  Antiferromagnets are attractive platforms for the propagation of information
via spin waves, offering advantages over ferromagnets in speed of response and
immunity to external fields. A recent study of the quasi-2D antiferromagnet
CrSBr reported that spin wavepackets propagate with group velocities that are
orders of magnitude higher than expected from the magnon dispersion obtained by
inelastic neutron scattering [1,2]. Here we show that the anomalous magnitude
and anisotropy of the group velocity, v_g, are naturally explained by
considering the long-range magnetic dipole-dipole interaction. We also
demonstrate that v_g can be tuned over orders of magnitude by applying an
external magnetic field or varying the sample thickness. Beyond spin wavepacket
propagation, the dipolar interaction creates the possibility of non-equilibrium
Bose-Einstein condensation in antiferromagnets, previously thought to be a
property unique to ferromagnets.
