  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 NiGa$_2$S$_4$.
  From the temperature evolution of the ESR absorption linewidth, we find a
distinct disturbing of the development of the spin correlation by
$Z_2$-vortices between 23 K and 8.5 K. Below $T_{\rm{v}}=8.5$ K, spin-wave
calculations based on a 57$^{\circ}$ spiral spin order well explains the
frequency dependence of the ESR resonance fields and high field magnetization
processes for $H$$\parallel$$c$ and $H$$\perp$$c$, although the magnetization
for $H$$\perp$$c$ at high fields is different from the calculated one.
Furthermore, we explain the field independent specific heat with
$T^2$-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 $Z_2$ vortex-induced topological transition at
$T_{\rm{v}}$ and may indicate quantum effects beyond the descriptions based on
the above classical spin models.
