Short-range magnetic ordering process for the triangular lattice compound NiGa2S4: a positive muon spin rotation and relaxation study
Abstract
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 Tc = 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 Tc < T < Tcross where Tcross = 12.6 K, which is a signature of a multichannel relaxation for this temperature range; (iii) above Tcross, the relaxation is exponential as expected for a conventional compound. The transition at Tc 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 Tc.