TY - JOUR
T1 - Spin dynamics and spin freezing in the triangular lattice antiferromagnets FeGa 2S 4 and NiGa 2S 4
AU - Zhao, Songrui
AU - Dalmas De Réotier, P.
AU - Yaouanc, A.
AU - MacLaughlin, D. E.
AU - MacKie, J. M.
AU - Bernal, O. O.
AU - Nambu, Y.
AU - Higo, T.
AU - Nakatsuji, S.
PY - 2012/8/27
Y1 - 2012/8/27
N2 - 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.
AB - 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.
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U2 - 10.1103/PhysRevB.86.064435
DO - 10.1103/PhysRevB.86.064435
M3 - Article
AN - SCOPUS:84865654217
VL - 86
JO - Physical Review B - Condensed Matter and Materials Physics
JF - Physical Review B - Condensed Matter and Materials Physics
SN - 0163-1829
IS - 6
M1 - 064435
ER -