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# Interplay between magnetism and superconductivity in EuFe2-xCoxAs2 studied by 57Fe and 151Eu Mössbauer spectroscopy

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(Impact Factor: 3.66). 07/2011; 84.

ABSTRACT The compound EuFe2-xCoxAs2 was investigated by means of 57Fe and 151Eu Mössbauer spectroscopy versus temperature (4.2-300 K) for x = 0 (parent), x = 0.34-0.39 (superconductor), and x = 0.58 (overdoped). It was found that the spin density wave (SDW) is suppressed by Co substitution; however, it survives in the region of superconductivity, but iron spectra exhibit some nonmagnetic components in the superconducting region. Europium orders magnetically, regardless of the cobalt concentration, with the spin reorientation from the a-axis in the parent compound toward the c-axis with increasing replacement of iron by cobalt. The reorientation takes place close to the a-c plane. Some trivalent europium appears in EuFe2-xCoxAs2 versus substitution due to the chemical pressure induced by Co atoms, and it experiences some transferred hyperfine field from Eu2+. Iron experiences some transferred field due to the europium ordering for substituted samples in the SDW and nonmagnetic state both, while the transferred field is undetectable in the parent compound. Superconductivity coexists with the 4f-europium magnetic order within the same volume. It seems that superconductivity has some filamentary character in EuFe2-xCoxAs2, and it is confined to the nonmagnetic component seen by the iron Mössbauer spectroscopy.

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• ##### Article: Interplay between spin density wave and superconductivity in '122' iron pnictides: 57Fe M\
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ABSTRACT: Iron-based superconductors Ba0.7Rb0.3Fe2As2 and CaFe1.92Co0.08As2 of the '122' family have been investigated by means of the 14.41-keV Moessbauer transition in 57Fe versus temperature ranging from the room temperature till 4.2 K. A comparison is made with the previously investigated parent compounds BaFe2As2 and CaFe2As2. It has been found that Moessbauer spectra of these superconductors are composed of the magnetically split component due to development of spin density wave (SDW) and non-magnetic component surviving even at lowest temperatures. The latter component is responsible for superconductivity. Hence, the superconductivity occurs in the part of the sample despite the sample is single phase. This phenomenon is caused by the slight variation of the dopant concentration across the sample (crystal).
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##### Article: Tuning superconductivity by magnetic fields in Eu(Fe0.81Co0.19)2As2
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ABSTRACT: The distinct difference between BCS-type and unconventional triplet superconductivity manifests itself in their response to external magnetic fields. An applied field easily extinguishes s-wave singlet superconductivity by both the paramagnetic or orbital pair-breaking effects. However, it hardly destroys triplet state because the paramagnetic effect, owing to spins of the Cooper pairs readily aligned with the field, is not so efficacious. This suggests that the triplet superconductivity may be affected mostly by the orbital effect. Conversely, if one can break down the orbital effect then one can recover the superconductivity. Here, we show that superconductivity can be induced with magnetic fields applied parallel to the ab plane of crystals of the magnetic Eu(Fe0.81Co0.19)2As2 superconductor. We argue that the tuning superconductivy may be actuated by relative enhancement of ferromagnetic interactions between the Eu2+ moments lying in adjacent layers and removal of their canting toward c axis that is present in zero field.
Physical review. B, Condensed matter 12/2011; 85(5). · 3.66 Impact Factor
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##### Article: The electronic phase diagrams of the Eu(Fe0.81Co0.19)2As2 superconductor
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ABSTRACT: Magnetic and superconducting properties of \Eu8~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 \emph{c}-axis. The compound exhibits the coexistence of magnetism and superconductivity, characterized by the structural distortion (SD) and/or spin-density-wave (SDW) ordering at $T_{SD/SDW}$ = 78$\pm$4 K, the canted-antiferromagnetic (C-AF) ordering at the N\'{e}el temperature $T_N$ = 16.5$\pm$0.5 K and superconductivity (SC) at the critical temperature $T_c$= 5.3$\pm$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; C-AF and ferromagnetic (F) ones. Unusual behavior of the SC state deserves the observation of field-induced superconductivity in the $H\perp c$ configuration as an outcome of weakening orbital pair-breaking effect. From the experimental data we derive the field-temperature phase diagrams for \Eu8. Comparison of experimental results is made with theory developed for the type-II superconductors, and then we deduce some important thermodynamic parameters characteristic for the superconducting state of \Eu8, such as the specific heat jump at $T_c$, $\Delta C_p(T_c)/ \gamma_n T_c$, electron – phonon coupling constant, $\lambda_{e-ph}$, upper critical field $H_{c2}$, coherence length $\xi$, Fermi wave-vector $k_F$, effective mass $m^*$, Hall mobility $\mu_H$, magnetic penetration depth $\lambda$, and Ginzburg - Landau parameter $\kappa$.
New Journal of Physics 07/2012; 14:073052. DOI:10.1088/1367-2630/14/7/073052 · 3.67 Impact Factor