  The pinning of quantized flux lines, or vortices, in the mixed state is used
to quantify the effect of impurities in iron-based superconductors (IBS).
Disorder at two length scales is relevant in these materials. Strong flux
pinning resulting from nm-scale heterogeneity of the superconducting properties
leads to the very disordered vortex ensembles observed in the IBS, and to the
pronounced maximum in the critical current density jc at low magnetic fields.
Disorder at the atomic scale, most likely induced by the dopant atoms, leads to
"weak collective pinning" and a magnetic field-independent contribution jcoll.
The latter allows one to estimate quasi-particle scattering rates.
