Competing superconducting and magnetic order parameters and   field-induced magnetism in electron doped   Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$
  We have studied the magnetic and superconducting properties of Ba(Fe$_{0.95}$Co$_{0.05}$)$_{2}$As$_{2}$ as a function of temperature and external magnetic field using neutron scattering and muon spin rotation. Below the superconducting transition temperature the magnetic and superconducting order parameters coexist and compete. A magnetic field can significantly enhance the magnetic scattering in the superconducting state, roughly doubling the Bragg intensity at 13.5 T. We perform a microscopic modelling of the data by use of a five-band Hamiltonian relevant to iron pnictides. In the superconducting state, vortices can slow down and freeze spin fluctuations locally. When such regions couple they result in a long-range ordered antiferromagnetic phase producing the enhanced magnetic elastic scattering in agreement with experiments. 
J. Larsen, B. Mencia Uranga, G. Stieber, S. L. Holm, C. Bernhard, T.   Wolf, K. Lefmann, B. M. Andersen and C. Niedermayer
cond-mat.supr-con