  The microscopic origin of the topological magnon band gap in CrI$_3$
ferromagnets has been a subject of controversy for years since two main models
with distinct characteristics, i.e., Dzyaloshinskii-Moriya (DM) and Kitaev,
provided possible explanations with different outcome implications. Here we
investigate the angular magnetic field dependence of the magnon gap of CrI$_3$
by elucidating what main contributions play a major role in its generation. We
implement stochastic atomistic spin dynamics simulations to compare the impact
of these two spin interactions on the magnon spectra. We observe three distinct
magnetic field dependencies between these two gap opening mechanisms. First, we
demonstrate that the Kitaev-induced magnon gap is influenced by both the
direction and amplitude of the applied magnetic field, while the DM-induced gap
is solely affected by the magnetic field direction. Second, the position of the
Dirac cones within the Kitaev-induced magnon gap shifts in response to changes
in the magnetic field direction, whereas they remain unaffected by the magnetic
field direction in the DM-induced gap scenario. Third, we find a
direct-indirect magnon band-gap transition in the Kitaev model by varying the
applied magnetic field direction. These differences may distinguish the origin
of topological magnon gaps in CrI$_3$ and other van der Waals magnetic layers.
Our findings pave the way for exploration and engineering topological gaps in
van der Waals materials.
