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

TL;DR

This study uses a mean-field approach to analyze the magnetic properties of NiGa$_2$S$_4$, revealing a stable spiral magnetic state consistent with experiments and providing insights into exchange interactions and spin dynamics.

Contribution

It introduces a spiral mean-field theory applied to a 17-band model for NiGa$_2$S$_4$, offering a detailed itinerant-model perspective on its magnetic properties.

Findings

Stable spiral magnetic state with wave vector near (0.15, 0.15, 0)

Nearest-neighbor exchange is ferromagnetic and dominant

Calculated spin excitation spectrum differs from conventional spin waves

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