  MnBi2Te4(MBT) is a promising van der Waals layered antiferromagnetic (AF)
topological insulator that combines a topologically non-trivial inverted Bi-Te
band gap with ferromagnetic (FM) layers of Mn ions. We perform inelastic
neutron scattering (INS) on co-aligned single crystals to study the magnetic
interactions in MBT. Consistent with previous work, we find that the AF
interlayer exchange coupling and uniaxial magnetic anisotropy have comparable
strength, which supports metamagnetic transitions that allow access to
different magnetic symmetries in applied fields. Modelling of the
two-dimensional intralayer FM spin waves requires the introduction of
long-range and competing Heisenberg FM and AF interactions, up to at least the
seventh nearest-neighbor, and possess anomalous damping, especially near the
Brillouin zone boundary. First-principles calculations of insulating MBT find
that both interlayer and intralayer magnetic interactions are long-ranged. We
discuss the potential roles that bulk $n$-type charger carriers and chemical
disorder play in the magnetism of MBT.
