Interplay between magnetism and superconductivity in EuFe_2−xCo_xAs₂ studied by ⁵⁷Fe and ¹⁵¹Eu Mössbauer spectroscopy

The compound EuFe_2−xCo_xAs₂ 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 EuFe_2−xCo_xAs₂ 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 EuFe_2−xCo_xAs₂, and it is confined to the nonmagnetic component seen by the iron Mössbauer spectroscopy.