  Monolayer NiI$_2$ garners large research interest due to its multiferroic
behavior stemming from the interplay between its non-collinear magnetic order
and the spin-orbit coupling. This prompts an investigation into the stability
of the magnetic order in NiI$_2$ and similar materials under external stimuli.
In this work, we report the effect of biaxial and uniaxial strain on the
magnetic ground state, the critical temperature, and the magnonic properties of
the NiX$_2$ (X = I, Br, Cl) monolayers. For all three materials, we reveal
intricate strain-dependent phase diagrams, including ferromagnetic,
helimagnetic, and skyrmionic phases. Moreover, we discuss the necessity of
considering the biquadratic exchange interaction in the latter analysis. We
reveal that the biquadratic exchange significantly alters both the magnetic
ground state and the critical temperature of the magnetic order, and we
demonstrate that its importance becomes even more explicit when monolayer
Ni-dihalides are strained. Finally, we calculate the magnonic dispersion for
the predicted magnetic states, showing that the skyrmionic phase functions as a
magnonic crystal, and demonstrate the presence of strain-tunable soft magnon
modes at finite wavevectors in the helimagnetic phase.
