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.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - 2012 Aug 27|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics