Strain-tunable magnetic and magnonic states in Ni-dihalide monolayers

Monolayer ${\mathrm{NiI}}_{2}$ garners large research interest because of its multiferroic behavior stemming from the interplay between its noncollinear magnetic order and the spin-orbit coupling. This prompts an investigation into the stability of the magnetic order in ${\mathrm{NiI}}_{2}$ and similar materials under external stimuli. In this paper, we report the effect of biaxial and uniaxial strain on the magnetic ground state, the critical temperature, and the magnonic properties of the ${\mathrm{Ni}\mathit{X}}_{2}$ ($X$ = 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.