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.

We present a comprehensive overview of vortex pinning in single crystals of the isovalently substituted iron-based superconductor BaFe2(As1−xPx)2, a material that qualifies as an archetypical clean superconductor, containing only sparse strong point-like pins [in the sense of C.J. van der Beek et al., Phys. Rev. B 66, 024523 (2002)]. Widely varying critical current values for nominally similar compositions show that flux pinning is of extrinsic origin. Vortex configurations, imaged using the Bitter decoration method, show less density fluctuations than those previously observed in charge-doped Ba(Fe1−xCox)2As2 single crystals. Analysis reveals that the pinning force andenergy distributions depend on the P-content x. However, they are always much narrower than in Ba(Fe1−xCox)2As2, a result that is attributed to the weaker temperature dependence of the superfluid density on approaching Tc in BaFe2(As1−xPx)2. Critical current density measurements and pinning force distributions independently yield a mean distance between effective pinning centers L ∼ 90 nm, increasing with increasing P-content x. This evolution can be understood as being the consequence of the P-dependence of the London penetration depth. Further salient features are a wide vortex free "Meissner belt", observed at the edge of overdoped crystals, and characteristic chain-like vortex arrangements, observed at all levels of P-substitution.