Detailed magnetic and structural analysis mapping a robust magnetic C4 dome in Sr1-xNaxFe2As2

الأصل · EN

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 Δx 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.