  We report a study of the triangular lattice Heisenberg magnet NiGa2S4 by the
positive muon spin rotation and relaxation technique. We unravel three
temperature regimes: (i) below T_c = 9.2(2) K a spontaneous static magnetic
field at the muon site is observed and the spin dynamics is appreciable: the
time scale of the modes we probe is ~ 7 ns; (ii) an unconventional stretched
exponential relaxation function is found for T_c < T < T_{cross} where
T_{cross} = 12.6 K, which is a signature of a multichannel relaxation for this
temperature range; (iii) above T_{cross}, the relaxation is exponential as
expected for a conventional compound. The transition at T_c is of the
continuous type. It occurs at a temperature slightly smaller than the
temperature at which the specific heat displays a maximum at low temperature.
This is reminiscent of the behavior expected for the
Berezinskii-Kosterlitz-Thouless transition. We argue that these results reflect
the presence of topological defects above T_c.
