Fig. 3. Magnetic structures for NiBr₂ monolayer.

a, b Snapshots of spin configurations at T = 1 K from MC simulations on a 30 × 30 supercell obtained for B_z/J^1iso = 0 and  ≃ 0.3, respectively. Insets show the spin structure factor S(q), which corresponds to a single-q state along q₃ = (−2δ, δ) in a and q₂ = (δ, − 2δ) in b, with δ ≃ 0.10. c, d Decomposition of the spin structure factor in the cartesian components S_x(q), S_y(q) and S_z(q). The spin configuration displayed in a is composed of a spin-spiral, with spins rotating in a plane perpendicular to the propagation vector q₃, and of a spin-cycloidal component, with spins rotating in a plane containing the propagation vector, as highlighted by the small peak in S_x(q). Such noncoplanar helimagnetic single-q state arises from the small anisotropic symmetric exchange reported in Table 1. Under an applied magnetic field, a conical helix state is stabilized (b), where a ferromagnetic component parallel to the field is superimposed to a cycloidal state, as evidenced by in-plane and out-of-plane components of the spin structure factor shown in d.