  Measurements of magneto-resistivity and magnetic susceptibility were
performed on single crystals of superconducting
Ba(Fe$_{0.9}$Co$_{0.1}$)$_{2}$As$_{2}$ close to the conditions of optimal
doping. The high quality of the investigated samples allows us to reveal a
dynamic scaling behaviour associated with a vortex-glass phase transition in
the limit of weak degree of quenched disorder. Accordingly, the dissipative
component of the ac susceptibility is well reproduced within the framework of
Havriliak-Negami relaxation, assuming a critical power-law divergence for the
characteristic correlation time $\tau$ of the vortex dynamics. Remarkably, the
random disorder introduced by the Fe$_{1-x}$Co$_{x}$ chemical substitution is
found to act on the vortices as a much weaker quenched disorder than previously
reported for cuprate superconductors such as, e.g.,
Y$_{1-x}$Pr$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$.
