M\"ossbauer 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

TL;DR

This study uses Mössbauer spectroscopy to analyze magnetic and electronic properties of Eu-Ca-Fe-As compounds, revealing how substitution affects magnetic order and superconductivity, with implications for understanding iron-based superconductors.

Contribution

It provides detailed Mössbauer spectra analysis of Eu-Ca-Fe-As compounds, showing effects of substitution on magnetic moments and superconducting states, and compares these with parent compounds.

Findings

Substitution does not cause Eu2+ moment canting or Eu3+ formation.

Re-entrant superconductor exhibits Eu2+ moment canting along the c-axis.

The superconducting phase is inhomogeneous, with coexisting magnetic and non-magnetic regions.

Abstract

57Fe and 151Eu Moessbauer spectra were obtained versus temperature for Eu0.57Ca0.43Fe2As2 compound with 3d and 4f magnetic order and Eu0.73Ca0.27(Fe0.87Co0.13)2As2 re-entrant superconductor, where the finite resistivity reappears while approaching the ground state. They were compared with previously obtained spectra for parent compounds EuFe2As2 and CaFe2As2. It was found that substitution beyond the Fe-As layers does not lead to the rotation (canting) of the Eu2+ magnetic moments and does not generate Eu3+ states. On the other hand, re-entrant superconductor exhibits rotation (canting) of the Eu2+ 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.