RbEu(Fe1−xNix)4As4 : From a ferromagnetic superconductor to a superconducting ferromagnet

Abstract

The intrinsically hole-doped RbEuFe4As4 exhibits bulk superconductivity at Tsc = 36.5 K and ferromagnetic ordering in the Eu sublattice at Tm = 15 K. Here we present a hole-compensation study by introducing extra itinerant electrons via a Ni substitution in the ferromagnetic superconductor RbEuFe4As4 with Tsc > Tm. With the Ni doping, Tsc decreases rapidly, and the Euspin ferromagnetism and its Tm remain unchanged. Consequently, the system RbEu(Fe1−xNix)4As4 transforms into a superconducting ferromagnet with Tm > Tsc for 0.07 ≤ x ≤ 0.08. The occurrence of superconducting ferromagnets is attributed to the decoupling between Eu 2+ spins and superconducting Cooper pairs. The superconducting and magnetic phase diagram is established, which additionally includes a recovered yet suppressed spin-density-wave state.

Microscopic Modeling of Correlated Systems Under Pressure: Representative Examples

“…This kind of spin fluctuations were later on confirmed for CaKFe 4 As 4 by NMR measurements . For the case of superconducting EuRbFe 4 As 4 and EuCsFe 4 As 4 , experimental signatures of long‐range Eu magnetic order are found below 15 K at zero pressure . Similarly to EuFe 2 As 2 , Eu order in 1144 systems is expected to be sensitive to pressure, as suggested by a recent enthalpy analysis and high‐pressure measurements that reveal the persistence of Eu order and a significant raise of ordering temperature up to 40–45 K around 20 GPa .…”

“…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.…”

“…Further below, we discuss the results of pressure simulations and analyze the electronic structure of CaKFe 4 As 4 .In the next step, we review the general trends in the whole 1144 family with a focus on the hcT transitions . In addition, we discuss the magnetic behavior of Eu‐based 1144 systems that are attracting increasing attention due to the coexistence and complex interplay of ferromagnetism and superconductivity …”

“…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.…”