Short-range magnetic ordering process for the triangular-lattice compound Ni Ga2 S4: A positive muon spin rotation and relaxation study

Abstract

We report a study of the triangular-lattice Heisenberg magnet Ni Ga2 S4 by the positive muon spin 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 < Tcross, where Tcross =12.6 K, which is a signature of a multichannel relaxation for this temperature range; and (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. © 2008 The American Physical Society.