Magnon dispersion in bilayers of two-dimensional ferromagnets

We determine magnon spectra of an atomic bilayer magnet with ferromagnetic intra- and both ferro- and antiferromagnetic interlayer coupling. Analytic expressions for the full magnon band of the latter case reveal that both exchange interactions govern the fundamental magnon gap. The inter- and intralayer magnetic ordering are not independent: a stronger ferromagnetic intralayer coupling effectively strengthens the antiferromagnetic interlayer coupling as we see from comparison of two bilayer systems. The trivial topology of these exchange-anisotropy spin models without spin-orbit interaction excludes a magnon thermal Hall effect.