Argonne scientists discover new magnetic phase in iron-based superconductors
A neutron diffraction image giving evidence for the new magnetic phase in iron-based superconductors discovered by Argonne scientists. It shows the scattering results from a sample of barium iron arsenide with sodium ions added to 24% of the barium sites. Nematic order sets in below 90 K (about -300°F), but four-fold symmetry is restored below 40 K (-387°F). The resulting atomic and magnetic structures are illustrated in the figure on the right, in which the blue spheres represent iron atoms and the red arrows show the direction of their magnetic moments.
Neutron diffraction from a polycrystalline sample of BaFe2As2 with sodium doped onto 24% of the barium sites. The panels show how the intensity of three diffraction peaks vary with temperature as the atomic and magnetic structures change. These structures are shown schematically on the right, with the blue balls representing iron atoms and the red arrows the direction of their magnetic moments.
The first peak, labelled (112), is determined by the Fe-Fe bond lengths, which split into two when nematic order is established at TN, but become equal again when nematic order is suppressed below Tr. The second peak, labelled (½½3), shows the onset at TN of magnetic order with two-fold symmetry, while the third peak, labelled (½½1), shows that the magnetic moments are reoriented below Tr to restore four-fold symmetry. The measurements show that magnetic order coexists with superconductivity below Tc.).
Credit: Image by Jared Allred / Argonne National Laboratory.