Small-angle neutron scattering study of vortex matter in superconducting Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$*

We present small-angle neutron scattering studies of the superconducting vortices Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$. At all fields measured a ring of scattering was observed, indicating a highly disordered vortex configuration, and no discernable rocking curve could be measured. The field dependence of the magnitude of the scattering vector indicates vortex lattice domains of (distorted) hexagonal symmetry. An analysis of the scattered intensity due to the vortices shows a rapid decrease with increasing applied magnetic field, significantly exceeding what would be expected based on estimates of the upper critical field. These results are consistent with the existence of a vortex glass or Bragg glass phase in Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$.