NiGa2S4, FeGa2S4, and Fe 2Ga2S5 are layered triangular lattice antiferromagnets. Although the singlelayer systems NiGa2S4 with S = 1 and FeGa2S4 with S = 2 are both insulators and have two-dimensional (2D) spin-disordered states, the bilayer system Fe2Ga2S 5, which has an effective buckled honeycomb lattice of S = 2, is a semiconductor and exhibits an antiferromagnetic long-range order at 110 K. Here, we present our results of the resistivity measurements of single crystals of FeGa2S4 and Fe2Ga2S5 under pressures of up to 8 GPa. We have observed a kink in the temperature dependence of the resistivity ρ(T) of FeGa2S4 under a pressure of 8 GPa, which is attributable to a transition from a 2D frozen spindisordered state to a three-dimensional (3D) spin-ordered state. In either FeGa2S4 or Fe2Ga2S5, we have observed no transition into a metallic state within pressure range of up to 8 GPa, despite the fact that the resistivities of both FeGa2S4 and Fe2Ga2S5 show decreases with an increase in pressure at room temperature. The energy gap of FeGa2S4 estimated from the temperature dependences of the resistivities show negative pressure dependences.
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