The electronic phase diagrams of the Eu(Fe_0.81Co_0.19)₂As₂superconductor

Abstract

The magnetic and superconducting properties of an Eu(Fe 0.81 Co 0.19 ) 2 As 2 single crystal are investigated by means of ac magnetic susceptibility, dc magnetization, specific heat, transverse resistivity and Hall effect measurements in magnetic fields up to 9 T, applied parallel and perpendicular to the c-axis. The compound exhibits the coexistence of magnetism and superconductivity (SC), characterized by structural distortion (SD) and/or spin-density-wave (SDW) ordering at T SD/SDW = 78 ± 4 K, cantedantiferromagnetic (C-AF) ordering at the Néel temperature T N = 16.5 ± 0.5 K and SC at the critical temperature T c = 5.3 ± 0.2 K at zero field. Upon applying fields both the C-AF and SC states evolve in an unconventional manner. Magnetic field distinctly affects the spin canting, resulting in separation of the C-AF into two new phases: the C-AF and ferromagnetic (F) ones. The unusual behavior of the SC state produces field-induced SC in the H ⊥c configuration as an outcome of the weakening orbital pair-breaking effect. From the experimental data we derive the field-temperature phase diagrams for Eu(Fe 0.81 Co 0.19 ) 2 As 2 . A comparison of experimental results is made with theory developed for type II superconductors and then some important thermodynamic parameters characteristic of the superconducting state of Eu(Fe 0.81 Co 0.19 ) 2 As 2 are deduced such as the specific heat jump at T c , C p (T c )/γ n T c , the electron-phonon coupling constant e-ph , the upper critical field H c2 , coherence length ξ , the Fermi wave-vector k F , effective mass m * , Hall mobility µ H , magnetic penetration depth and the Ginzburg-Landau parameter κ.

Magnetic structure of superconducting Eu(Fe0.82Co0.18)2As<

“…1.The SC transition in Co-doped EuFe 2 As 2 is usually more susceptible to the adverse effect of the Eu-magnetic ordering and zero resistivity is only realized within a quite narrow Co-concentration window around 20%. 26 The R-T behavior here is very similar to that reported by Tran et al on a single crystal with a similar composition Eu(Fe 0.81 Co 0.19 ) 2 As 2 , 39 in which also multiple phase transitions with comparable transition temperatures were found.…”

“…(Space group: F mmm) of the Eu 2+ spins is slightly suppressed from T N = 19 K for the parent compound to T C = 17 K with x = 0.18. The c-direction FM ordering of Eu 2+ presented here contrasts with the canted-AFM ordering with an easy a-b plane proposed by Tran et al in a single crystal with similar Co-concentration x = 0.19 based on pure macroscopic measurements 39. The microspopic bulk probe of neutron diffraction in our study, nevertheless, provides convincing evidence for the ferromagnetic ordering of the Eu 2+ moments.…”

“…The occupancies of Fe and Co atoms are fixed to 82% and 18%, respectively, according to the chemical composition determined from WDS. (Space group: F mmm) 39 The microspopic bulk probe of neutron diffraction in our study, nevertheless, provides convincing evidence for the ferromagnetic ordering of the Eu 2+ moments. Regarding the magnetic structure of the Fe sublattice, the moment size of Fe is significantly suppressed, from 0.98(8) µ B in the parent compound to ~0.15(1) µ B with x = 0.18, while probably keeping the moment direction of Fe unchanged with the Co-doping.…”

Magnetic phase diagram of Ca-substituted EuFe2As2

“…It was shown that in EuFe 2 As 2 -based systems the saturation state is equivalent to a field-induced ferromagnetic state. 5,11,29,32 The field at which the system "switches" from an antiferromagnetic (AF) to a field-induced ferromagnetic state (FI-FM) is represented by the "crossover" field H cr . Such H cr values can, therefore, be calculated by determining the minimum in the second derivative of magnetization d 2 M /dH 2 .…”

Mössbauer study of Eu0.57Ca0.43Fe2As2 and Eu0.73Ca0.27(Fe0.87Co0.13)2As2: A comparison to ‘122’ iron-based superconductors parent compounds EuFe2As2 and CaFe2As2

“…phosphorus, i.e., one has to perturb significantly Fe-As layers with strong perturbation of the iron environment, as iron atoms are tetrahedrally coordinated by pnictogen [10]. It is interesting to note, that 4f antiferromagnetic order occurs within superconducting material as well, and doping leading to superconductivity usually causes some 4f magnetic moment reorientation (canting) with a generation of the 4f ferromagnetic component [11][12][13][14].…”