High-field electron spin resonance in the two-dimensional triangular-lattice antiferromagnet NiGa₂S₄

We report experimental results of high-field electron spin resonance (ESR) measurements in magnetic fields up to about 53 T on the quasi-two-dimensional triangular-lattice antiferromagnet NiGa2 S4. From the temperature evolution of the ESR absorption linewidth, we find three distinct temperature regions: those (i) above 23 K, (ii) between 23 K and 8.5 K, and (iii) below 8.5 K. The linewidth is affected by the dynamics of Z2 vortices below 23 K and one of the conventional spiral resonance modes explains well the frequency dependence of the ESR resonance fields far below 8.5 K. Furthermore, we found an anomaly in the magnetization curve around one third of the saturation magnetization for H⊥c, corresponding to softening of another resonance mode. These results suggest the occurrence of a Z2 vortex-induced topological transition at 8.5 K.