  We investigate the phase transition dynamics of a quasi-2D antiferromagnetic
spin-1 Bose-Einstein condensate from the easy-axis polar phase to the
easy-plane polar phase, which is initiated by suddenly changing the sign of the
quadratic Zeeman energy $q$. We observe the emergence and decay of spin
turbulence and the formation of half-quantum vortices (HQVs) in the quenched
condensate. The characteristic time and length scales of the turbulence
generation dynamics are proportional to $|q|^{-1/2}$ as inherited from the
dynamic instability of the initial state. In the evolution of the spin
turbulence, spin wave excitations develop from large to small length scales,
suggesting a direct energy cascade, and the spin population for the axial polar
domains exhibit a nonexponential decay. The final equilibrated condensate
contains HQVs, and the number is found to increase and saturate with increasing
$|q|$. Our results demonstrate the time-space scaling properties of the phase
transition dynamics near the critical point and the peculiarities of the spin
turbulence state of the antiferromagnetic spinor condensate.
