  We use polarized neutron scattering to demonstrate that in-plane spin
excitations in electron doped superconducting BaFe1.904Ni0.096As2 (Tc=19.8 K)
change from isotropic to anisotropic in the tetragonal phase well above the
antiferromagnetic (AF) ordering and tetragonal-to-orthorhombic lattice
distortion temperatures (Tn=Ts=33 K) without an uniaxial pressure. While the
anisotropic spin excitations are not sensitive to the AF order and
tetragonal-to-orthorhombic lattice distortion, superconductivity induces
further anisotropy for spin excitations along the [1,1,0] and [1,-1,0]
directions. These results indicate that the spin excitation anisotropy is a
probe of the electronic anisotropy or orbital ordering in the tetragonal phase
of iron pnictides.
