Elucidating the nature of the magnetic ground state of iron-based superconductors is of paramount importance in unveiling the mechanism behind their high temperature superconductivity. Until recently, it was thought that superconductivity emerges only from an orthorhombic antiferromagnetic stripe phase, which can in principle be described in terms of either localized or itinerant spins. However, we recently reported that tetragonal symmetry is restored inside the magnetically ordered state of a hole-doped BaFe2As2. This observation was interpreted as indirect evidence of a new double-Q magnetic structure, but alternative models of orbital order could not be ruled out. Here, we present Mössbauer data that show unambiguously that half of the iron sites in this tetragonal phase are non-magnetic, establishing conclusively the existence of a novel magnetic ground state with a non-uniform magnetization that is inconsistent with localized spins. We show that this state is naturally explained as the interference between two spin-density waves, demonstrating the itinerant character of the magnetism of these materials and the primary role played by magnetic over orbital degrees of freedom.

Combined neutron and x-ray diffraction experiments demonstrate the formation of a lowtemperature minority magnetic tetragonal phase in Ba0.76K0.24Fe2As2 in addition to the majority magnetic, orthorhombic phase. The coincident enhancement in the magnetic ( 1 2 1 2 1) peaks shows that this minority phase is of the same type that was observed in Ba1−xNaxFe2As2 (0.24 ≤ x ≤ 0.28), in which the magnetic moments reorient along the c-axis. This is evidence that the tetragonal magnetic phase is a universal feature of the hole-doped iron-based superconductors. The observations suggest that in this regime the energy levels of the C2 and C4 symmetric magnetic phases are very close.

The recently discovered C4 tetragonal magnetic phase in hole-doped members of the iron-based superconductors provides new insights into the origin of unconventional superconductivity. Previously observed in Ba1-xAxFe2As2 (with A = K, Na), the C4 magnetic phase exists within the well studied C2 spin-density wave (SDW) dome, arising just before the complete suppression of antiferromagnetic (AFM) order but after the onset of superconductivity. Here, we present detailed x-ray and neutron diffraction studies of Sr1−xNaxFe2As2 (0.10 ≤ x ≤ 0.60) to determine their structural evolution and the extent of the C4 phase. Spanning ∆x ∼ 0.14 in composition, the C4 phase is found to extend over a larger range of compositions, and to exhibit a significantly higher transition temperature, Tr ∼ 65K, than in either of the other systems in which it has been observed. The onset of this phase is seen near a composition (x ∼ 0.30) where the bonding angles of the Fe2As2 layers approach the perfect 109.46• tetrahedral angle. We discuss the possible role of this return to a higher symmetry environment for the magnetic iron site in triggering the magnetic reorientation and the coupled re-entrance to the tetragonal structure. Finally, we present a new phase diagram, complete with the C4 phase, and use its observation in a third hole-doped 122 system to suggest the universality of this phase.