Unconventional spin freezing and fluctuations in the frustrated antiferromagnet NiGa2S4

Muon spin rotation (mu SR) experiments reveal unconventional spin freezing and dynamics in the two-dimensional (2D) triangular lattice antiferromagnet NiGa2S4. Long-lived disordered Ni-spin freezing (correlation time greater than or similar to 10(-6) s at 2 K) sets in below T-f=8.5 +/- 0.5 K with a mean-field-like temperature dependence. The observed exponential temperature dependence of the muon spin relaxation above T-f is strong evidence for 2D critical spin fluctuations. Slow Ni-spin fluctuations coexist with quasistatic magnetism at low temperatures but are rapidly suppressed for fields greater than or similar to 10 mT, in marked contrast with the field-independent specific heat. The mu SR and bulk susceptibility data indicate a well-defined 2D phase transition at T-f, below which NiGa2S4 is neither a conventional magnet nor a singlet spin liquid.