  The interrelation between heterogeneity and flux pinning is studied in
Ba(Fe1-xCox)2As2 single crystals with widely varying Co-content x. Magnetic
Bitter decoration of the superconducting vortex ensemble in crystals with x =
0:075 and x = 0:1 reveal highly disordered vortex structures. The width of the
Meissner belt observed at the edges of the crystals, and above the surface
steps formed by cleaving, as well as the width of the intervortex distance
distribution, indicate that the observed vortex ensemble is established at a
temperature just below the critical temperature Tc. The vortex interaction
energy and pinning force distributions extracted from the images strongly
suggest that the vortex lattice disorder is attributable to strong pinning due
to spatial fluctuations of Tc and of the superfluid density. Correlating the
results with the critical current density yields a typical length scale of the
relevant disorder of 40 - 60 nm.
