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^-13-1 sec) 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*=8.5 K, but fluctuations on the microsecond time scale then persist in an anomalous dynamical regime for $4 K