The fundamental pairing mechanism causing high-T superconductivity in Febased superconductors remains controversial. Superconductivity is only one of several phases exhibited by these materials, and it is widely believed that the mechanism responsible for pairing may be closely linked to the existence of other proximate ordered phases. Most of these materials are obtained by chemical doping, which besides extra carriers, introduces disorder in the system. Therefore, understanding the role of these impurities is crucial, and is a main focus of our work.

In this thesis, we start with the assumption that the observed electronic phases are captured within the itinerant electron scenario. We investigate competing spin density wave phases in homogeneous systems, but also in disordered systems where the interaction between impurity moments becomes relevant. The theory of emergent states around potentials in multi-band systems is introduced, and we propose strongly anisotropic defect states as a source of the reported transport anisotropy. Finally, we discuss unconventional correlation-driven disorder phenomena in the superconducting state, revealing a highly unusual impurity response.