Spin dynamics and spin freezing in the triangular lattice antiferromagnets FeGa ₂S ₄ and NiGa ₂S ₄

Magnetic susceptibility and muon spin relaxation (μSR) experiments have been carried out on the quasi-2D triangular-lattice spin S=2 antiferromagnet FeGa ₂S ₄. The μSR data indicate a sharp onset of a frozen or nearly frozen spin state at T ^*=31(2) K, twice the spin-glass-like freezing temperature T _f=16(1) K. The susceptibility becomes field dependent below T ^*, but no sharp anomaly is observed in any bulk property. A similar transition is observed in μSR data from the spin-1 isomorph NiGa ₂S ₄. In both compounds the dynamic muon spin relaxation rate λ _d(T) above T ^* agrees well with a calculation of spin-lattice relaxation by Chubukov, Sachdev, and Senthil in the renormalized classical regime of a 2D frustrated quantum antiferromagnet. There is no firm evidence for other mechanisms. At low temperatures, λ _d(T) becomes temperature independent in both compounds, indicating persistence of spin dynamics. Scaling of λ _d(T) between the two compounds is observed from ∼T _f to ∼1.5T ^*. Although the μSR data by themselves cannot exclude a truly static spin component below T ^*, together with the susceptibility data they are consistent with a slowly fluctuating "spin gel" regime between T _f and T ^*. Such a regime and the absence of a divergence in λ _d(T) at T ^* are features of two unconventional mechanisms: (1) binding/unbinding of Z ₂ vortex excitations, and (2) impurity spins in a nonmagnetic spin-nematic ground state. The absence of a sharp anomaly or history dependence at T ^* in the susceptibility of FeGa ₂S ₄, and the weakness of such phenomena in NiGa ₂S ₄, strongly suggest transitions to low-temperature phases with unconventional dynamics.