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 NiGa2S4. From the temperature evolution of the ESR absorption linewidth, we find a distinct disturbance of the development of the spin correlation by Z(2)-vortices between 23 and 8.5 K. Below T-v = 8.5 K, spin-wave calculations based on a 57 degrees spiral spin order well explain the frequency dependence of the ESR resonance fields and high-field magnetization processes for H parallel to c and H perpendicular to c, although the magnetization for H perpendicular to c at high fields is different from the calculated one. Furthermore, we explain the field-independent specific heat with a T-2 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) = 0.8 K is the same order of T-V(xi/a)(-2) = 0.3-0.4 K where xi and a are the correlation length and the lattice constant of the triangular plane in NiGa2S4, respectively. The relation D/k(B) <= T-V(xi/ 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 Z(2)-vortex-induced topological transition, which would be destroyed by an extremely small D not equal 0 in a classical nearest-neighbor spin model owing to the large xi. Accordingly, these results suggest the occurrence of a Z(2)-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.