Fig. 4: Magnon dispersion curves of the two magnon modes of an easy-plane antiferromagnet.

a Low (close to the Morin transition) and b high anisotropy (far from the Morin transition, here +30 K29). The pairs of magnons which satisfy energy conservation have small (large) Δk for low (high) anisotropy. For the calculations, we use a limiting magnon velocity c = 5.104 m/s as estimated from the magnon dispersion of Martin et al.40.