Vortices in superconducting Ba(Fe_0.93Co_0.07)₂As₂ studied via small-angle neutron scattering and Bitter decoration

We present small-angle neutron scattering (SANS) and Bitter decoration studies of the superconducting vortices in Ba(Fe_0.93Co_0.07)₂As₂. A highly disordered vortex configuration is observed at all measured fields and is attributed to strong pinning. This conclusion is supported by the absence of a Meissner rim in decoration images obtained close to the sample edge. The field dependence of the magnitude of the SANS scattering vector indicates vortex lattice domains of (distorted) hexagonal symmetry, consistent with the decoration images which show primarily sixfold coordinated vortex domains. An analysis of the scattered intensity shows that this decreases much more rapidly than expected from estimates of the upper critical field, consistent with the large degree of disorder.