMIs particularly at nanometer distances due to the lack of sufficiently fast probes. Here, we report the direct time-domain measurement of the velocity of antiferromagnetic magnons in NiO with optical-driven THz emission [1]. We find the magnons propagate in nonmagnetic Bi₂Te₃/antiferromagnetic insulator NiO/ferromagnetic Co trilayers at a superluminal velocity (up to 650 km/s) at nanoscales in NiO (≤ 50 nm), which exceeds far beyond the limiting magnon group velocity (~40 km/s) obtained by dispersion relation using inelastic neutron scattering. We attribute this finding to the fact that a finite damping makes the dispersion anomalous at small wavenumbers and yields the superluminal magnon propagation. Our observation suggests the prospects of energy-efficient nanodevices using AFMIs considering finite dissipation in real materials.