# Hydrostatic pressure effects on the static magnetism in Eu(Fe$_{0.925}$Co$_{0.075}$)$_{2}$As$_{2}$

## TL;DR

This study explores how hydrostatic pressure influences the magnetic phases in Eu(Fe$_{0.925}$Co$_{0.075}$)$_{2}$As$_{2}$, revealing a transition from canted antiferromagnetic to ferromagnetic and possibly novel antiferromagnetic states, alongside pressure-induced superconductivity.

## Contribution

It provides a detailed pressure-temperature phase diagram and uncovers pressure-driven magnetic state transitions in Eu(Fe$_{0.925}$Co$_{0.075}$)$_{2}$As$_{2}$, highlighting the role of RKKY interactions.

## Key findings

- Suppression of structural transition and Fe spin-density-wave above 2.0 GPa.
- Persistence of Eu magnetic order up to 14 GPa with non-monotonic variation.
- Coexistence of strong Eu ferromagnetism and superconductivity around 2 GPa.

## Abstract

The effects of hydrostatic pressure on the static magnetism in Eu(Fe$_{0.925}$Co$_{0.075}$)$_{2}$As$_{2}$ are investigated by complementary electrical resistivity, ac magnetic susceptibility and single-crystal neutron diffraction measurements. A specific pressure-temperature phase diagram of Eu(Fe$_{0.925}$Co$_{0.075}$)$_{2}$As$_{2}$ is established. The structural phase transition, as well as the spin-density-wave order of Fe sublattice, is suppressed gradually with increasing pressure and disappears completely above 2.0 GPa. In contrast, the magnetic order of Eu sublattice persists over the whole investigated pressure range up to 14 GPa, yet displaying a non-monotonic variation with pressure. With the increase of the hydrostatic pressure, the magnetic state of Eu evolves from the canted antiferromagnetic structure in the ground state, via a pure ferromagnetic structure under the intermediate pressure, finally to a possible "novel" antiferromagnetic structure under the high pressure. The strong ferromagnetism of Eu coexists with the pressure-induced superconductivity around 2 GPa. The change of the magnetic state of Eu in Eu(Fe$_{0.925}$Co$_{0.075}$)$_{2}$As$_{2}$ upon the application of hydrostatic pressure probably arises from the modification of the indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the Eu$^{2+}$ moments tuned by external pressure.