Muon spin rotation $\left(\mu \text{SR}\right)$ experiments reveal unconventional spin freezing and dynamics in the two-dimensional (2D) triangular lattice antiferromagnet ${\text{NiGa}}_2{\text{S}}_4$. Long-lived disordered Ni-spin freezing (correlation time $\gtrsim {10}^{-6}\text{s}$ at 2 K) sets in below $T_f=8.5\pm 0.5\text{K}$ 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 $\gtrsim 10\text{mT}$, in marked contrast with the field-independent specific heat. The $\mu \text{SR}$ and bulk susceptibility data indicate a well-defined 2D phase transition at $T_f$, below which ${\text{NiGa}}_2{\text{S}}_4$ is neither a conventional magnet nor a singlet spin liquid.

• Received 30 October 2008

DOI:https://doi.org/10.1103/PhysRevB.78.220403