High-field ESR and magnetization of the triangular lattice antiferromagnet NiGa₂S₄

We report the experimental and analytical results of electron spin resonance (ESR) and magnetization at high magnetic fields of up to about 68 T of the quasi two-dimensional triangular lattice antiferromagnet NiGa ₂S₄. From the temperature evolution of the ESR absorption linewidth, we find a distinct disturbance of the development of the spin correlation by Z₂-vortices between 23 and 8.5 K. Below T_v = 8.5 K, spin-wave calculations based on a 57° spiral spin order well explain the frequency dependence of the ESR resonance fields and high-field magnetization processes for H ∥ c and H ⊥ c, although the magnetization for H ⊥ c at high fields is different from the calculated one. Furthermore, we explain the field-independent specific heat with a T² dependence by the same spin-wave calculation, but the magnitude of specific heat is much less than the observed one. The single ion anisotropy constant D/k_B = 0.8K is the same order of T_V(ξ/a)^-2 = 0.3-0.4K where ξ and a are the correlation length and the lattice constant of the triangular plane in NiGa₂S₄, respectively. The relation D/k_B ≤ T_V(ξ/a)^-2 recently derived by Kawamura et al. [H. Kawamura et al.: J. Phys. Soc. Jpn. 79 (2010) 023701] is a necessary condition for the realization of a Z₂-vortex- induced topological transition, which would be destroyed by an extremely small D ≠ 0 in a classical nearest-neighbor spin model owing to the large ξ. Accordingly, these results suggest the occurrence of a Z₂-vortex- induced topological transition at T_v and indicate quantum effects beyond the descriptions based on the above classical spin models, for example, those due to quadrupolar correlations.