Spin Fluctuations from Hertz to Terahertz on a Triangular Lattice

The temporal magnetic correlations of the triangular-lattice antiferromagnet NiGa2S4 are examined through 13 decades (10-13-1sec) using ultrahigh-resolution inelastic neutron scattering, muon spin relaxation, and ac and nonlinear susceptibility measurements. Unlike the short-ranged spatial correlations, the temperature dependence of the temporal correlations show distinct anomalies. The spin fluctuation rate decreases precipitously upon cooling towards T∗=8.5K, but fluctuations on the microsecond time scale then persist in an anomalous dynamical regime for 4K<T≤T∗. As this time scale exceeds that of single-site dynamics by 6 orders of magnitude, these fluctuations bear evidence of emergent degrees of freedom within the short-range correlated incommensurate state of NiGa2S4.