Europium-based iron pnictides: a unique laboratory for magnetism, superconductivity and structural effects

Strain-switchable field-induced superconductivity

Field-induced superconductivity is a rare phenomenon where an applied magnetic field enhances or induces superconductivity. Here, we use applied stress as a control switch between a field-tunable superconducting state and a robust non-field-tunable state. This marks the first demonstration of a strain-tunable superconducting spin valve with infinite magnetoresistance. We combine tunable uniaxial stress and applied magnetic field on the ferromagnetic superconductor Eu(Fe0.88Co0.12)2As2 to shift the field-induced zero-resistance temperature between 4 K and a record-high value of 10 K. We use x-ray diffraction and spectroscopy measurements under stress and field to reveal that strain tuning of the nematic order and field tuning of the ferromagnetism act as independent control parameters of the superconductivity. Combining comprehensive measurements with DFT calculations, we propose that field-induced superconductivity arises from a novel mechanism, namely, the uniquely dominant effect of the Eu dipolar field when the exchange field splitting is nearly zero.

Pressure effect on magnetism and valence in ferromagnetic superconductor Eu(Fe0.75Ru0.25)2As2

Eu(Fe_0.75Ru_0.25)₂As₂is an intriguing system with unusual coexistence of superconductivity and ferromagnetism, providing a unique platform to study the nature of such coexistence. To establish a magnetic phase diagram, time-domain synchrotron Mössbauer experiments in¹⁵¹Eu have been performed on a single crystalline Eu(Fe_0.75Ru_0.25)₂As₂sample under hydrostatic pressures and at low temperatures. Upon compression the magnetic ordering temperature increases sharply from 20 K at ambient pressure, reaching ∼49 K at 10.1 GPa. With further compression, the magnetic order is suppressed and eventually collapses. Isomer shift values from Mössbauer measurements and x-ray absorption spectroscopy data at EuL₃edge show that pressure drives Eu ions to a homogeneous intermediate valence state with mean valence of ∼2.4 at 27.4 GPa, possibly responsible for the suppression of magnetism. Synchrotron powder x-ray diffraction experiment reveals a tetragonal to collapsed-tetragonal structural transition around 5 GPa, a lower transition pressure than in the parent compound. These results provide guidance to further work investigating the interplay of superconductivity and magnetism.

Magnetic Phase Separation in the Oxypnictide Sr2Cr1.85Mn1.15As2O2

Layered Sr₂M₃As₂O₂-type oxypnictides are composed of tetrahedral M₂Pn₂ and square planar MO₂ layers, the building blocks of iron-based and cuprate superconductors. To further expand our understanding of the chemical and magnetic properties of the Sr₂Cr_3–xMn_xAs₂O₂ solid solution, Sr₂Cr₂MnAs₂O₂ has been synthesized. The compound crystallizes in the I4/mmm tetragonal space group with a refined stoichiometry of Sr₂Cr_1.85Mn_1.15As₂O₂. The M(2) site within the M₂Pn₂ slab is occupied by 42.7% Cr and 57.3% Mn, and the magnetic moments order antiferromagnetically below T_N(M2) = 540 K with a C-type antiferromagnetic structure. The M(1) site within the MO₂ layers is fully occupied by Cr, and antiferromagnetic order is observed below T_N(M1) = 200 K. Along c, there are two possible interplanar arrangements: ferromagnetic with the (1/2, 1/2, 0) propagation vector and antiferromagnetic with the (1/2, 1/2, 1/2) propagation vector. Magnetic phase separation arises so that both propagation vectors are observed below 200 K. Such magnetic phase separation has not been previously observed in Sr₂M₃As₂O₂ phases (M = Cr, Mn) and shows that there are several competing magnetic structures present in these compounds.

Layered Sr₂M₃As₂O₂-type oxypnictides are composed of tetrahedral M₂Pn₂ and square planar MO₂ layers, the building blocks of iron-based and cuprate superconductors. Sr₂Cr_1.85Mn_1.15As₂O₂ is antiferromagnetic and semiconducting. Surprisingly magnetic phase segregation is observed in the oxypnictide Sr₂Cr_1.85Mn_1.15As₂O₂ because of competing magnetic exchange interactions along c.

Pressured-induced superconducting phase with large upper critical field and concomitant enhancement of antiferromagnetic transition in EuTe2

We report an unusual pressure-induced superconducting state that coexists with an antiferromagnetic ordering of Eu²⁺ moments and shows a large upper critical field comparable to the Pauli paramagnetic limit in EuTe₂. In concomitant with the emergence of superconductivity with T_c ≈ 3–5 K above P_c ≈ 6 GPa, the antiferromagnetic transition temperature T_N(P) experiences a quicker rise with the slope increased dramatically from dT_N/dP = 0.85(14) K/GPa for P ≤ P_c to 3.7(2) K/GPa for P ≥ P_c. Moreover, the superconducting state can survive in the spin-flop state with a net ferromagnetic component of the Eu²⁺ sublattice under moderate magnetic fields μ₀H ≥ 2 T. Our findings establish the pressurized EuTe₂ as a rare magnetic superconductor possessing an intimated interplay between magnetism and superconductivity.

Here, the authors report pressure-induced superconductivity with concomitant enhancement of antiferromagnetic transition in layered EuTe₂. The superconductivity is distinctly characterized by the high upper critical fields exceeding the Pauli limit among binary tellurides, a prerequisite of the coexistence of ferromagnetism with superconductivity.

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.

Superconductivity-driven ferromagnetism and spin manipulation using vortices in the magnetic superconductor EuRbFe4As4

Magnetic superconductors are specific materials exhibiting two antagonistic phenomena, superconductivity and magnetism, whose mutual interaction induces various emergent phenomena, such as the reentrant superconducting transition associated with the suppression of superconductivity around the magnetic transition temperature (T m), highlighting the impact of magnetism on superconductivity. In this study, we report the experimental observation of the ferromagnetic order induced by superconducting vortices in the high-critical-temperature (high-T c) magnetic superconductor EuRbFe4As4 Although the ground state of the Eu2+ moments in EuRbFe4As4 is helimagnetism below T m, neutron diffraction and magnetization experiments show a ferromagnetic hysteresis of the Eu2+ spin alignment. We demonstrate that the direction of the Eu2+ moments is dominated by the distribution of pinned vortices based on the critical state model. Moreover, we demonstrate the manipulation of spin texture by controlling the direction of superconducting vortices, which can help realize spin manipulation devices using magnetic superconductors.

Pressure-Tuned Superconducting Dome in Chemically-Substituted κ-(BEDT-TTF)2Cu2(CN)3

The quantum spin liquid candidate κ-(BEDT-TTF)2Cu2(CN)3 has been established as the prime example of a genuine Mott insulator that can be tuned across the first-order insulator–metal transition either by chemical substitution or by physical pressure. Here, we explore the superconducting state that occurs at low temperatures, when both methods are combined, i.e., when κ-[(BEDT-TTF)1−x(BEDT-STF)x]2Cu2(CN)3 is pressurized. We discovered superconductivity for partial BEDT-STF substitution with x = 0.10–0.12 even at ambient pressure, i.e., a superconducting state is realized in the range between a metal and a Mott insulator without magnetic order. Furthermore, we observed the formation of a superconducting dome by pressurizing the substituted crystals; we assigned this novel behavior to disorder emanating from chemical tuning.

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.

Observing the Suppression of Superconductivity in RbEuFe_{4}As_{4} by Correlated Magnetic Fluctuations

In this Letter, we describe quantitative magnetic imaging of superconducting vortices in RbEuFe_{4}As_{4} in order to investigate the unique interplay between the magnetic and superconducting sublattices. Our scanning Hall microscopy data reveal a pronounced suppression of the superfluid density near the magnetic ordering temperature in good qualitative agreement with a recently developed model describing the suppression of superconductivity by correlated magnetic fluctuations. These results indicate a pronounced exchange interaction between the superconducting and magnetic subsystems in RbEuFe_{4}As_{4}, with important implications for future investigations of physical phenomena arising from the interplay between them.

Eliashberg Theory of a Multiband Non-Phononic Spin Glass Superconductor

I solved the Eliashberg equations for a multiband non-phononic s± wave spin-glass superconductor, calculating the temperature dependence of the gaps and of superfluid density. Their behaviors were revealed to be unusual: showing non-monotonic temperature dependence and reentrant superconductivity. By considering particular input parameters values that could describe the iron pnictide EuFe2(As1−xPx)2, a rich and complex phase diagram arises, with two different ranges of temperature in which superconductivity appears.

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.

Evolution of the magnetic order of Fe and Eu sublattices in Eu1-x Ca x Fe2As2 (0 ⩽ x ⩽ 1) single crystals

Single crystals of Eu_1-x Ca _x Fe₂As₂ ([Formula: see text]) are grown using the high-temperature solution-growth method employing FeAs self-flux. Structural and chemical analysis indicates that these crystals are homogeneous and their lattice parameters exhibit a gradual monotonic decrease with increasing Ca concentration. Detailed magnetic, specific heat and resistivity data were used to construct a phase diagram which depicts the evolution of the structural/spin-density-wave transition at T ₀, and of the antiferromagnetic (AFM) ordering temperature of the Eu moments at T _N. We found out that while T _N decreases monotonically from 19.1 K (for x  =  0) to below 2 K (for [Formula: see text]), T ₀ remains almost constant up to x  =  x _c and decreases steadily for higher values of x. Annealing at low temperatures for several days leads to enhancement of T _N and T ₀ by a few kelvin and sharpened the anomalies associated with these transitions. However, annealing did not change the variation of T _N and T ₀ across the series. The observation that T ₀ is almost constant until the long-range AFM ordering of the Eu²⁺ moments gets destroyed, suggests a subtle interrelationship between the Eu²⁺ and Fe²⁺ magnetic sublattices.