Neutron scattering studies of competing magnetic and superconducting order parameters in Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$

Neutron and x-ray diffraction studies show that the simultaneous first-order transition to an orthorhombic and antiferromagnetic (AFM) ordered state in BaFe$_{2}$As$_{2}$ splits into two transitions, and T$_{N }$as well as the ordered moment decrease as x ({\%}Co) increases.~ Recent experiments have demonstrated that for a range of doping, superconductivity and magnetism co-exist homogeneously and compete for electrons.~ The magnetic order parameters have been measured for a series of Co doping from x = 0 to 6.3{\%} and a clear drop in sublattice magnetization is observed at T$_{C}$ for each compound.~ Comparison of these measurements to mean field calculations has provided insight into these competing order parameters.~ The good agreement between the measurement and calculations show that neutron scattering is a useful probe for understanding the symmetry of the superconducting gap in the iron-arsenide superconductors.$^{ }$ The work at the Ames Laboratory was supported by the US DOE, office of science, under contract No. DE-AC02-07CH11358.