Magnetism of the 35 K superconductor CsEuFe4As4

Iron-based magnetic superconductorsAEuFe4As4(ARb, Cs): natural superconductor-ferromagnet hybrids

Superconductivity (SC) and ferromagnetism (FM) are normally antagonistic, and their coexistence in a single crystalline material appears to be very rare. Over a decade ago, the iron-based pnictides of doped EuFe₂As₂were found to render such a coexistence, primarily because of the Fe-3dmulti-orbitals which simultaneously satisfy the superconducting pairing and the ferromagnetic exchange interaction among Eu local spins. In 2016, the discovery of the iron-based superconductorsAEuFe₄As₄(A= Rb, Cs) provided an additional and complementary material basis for the study of the coexistence and the interplay between SC and FM. The two sibling compounds, which can be viewed as an intergrowth or a hybrid betweenAFe₂As₂and EuFe₂As₂, show SC in the FeAs bilayers atT_c= 35-37 K and magnetic ordering atT_m∼ 15 K in the sandwiched Eu²⁺-ion sheets. BelowT_m, the Eu²⁺spins align ferromagnetically within each Eu plane, making the system as a natural atomic-thick superconductor-ferromagnet superlattice. This paper reviews the main research progress in the emerging topic during the past five years. An outlook for the future research opportunities is also presented.

Spin polarized density functional theory calculations of the electronic structure and magnetism of the 112 type iron pnictide compound [Formula: see text]

Using density-functional theory, we investigate the electronic, magnetic, and hyperfine-interaction properties of the 112-type iron-pnictide compound [Formula: see text], which is isostructural to the high-temperature iron-based superconductor [Formula: see text]. We show that the band structure of [Formula: see text] is similar to that of the 112-type compounds' family, with hole-like and electron-like bands at the Brillouin-zone center and corners, respectively. We demonstrate that the bands near the Fermi level originate mainly from the Fe atoms. The presence of a mixture of ionic and covalent bonding is predicted from the charge-density and atom-resolved density-of-states calculations. There is good agreement between the calculated hyperfine-interaction parameters with those obtained from the [Formula: see text]Fe and [Formula: see text]Eu Mössbauer measurements. The spatial distribution of atoms in [Formula: see text] leads to an in-plane 2D magnetism. Moreover, ab-initio calculations predict the compound's magnetic moment and the magnetic moments of each constituent atom. Also, the density of states profile provides insight into the relative magnitude of these moments. Electronic structure calculations and Fermi surface topology reveal various physical and chemical properties of [Formula: see text]. Valence electron density maps indicate the co-existence of a wide range of chemical bonds in this system, and based on structural properties, the transport characteristics are deduced and discussed. A thorough analysis of the atomic structure of [Formula: see text] and its role in the bond formation is presented.

57Fe and 151Eu Mössbauer studies of 3d-4f spin interplay in EuFe2-xNixAs2

The EuFe_2−xNi_xAs₂ (with 0 ≤ x ≤ 0.4) compounds exhibiting 3d and/or 4f magnetic order were investigated by means of ⁵⁷Fe and ¹⁵¹Eu Mössbauer spectroscopy. Additionally, results for EuNi₂As₂ are reported for comparison. It was found that spin-density-wave order of the Fe itinerant moments is monotonically suppressed by Ni-substitution. However, the 3d magnetic order survives at the lowest temperature up to at least x = 0.12 and it is certainly completely suppressed for x = 0.20. The Eu localized moments order regardless of the Ni concentration, but undergo a spin reorientation with increasing x from alignment parallel to the a-axis in the parent compound, toward c-axis alignment for x > 0.07. Change of the 4f spins ordering from antiferromagnetic to ferromagnetic takes place simultaneously with a disappearance of the 3d spins order what is the evidence of a strong coupling between magnetism of Eu²⁺ ions and the conduction electrons of [Fe_2−xNi_xAs₂]^2- layers. The Fe nuclei experience the transferred hyperfine magnetic field due to the Eu²⁺ ordering for Ni-substituted samples with x > 0.04, while the transferred field is undetectable in EuFe₂As₂ and for compound with a low Ni-substitution level. It seems that the 4f ferromagnetic component arising from a tilt of the Eu²⁺ moments to the crystallographic c-axis leads to the transferred magnetic field at the Fe atoms. Superconductivity is not observed down to 1.8 K, although a comparison with ⁵⁷Fe and ¹⁵¹Eu Mössbauer data for EuFe₂As₂-based superconductors indicates a similar magnetic structure.

Physical properties of the layered oxypnictide Sr2ScFeAsO3: a Mössbauer study down to 1.7 K

A polycrystalline sample of Sr2ScFeAsO3was studied by57Fe Mössbauer spectroscopy down to 1.7 K. In contrast to the earlier Mössbauer data, the obtained in this work results indicate that Sr2ScFeAsO3is in paramagnetic state down to 10 K, while the spectra recorded at 4.6 K and 1.7 K show a weak magnetic order of Fe moments in the Fe2As2layers. Temperature dependences of isomer shift and quadrupole splitting/shift are compared with specific heat and electrical resistivity data from earlier investigations revealing different local Debye temperatures for the Fe2As2and perovskite-related Sr2ScO3layers. Finally, a fast decrease of the carrier density was observed below 80 K and this effect seems to be responsible for the absence of superconductivity in the studied compound.

Superconductivity and magnetism in RbEu(Fe1-x Co x )4As4

We studied the cobalt-doping effect on superconductivity and magnetism in a hole self-doped RbEuFe₄As₄ magnetic superconductor which shows superconductivity at [Formula: see text] 36.5 K and Eu²⁺ -spin ordering at [Formula: see text] 15 K. The Co solubility limit in RbEu(Fe_1-x Co _x )₄As₄ achieves x  =  0.21 for the solid-state reaction at 880 °C. With increasing x, [Formula: see text] decreases gradually, and superconductivity eventually disappears at [Formula: see text]. A spin-density-wave transition at [Formula: see text] 35-40 K is recovered for [Formula: see text], which can be understood in terms of the hole-depletion and the disorder effects. On the other hand, [Formula: see text] remains unchanged despite the Co doping and, consequently, an intriguing superconducting ferromagnet without Meissner state is realized in the range of 0.125 [Formula: see text] 0.155. Our results indicate that the Eu²⁺ spins essentially decouple with superconductivity over a wide doping range, making the coexistence of superconductivity and ferromagnetism possible in the 1144-type system.