The spin spiral (SS) state in monolayer (ML) NiI2 presents a promising avenue for the exploration of two-dimensional multiferroics, but the mechanisms underlying this SS state and its effective modulation remain insufficiently understood. In this study, we employ first-principles calculations to reveal the major features and physical mechanisms governing the magnetism of ML-NiI2 under in-plane strain, explicitly considering the effects of higher-order interactions (HOIs), which have been overlooked in prior research. Our findings identify the magnetic ground state of ML-NiI2 as an SS state characterized by a propagation vector q = 0.23, and demonstrate that in-plane biaxial strain serves as an effective method for tuning the period and stability of the SS state, the frustrated ratio, magnetic anisotropy energy and the strength of HOIs. Notably, the strength of HOIs significantly increases under higher strain, particularly for the four-site-four-spin exchange interaction, which is substantial and crucial to the properties of ML-NiI2. Additionally, through atomistic spin dynamics simulations, the coexistence of skyrmions and anti-skyrmions with diameters of at least 2 nm are achieved in ML-NiI2 via in-plane biaxial strain, without the necessity for an external magnetic field, which is more conducive to applications for spintronic applications. The presented results establish ML-NiI2 as a prospective candidate for next-generation spintronic devices and contribute to a deeper understanding of HOIs in two-dimensional magnets.
