Magnetic phase diagram of Ca-substituted EuFe$_2$As$_2$

TL;DR

This study investigates how calcium substitution affects the magnetic order in EuFe$_2$As$_2$, revealing a localized Eu antiferromagnetism with weak coupling to Fe and a rapid decrease in Eu magnetic interactions.

Contribution

It provides detailed experimental insights into the evolution of Eu magnetic order and interactions upon Ca doping in EuFe$_2$As$_2$, highlighting the localized nature of Eu magnetism and weak Eu-Fe coupling.

Findings

Eu magnetic order decreases with Ca substitution.

Eu antiferromagnetism is localized and mediated by RKKY interactions.

Ca doping weakens Eu magnetic interactions and reduces magnetic volume fraction.

Abstract

The simultaneous presence of a Fe-related spin-density wave and antiferromagnetic order of Eu$^{2+}$ moments ranks EuFe$_2$As$_2$ among the most interesting parent compounds of iron-based pnictide superconductors. Here we explore the consequences of the dilution of Eu$^{2+}$ magnetic lattice through on-site Ca substitution. By employing macro- and microscopic techniques, including electrical transport and magnetometry, as well as muon-spin spectroscopy, we study the evolution of Eu magnetic order in both the weak and strong dilution regimes, achieved for Ca concentration $x(\mathrm{Ca}) = 0.12$ and 0.43, respectively. We demonstrate the localized character of the Eu antiferromagnetism mediated via RKKY interactions, in contrast with the largely itinerant nature of Fe magnetic interactions. Our results suggest a weak coupling between the Fe and Eu magnetic sublattices and a rapid decrease of the Eu magnetic interaction strength upon Ca substitution. The latter is confirmed both by the depression of the ordering temperature of the Eu$^{2+}$ moments, $T_\mathrm{N}$, and the decrease of magnetic volume fraction with increasing $x(\mathrm{Ca})$. We establish that, similarly to the EuFe$_2$As$_2$ parent compound, the investigated Ca-doped compounds have a twinned structure and undergo a permanent detwinning upon applying an external magnetic field.