Detailed magnetic and structural analysis mapping a robust magneticC4dome inSr1−xNaxFe2

Abstract

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.

Observation of the magnetic C4 phase in Ca1−xNaxFe2As2

“…12,20,30,31 In these reports we characterized the C 4 phase by an abrupt first order re-entrance at temperatures T < T N to the tetragonal I4/mmm structure from within the well known antiferromagnetically ordered orthorhombic F mmm structure. This structural transition is coupled to a magnetic transition where the previously in-plane Fe-site moments rotate to be co-linear to the c-axis forming a double-Q magnetic structure with P C 4 2 /ncm magnetic space group.…”

“…20 In Sr 1−x Na x Fe 2 As 2 the C 4 phase was found to occur in a small range of As-Fe-As bond angles where the FeAs 4 approached perfect tetrahedra. We suggested that the return locally to a higher symmetry in the Fe 2 As 2 layers might create an electronic instability which contributes to the recovery of magnetic degeneracy and a consequent return to C 4 symmetry.…”

“…30,38,40 Setting aside the composition dependence, it is somewhat unsurprising that the maximum T r for each system appears to scale with T N going from 45K to 65K to 52K for Ba, Sr and Ca. 20,30,41 Naively, the scaling of the T N of the parent compound with T r seems reasonable assuming a higher magnetic ordering temperature would require a larger amount of charge doping to disrupt. This would then extend the C 2 dome out to higher compositions and consequently allow for a more fully formed C 4 dome.…”

“…Such studies have so far found C 4 in both the well-studied Ba 1−x K x Fe 2 As 2 and Sr 1−x Na x Fe 2 As 2 materials. 14,20 The robust C 4 phase found in this latter system informs a return to the Na + doped CaFe 2 As 2 . Several reported phase diagrams for Ca 1−x Na x Fe 2 As 2 exist notably in References 21 and 22 however, their authors' use of large steps in composition mean that any C 4 phase would likely have been missed.…”

Magnetic interactions in iron superconductors: A review

“…So far three compounds have shown this reentrant tetragonal phase: Ba 1−x Na x Fe 2 As 2 [292], Ba 1−x K x Fe 2 As 2 [293], and Sr 1−x Na x Fe 2 As 2 [294]. Ba 1−x Na x Fe 2 As 2 shows a C 4 phase coexistent with the orthorhombic one just at the antiferromagnetic transition suppression for a small range of doping [292], while this tetragonal region is fully inside the C 2 one in Sr 1−x Na x Fe 2 As 2 [294]. The tetragonal phase in Ba 1−x K x Fe 2 As 2 is also at the magnetic transition but, differently to the other two cases, it is also reentrant as a function of temperature, being suppressed as the superconductivity sets in [293].…”