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

Abstract 57 Fe and 151 Eu Mossbauer spectra were obtained versus temperature for Eu 0.57 Ca 0.43 Fe 2 As 2 compound with 3d and 4f magnetic order and Eu 0.73 Ca 0.27 (Fe 0.87 Co 0.13 ) 2 As 2 re-entrant superconductor, where the finite resistivity reappears while approaching the ground state. They were compared with previously obtained spectra for parent compounds EuFe 2 As 2 and CaFe 2 As 2 . It was found that substitution beyond the Fe-As layers does not lead to the rotation (canting) of the Eu 2+ magnetic moments and does not generate Eu 3+ states. On the other hand, re-entrant superconductor exhibits rotation (canting) of the Eu 2+ moments on the c-axis of the unit cell leading to the transferred hyperfine magnetic field on iron nuclei. Divalent europium orders magnetically within the bulk of the re-entrant superconducting phase. The re-entrant superconductor remains in the inhomogeneous state close to the ground state with about 27% of the volume being free of 3d magnetism, while the remainder exhibits weak spin density wave. Those two regions slightly differ by the electric field gradient and electron density on iron nuclei.