  Spin dynamics evolution of BaFe$_2$(As$_{1-x}$P$_x$)$_2$ was probed as a
function of P concentration via $^{31}$P NMR. Our NMR study reveals that
two-dimensional antiferromagnetic (AF) fluctuations are notably enhanced with
little change in static susceptibility on approaching the AF phase from the
superconducting dome. Moreover, magnetically ordered temperature $\theta$
deduced from the relaxation rate vanishes at optimal doping. These results
provide clear-cut evidence for a quantum-critical point (QCP), suggesting that
the AF fluctuations associated with the QCP play a central role in the
high-$T_c$ superconductivity.
