Muon spin rotation and relaxation experiments in the pyrochlore iridate Eu 2 Ir 2 O 7 yield a welldefined muon spin precession frequency below the metal-insulator/antiferromagnetic transition temperature T M = 120 K, indicative of long-range commensurate magnetic order and thus ruling out quantum spin liquid and spin-glass-like ground states. The dynamic muon spin relaxation rate is temperature-independent between 2 K and ∼T M and yields an anomalously long Ir 4+ spin correlation time, suggesting a singular density of low-lying spin excitations. Similar behavior is found in other pyrochlores and geometrically frustrated systems, but also in the unfrustrated iridate BaIrO 3 . Eu 2 Ir 2 O 7 may be only weakly frustrated; if so, the singularity might be associated with the small-gap insulating state rather than frustration.

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 * ≃ 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 ≃ 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 Z2 topological transition in this latter system, is probably different in FeGa2S4.

Magnetic susceptibility and muon spin relaxation (µSR) experiments have been carried out on the quasi-2D triangular-lattice spin S = 2 antiferromagnet FeGa2S4. 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 NiGa2S4. 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 Z2 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 FeGa2S4, and the weakness of such phenomena in NiGa2S4, strongly suggest transitions to low-temperature phases with unconventional dynamics.