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Weak-coupling Mean-field Theory of Magnetic Properties of NiGa$_2$S$_4$

We report a mean-field theoretical study of a triangular lattice magnet NiGa$_2$S$_4$. Specifically, spiral mean-field theory is applied to a 17-band $d$-$p$ model constructed from the maximally localized Wannier functions. Our itinerant-model approach shows that the most stable spiral magnetic state has an ordering vector near ${\bf Q}=(0.15,0.15,0)$, consistent with neutron scattering experiments, when we assume the Ni-site Coulomb interaction is not so strong ($U \approx 2$ eV). To map onto a classical Heisenberg spin model, we estimate spin exchange interactions from the mean-field results, and find that the nearest-neighbor exchange is ferromagnetic and the largest (larger than the third nearest-neighbor exchange, in contrast to early studies). We also calculate the dynamical spin correlation function $S({\bf q}, ω)$, using the same model within the random-phase approximation (RPA). Calculated $S({\bf q}, ω)$ has a spectral structure quite different from that of conventional spin-wave excitations.