Antiferromagnetic order in CaK(Fe1−xNix)4As4 and its interplay with s

Abstract

The magnetic order in CaK(Fe1−xNix)4As4 (1144) single crystals (x = 0.051 and 0.033) has been studied by neutron diffraction. We observe magnetic Bragg peaks associated to the same propagation vectors as found for the collinear stripe antiferromagnetic (AFM) order in the related BaFe2As2 (122) compound. The AFM state in 1144 preserves tetragonal symmetry and only a commensurate, non-collinear structure with a hedgehog spin-vortex crystal (SVC) arrangement in the Fe plane and simple AFM stacking along the c direction is consistent with our observations. The SVC order is promoted by the reduced symmetry in the FeAs layer in the 1144 structure. The long-range SVC order coexists with superconductivity, however, similar to the doped 122 compounds, the ordered magnetic moment is gradually suppressed with the developing superconducting order parameter. This supports the notion that both collinear and non-collinear magnetism and superconductivity are competing for the same electrons coupled by Fermi surface nesting in iron arsenide superconductors.PACS numbers:

Microscopic Modeling of Correlated Systems Under Pressure: Representative Examples

“…Interestingly, the glide‐plane symmetry, typical for the 122 systems, is broken in the 1144 structure and two types of As sites are present. This symmetry breaking effect was argued to stabilize the novel magnetic state in electron‐doped CaKFe 4 As 4 – the spin‐vortex order (Figure b) – which is in contrast to the stripe order commonly observed in 122 systems (Figure a). Although the parent CaKFe 4 As 4 is not magnetically ordered, it is believed to be close to the antiferromagnetic quantum critical point and shows spin‐vortex‐like fluctuations of local Fe moments, confirmed by NMR experiments and ab initio calculations .…”

“…Very recently a new AeA Fe 4 As 4 family (1144) of FeBSC (space group , No. 123) with mixed alkaline‐earth ( Ae ) and alkali ( A ) spacer layers has been synthesized and a few of its members have been studied in the context of superconductivity and structural transitions under pressure . Compared to the 122‐type systems, these new 1144‐type materials show a perfect periodic arrangement of two different spacer layers in an alternating order, e.g., Ca and K in CaKFe 4 As 4 , as shown in Figure b.…”

“…Hydrostatic pressure influences the degree of charge transfer between the donor TTF and acceptor CA molecules and modifies the ferroelectric properties of the material. ii) The Fe‐based superconductor AeA Fe 4 As 4 (1144) with an alternating order of the alkaline‐earth ( Ae ) and alkali ( A ) spacer layers with a ThCr 2 Si 2 structure which undergoes two half‐collapsed tetragonal transitions under pressure, as predicted by some of the authors of this review and shows unusual magnetic states upon doping . iii) The frustrated triangular‐lattice antiferromagnet Cs 2 CuCl 4 for which we provide an overview of the sensibility of the exchange coupling constants to structural details and the predicted changes in coupling constants as a function of pressure .…”

“…Also inorganic materials can show extreme sensitivity to pressure due to formation and breaking of certain covalent bonds in their crystal structures upon pressurizing. Some examples are systems with layered ThCr 2 Si 2 (so‐called “122”) structure where the formation of a XX covalent bond (X = Si, As, Se, S, P) across the cation spacer layer at a critical pressure is accompanied by a drastic volume collapse, as is the case for the Fe‐based Ae Fe 2 As 2 and the recently discovered AeA Fe 4 As 4 families of superconductors (both structures are shown in Figure 3 where Ae and A stand for alkaline‐earth and alkali elements) where the collapse transition preserves the tetragonal lattice symmetry. A further example of pressure‐sensitive inorganic materials are layered frustrated magnets such as triangular‐lattice‐based Mott insulators Cs 2 CuCl 4 , Cs 2 CuBr 4 , and honeycomb‐lattice‐based spin‐orbit‐coupled Mott insulators α‐RuCl 3 and A 2 IrO 3 , with A = Li, Na.…”

Neutron Powder Diffraction Study on the Non-Superconducting Phases of ThFeAsN1−xOx (x = 0.15, 0.6) Iron Pnictide*

“…In most families of iron pnictide or chalcogenide superconductors, a long-range antiferromagnetic order always emerges in the undoped parent compounds 4 , such as the collinear order in LaFeAsO (1111 family) 5 , BaFe 2 As 2 (122 family) 6 , Na 1−δ FeAs(111 family) 7 , the bicollinear order in Fe 1+x Te(11 family) 8 , and the √ 5 × √ 5 block order in K 2 Fe 4 Se 5 [9][10][11] . Even in the stoichiometrically hole-type superconducting system CaKFe 4 As 4 (1144 family), compensating electron doping by Ni or Co can induce a spin-vortex phase (hedgehog order) with C 4 rotation symmetry 12,13 . Another C 4 magnetic order can be found in the hole-doped Ba 1−x (K, Na) x Fe 2 As 2 systems near the optimal doping 14,15 , too.…”