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

Abstract

The recently discovered $C_4$ tetragonal magnetic phase in hole-doped members of the iron-based superconductors provides new insights into the origin of unconventional superconductivity. Previously observed in Ba${}_{1-x}{A}_x$Fe${}_2$As${}_2$ (with $A=$ K, Na), the $C_4$ magnetic phase exists within the well studied $C_2$ 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 Sr${}_{1-x}$Na${}_x$Fe${}_2$As${}_2$ ($0.10\le x\le 0.60$) to determine their structural evolution and the extent of the $C_4$ phase. Spanning $\Delta x\sim 0.14$ in composition, the $C_4$ phase is found to extend over a larger range of compositions, and to exhibit a significantly higher transition temperature, $T_r\sim 65$K, than in either of the other systems in which it has been observed. The onset of this phase is seen near a composition ($x\sim 0.30$) where the bonding angles of the Fe${}_2$As${}_2$ layers approach the perfect $109.46^{\circ }$ 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 $C_4$ phase, and use its observation in a third hole-doped 122 system to suggest the universality of this phase.

14 pages, 10 figures, 2 tables