High Field ESR and Magnetization of the Triangular Lattice Antiferromagnet NiGa2S4
Abstract
We report the experimental and the analytical results of electron spin resonance (ESR) and magnetization in high magnetic fields up to about 68 T of the quasi two-dimensional triangular lattice antiferromagnet NiGa2S4. From the temperature evolution of the ESR absorption linewidth, we find a distinct disturbing of the development of the spin correlation by Z2-vortices between 23 K and 8.5 K. Below Tv=8.5 K, spin-wave calculations based on a 57 spiral spin order well explains the frequency dependence of the ESR resonance fields and high field magnetization processes for H and H, although the magnetization for H at high fields is different from the calculated one. Furthermore, we explain the field independent specific heat with T2-dependence by the same spin-wave calculation, but the magnitude of the specific heat is much less than the observed one. Accordingly, these results suggest the occurrence of a Z2 vortex-induced topological transition at Tv and may indicate quantum effects beyond the descriptions based on the above classical spin models.