  Neutron and x-ray diffraction studies show that the simultaneous first-order
transition to an orthorhombic and antiferromagnetic (AFM) ordered state in
BaFe2As2 splits into two transitions with Co doping. For
Ba(Fe0.953Co0.047)2As2, a tetragonal-orthorhombic transition occurs at TS = 60
K, followed by a second-order transition to AFM order at TN = 47 K.
Superconductivity (SC) occurs in the orthorhombic state below TC = 15 K and
coexists with AFM. Below TC, the static Fe moment is reduced and a 4 meV spin
gap develops indicating competition between coexisting SC and AFM order.
