  In this Letter, we report the results of ESR measurements in high magnetic
fields up to about 53 T on single crystals of NiGa2S4 to clarify the spin
dynamics in more detail. We have found that the dynamics of Z2 vortices affects
the temperature dependence of the ESR absorption linewidth and the frequency
dependence of the ESR resonance fields at 1.3 K is well explained by a
conventional spin wave theory. These results suggest an occurrence of Z2
vortex-induced topological transition.
