  Bilayer Heisenberg ferromagnetic insulators hold degenerate terahertz Dirac
magnon modes associated with two opposite valleys of the hexagonal Brillouin
zone. We show that this energy degeneracy can be removed by breaking of the
inversion symmetry (I), leading to a topological magnon valley current. We show
furthermore that this current leads to valley Seebeck effect for magnons and is
thereby detectable. We perform calculations in the specific example of bilayer
CrBr3, where I can be broken by electrostatic doping.
