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Local magnetic measurements are used to quantitatively characterizeheterogeneity and flux line pinning in PrFeAsO_1-y and NdFeAs(O,F)superconducting single crystals. In spite of spatial fluctuations of thecritical current density on the macroscopic scale, it is shown that the majorcontribution comes from collective pinning of vortex lines by microscopicdefects by the mean-free path fluctuation mechanism. The defect densityextracted from experiment corresponds to the dopant atom density, which meansthat dopant atoms play an important role both in vortex pinning and inquasiparticle scattering. In the studied underdoped PrFeAsO_1-y and NdFeAs(O,F)crystals, there is a background of strong pinning, which we attribute tospatial variations of the dopant atom density on the scale of a few dozen toone hundred nm. These variations do not go beyond 5% - we therefore do not findany evidence for coexistence of the superconducting and the antiferromagneticphase. The critical current density in sub-T fields is characterized by thepresence of a peak effect, the location of which in the (B,T)-plane isconsistent with an order-disorder transition of the vortex lattice.

Flux pinning in (1111) iron-pnictide superconducting crystals