Extended Data Fig. 5: Extracting the magnon propagation distance.

Time-resolved polarization rotation originating from a propagating magnon, as obtained in the reflection geometry. The solid line represents a best fit of a damped sine, giving a lifetime of about 85 ps. With the largest estimated group velocities vg of the measured magnons of about 13 km/s, this gives a propagation distance lc = vgτ = 1.1 μm.