Unconventional spin freezing and fluctuations in the frustrated antiferromagnetNiGa2S4

Abstract

Muon spin rotation $(\ensuremath{\mu}\text{SR})$ 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 $\ensuremath{\gtrsim}{10}^{\ensuremath{-}6}\text{ }\text{s}$ at 2 K) sets in below ${T}_{f}=8.5\ifmmode\pm\else\textpm\fi{}0.5\text{ }\text{K}$ 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 $\ensuremath{\gtrsim}10\text{ }\text{mT}$, in marked contrast with the field-independent specific heat. The $\ensuremath{\mu}\text{SR}$ 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.