Symmetry and disorder of the vitreous vortex lattice in overdopedBaFe2−xCoxAs2: Indication for strong single-vortex pinning

The disordered flux line lattice in single crystals of the slightly overdoped BaFe2−xCoxAs2 (x=0.19, Tc=23 K) superconductor is studied by magnetization measurements, small-angle neutron scattering, 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 data sets lets us directly measure its radial and angular distribution functions and extract the radial correlation length ζ. 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 ζ decreases as (1.3±0.1)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 ζ 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 BaFe2−xCoxAs2 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.