Observation of the topological phase transition in the triangular Heisenberg antiferromagnet NiGa2S4

It has been shown in 2005 by neutron powder diffraction that the triangular layer Heisenberg compound NiGa2S4 is characterized by nano-scale magnetic correlations below similar to 18 K. Using the muon-spin rotation and relaxation (mu SR) techniques, we have evidenced that the compound displays a phase transition at T-c = 9.2 (2) K below which a coherent oscillation of the muon spins is detected. Remarkably, the magnetic specific heat C-m exhibits a rounded low-temperature maximum at similar to 13 K, i.e. at a temperature above T-c. This is consistent with the theory of Kawamura and Miyashita which attributes the transition at T-c to the unbinding of Z(2) vortices. NiGa2S4 is characterized by a wide spectrum of magnetic fluctuations as inferred, for example, from the difference in the temperature scale deduced from mu SR and neutron scattering.