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.

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

References in corpus (7)

• The quadrupolar phases of the S=1 bilinear-biquadratic Heisenberg model on the triangular lattice
• Possible ferro-spin nematic order in NiGa2S4
• Quadrupolar correlations and spin freezing in S = 1 triangular lattice antiferromagnets
• Spin dynamics and spin freezing behavior in the two-dimensional antiferromagnet NiGa${}_2$S${}_4$ revealed by Ga-NMR, NQR and $\mu$SR measurements
• Unusual superexchange pathways in a Ni triangular lattice of NiGa${}_2$S${}_4$ with negative charge-transfer energy
• Short-range magnetic ordering process for the triangular lattice compound NiGa2S4: a positive muon spin rotation and relaxation study
• Unconventional spin freezing and fluctuations in the frustrated antiferromagnet NiGa2S4