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

Abstract

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 $\mathbf{Q}=(0.15,0.15,0)$, 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 $S(\mathbf{q},\omega )$, using the same model within the random-phase approximation (RPA). Calculated $S(\mathbf{q},\omega )$ has a spectral structure quite different from that of conventional spin-wave excitations.

arXiv comment23 pages, 6 figures. To be published in J. Phys. Soc. Jpn

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