Transition from Three-Dimensional Anisotropic Spin Excitations to Two-Dimensional Spin Excitations by Electron Doping the FeAs-Based BaFe_1.96Ni_0.04As₂ Superconductor

We use neutron scattering to study the effect of electron doping on the structural or magnetic order in BaFe₂As₂. In the undoped state, BaFe₂As₂ exhibits simultaneous structural and magnetic phase transitions below 143 K. Upon electron doping to form BaFe_1.96Ni_0.04As₂, the system first displays the lattice distortion near ∼97  K, and then orders antiferromagnetically at 91 K before developing weak superconductivity below ∼15  K. The effect of electron doping is to reduce the c-axis exchange coupling in BaFe₂As₂ and induce quasi-two-dimensional (2D) spin excitations. These results suggest that the transition from 3D spin waves to quasi-2D spin excitations by electron doping is important for the separated structural and magnetic phase transitions in iron arsenides.