The interplay between geometric frustration and spin-orbit coupling (SOC) in two-dimensional van der Waals magnets has emerged as a fertile ground for exploring exotic quantum phases. Here, we elucidate the microscopic origin of the directional spin-spiral states in monolayer ${\mathrm{NiBr}}_2$ by systematically quantifying the complete set of magnetic parameters induced by SOC—specifically the Kitaev interaction, off-diagonal exchange, and single-ion anisotropy (SIA). By combining generalized Bloch calculations with an energy mapping and magnetic anisotropy analysis, we determine the full angular dependence of these interactions. Our analysis reveals that the Kitaev interaction serves as the dominant source of magnetic anisotropy. Crucially, while the isotropic Heisenberg frustration fundamentally drives the formation of the spin-spiral state, we find that the Kitaev interaction plays a decisive role in stabilizing its specific directional locking along the G–L direction. Incorporating this interaction substantially lowers the spin-spiral ground state energy at Hubbard $U=3$ and 4 eV and imposes a clear orientational selectivity. Specifically, the spiral plane is primarily driven by the Kitaev interaction to align with the Ni–Br–Ni–Br bond plane, but is significantly tilted towards the basal plane by the SIA. These results identify monolayer ${\mathrm{NiBr}}_2$ as a prototypical triangular-lattice magnet whose spin-spiral geometry is jointly dictated by the interplay of the Kitaev interaction and SIA, and demonstrate the broad applicability of the energy mapping and magnetic anisotropy energy method for characterizing bond-dependent interactions in two-dimensional van der Waals magnets.

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• Density of states
• Electronic structure
• Magnetic interactions
• Spin-orbit coupling
• 2-dimensional systems
• First-principles calculations