Long-lived zone-boundary magnons in an antiferromagnet

Antiferromagnetic (AFM) insulators exhibit many desirable features for spintronic applications such as fast dynamics in the THz range and robustness to fluctuating external fields. However, large damping typically associated with THz magnons presents a serious challenge for THz magnonic applications.

Here, we report long-lived short-wavelength zone boundary magnons in the honeycomb AFM insulator CoTiO₃, recently found to host topological magnons. We find that its zone-boundary THz magnons exhibit longer lifetimes than its zone-center magnons. This unusual momentum-dependent long magnon lifetime originates from several factors including the antiferromagnetic order, exchange anisotropy, a finite magnon gap, and magnon band dispersion. Our work suggests that magnon-magnon interaction may not be detrimental to magnon lifetimes and should be included in future searches for topological magnons.

Figure: Magnons in CoTiO3. a Spin lattice structure of the buckled honeycomb of Co2+ ions in the AFM phase. The spins are ordered ferromagnetically (anti-ferromagnetically) between intralayer (interlayer) spins coupled by the exchange interaction J1 (J2). b Calculated magnon dispersion (color solid lines) along the lines of Γ-K-M-Γ in the Brillouin zone. The green and purple dots are experimental data from our Raman measurements and inelastic neutron scattering, respectively. c boundary labeled with high symmetry points. c Schematic of magnon precession for M1 and M 2 modes involving two adjacent honeycomb lattices in the ab plane. Atoms in the top (bottom) layer are represented by dark (light) green balls and the arrows describe the phases of the precession. Three atoms (with both blue and red cones) in the two layers overlap in this top view along the c-axis.