Varying Eu$^{2+}$ magnetic order in EuFe$_2$As$_2$ by chemical pressure

TL;DR

This paper investigates how chemical pressure influences the magnetic order of Eu$^{2+}$ ions in EuFe$_2$As$_2$ compounds, revealing a transition from antiferromagnetic to ferromagnetic coupling as pressure varies.

Contribution

It introduces a novel scheme explaining the evolution of Eu$^{2+}$ magnetic order under chemical pressure, supported by magnetization measurements and literature observations.

Findings

Eu$^{2+}$ moments are slightly canted in EuFe$_2$As$_2$

Ferromagnetic contribution along the c-direction increases with pressure

Antiferromagnetic coupling turns ferromagnetic with reduced interlayer distance

Abstract

Among iron 122 pnictide superconductors, the EuFe$_2$As$_2$ series draws particular interest because, in addition to superconductivity or the long-range spin-density-wave order in the Fe subsystem, the localized Eu$^{2+}$ magnetic moments order at low temperatures. Here we present a novel scheme of how the spins align in the Eu compounds when pressure varies the coupling; we explain magnetization measurements on EuFe$_2$(As$_{1-x}$P$_x$)$_2$ single crystals as well as other observations of the Eu$^{2+}$ ordering previously reported in literature. The magnetic moments of the Eu$^{2+}$ ions are slightly canted even in the parent compound EuFe$_2$As$_2$, yielding a ferromagnetic contribution along the $c$-direction that becomes stronger with pressure. Reducing the interlayer distance even further, the antiferromagnetic coupling of the $ab$ planes finally turns ferromagnetic.