Anisotropic 2D Spin Fluctuations in Superconducting Ba(Fe$_{0.926}$Co$_{0.074}$)$_2$As$_2$*

We report inelastic neutron scattering from superconducting Ba(Fe$_{0.926}$Co$_{0.074}$)$_2$As$_2$. For energies up to 150 meV, magnetic excitations are found close to the 2D antiferromagnetic (AFM) wavevector $\mathbf{Q}_{AFM} = $ (0.5 0.5) in the tetragonal symmetry of the parent compound BaFe$_2$As$_2$. We observe an anisotropy of the in-plane spin fluctuations, allowing an estimate of the effective AFM spin exchange interactions: $J_2/J_1 = 1.0 (3)$ ($J_1$: NN; $J_2$: NNN). The 8 meV spin resonance displays a similar in-plane anisotropy. Above 100 meV, the anisotropy evolves into two separated maxima displaced transverse to $\mathbf{Q}_{AFM}$. While the low energy results can be interpreted in terms of competing exchange interactions, the more complex high energy/wavevector dependence requires an itinerant model.