Quasiclassical Theory of C4-Symmetric Magnetic Order in Disordered Multiband Metals

Recent experimental studies performed in the normal state of iron-based superconductors have discovered the existence of the C-4-symmetric (tetragonal) itinerant magnetic state. This state can be described as a spin density wave with two distinct magnetic vectors Q -> 1 and Q -> 2. Given an itinerant nature of magnetism in iron-pnictides, we develop a quasiclassical theory of tetragonal magnetic order in disordered three-band metal with anisotropic band structure. Within our model we find that the C-4-symmetric magnetism competes with the C-2-symmetric state with a single Q -> magnetic structure vector. Our main results is that disorder promotes tetragonal magnetic state which is in agreement with earlier theoretical studies.