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Evidence for an exotic magnetic transition in the triangular spin system FeGa2S4

We report positive muon spin relaxation measurements on the triangular lattice magnetic system FeGa2S4. A magnetic transition not previously detected by specific heat and magnetic susceptibility measurements is found in zero field at T^* \simeq 30 K. It is observed through the temperature dependencies of the signal amplitude and the spin-lattice relaxation rate. This transition is therefore not a conventional magnetic phase transition. Since persistent spin dynamics is observed down to 0.1 K, the ground state cannot be of the canonical spin-glass type, which could be suggested from hysteresis effects in the bulk susceptibility below T_f \simeq 16 K. These results are compared to those found for the isomorph NiGa2S4. It is argued that the fate of the transition, which has been interpreted in terms of the Z_2 topological transition in this latter system, is probably different in FeGa2S4.