Impact of local-moment fluctuations on the magnetic degeneracy of iron arsenide superconductors

We investigate the fate of the orthorhombic stripe-type magnetic state [ordering vectors (π,0)/(0,π)], observed in most iron-pnictide superconductors, in the presence of localized magnetic moments that tend to form a Néel state [ordering vector (π,π)]. We show that before long-range Néel order sets in, the coupling between the conduction electrons and the fluctuations of the local moments favors an unusual magnetic state consisting of a coherent superposition of the (π,0) and (0,π) orders that preserves tetragonal symmetry. The magnetization of this state is nonuniform and induces a simultaneous checkerboard charge order. We discuss signatures of this magnetic configuration on the electronic spectrum and its impact on the superconducting state, showing that its phase space for coexistence with the s+ state is smaller than the stripe-type state. Our results shed light on recent experimental observations on Ba(Fe1-xMnx)2As2 compounds, where the Néel-type local Mn moments interact with the Fe conduction electrons.