  The disordered flux line lattice in single crystals of the slightly overdoped
aFe_{2-x}Co_xAs_2 (x = 0.19, Tc = 23 K) superconductor is studied by
magnetization measurements, small-angle neutron scattering (SANS), and magnetic
force microscopy (MFM). In the whole range of magnetic fields up to 9 T, vortex
pinning precludes the formation of an ordered Abrikosov lattice. Instead, a
vitreous vortex phase (vortex glass) with a short-range hexagonal order is
observed. Statistical processing of MFM datasets lets us directly measure its
radial and angular distribution functions and extract the radial correlation
length \zeta. In contrast to predictions of the collective pinning model, no
increase in the correlated volume with the applied field is observed. Instead,
we find that \zeta decreases as 1.3*R1 ~ H^(-1/2) over four decades of the
applied magnetic field, where R1 is the radius of the first coordination shell
of the vortex lattice. Such universal scaling of \zeta implies that the vortex
pinning in iron arsenides remains strong even in the absence of static
magnetism. This result is consistent with all the real- and reciprocal-space
vortex-lattice measurements in overdoped as-grown aFe_{2-x}Co_xAs_2 published
to date and is thus sample-independent. The failure of the collective pinning
model suggests that the vortices remain in the single-vortex pinning limit even
in high magnetic fields up to 9 T.
