Impact of the lattice on magnetic properties in the S=1 triangular antiferromagnet NiGa₂S₄

NiGa₂S₄ is a triangular lattice S=1 system with strong two dimensionality of the lattice, and a competition between ferromagnetic nearest neigbor exchange J₁ and antiferromagnetic third nearest neigbor exchange J₃. It was actively discussed as a candidate to host spin-nematic order brought about by strong quadrupole coupling. Using Raman scattering spectroscopy we identify a phonon of E_g symmetry which can modulate magnetic exchange J₁ and produce quadrupole coupling. Additionally, our Raman scattering results demonstrate a gradual loss of local inversion symmetry on lowering the temperature of the sample. We associate it with a lattice distortion due to sulfur vacancies. This loss of inversion symmetry will lead to disordered Dzyaloshinskii-Moriya interactions, which can prevent long-range magnetic order. Using magnetic Raman scattering response we identify 160 K as a temperature of an upturn of magnetic correlations. The temperature range below 160 K, but above 50 K where antiferromagnetic correlations start to increase, is a candidate for spin-nematic regime.