RbEu(Fe$_{1-x}$Ni$_x$)$_4$As$_4$: From a ferromagnetic superconductor to a superconducting ferromagnet

TL;DR

This study explores how Ni doping in RbEuFe$_4$As$_4$ tunes the material from a ferromagnetic superconductor to a superconducting ferromagnet, revealing decoupling mechanisms between Eu spins and Cooper pairs.

Contribution

It demonstrates the transition from ferromagnetic superconductor to superconducting ferromagnet via Ni doping and elucidates the decoupling mechanism between Eu spins and superconductivity.

Findings

Ni doping suppresses $T_{sc}$ rapidly

Eu ferromagnetism remains unchanged with doping

System becomes a superconducting ferromagnet for $0.07 \\leq x \\leq 0.08$

Abstract

The intrinsically hole-doped RbEuFe$_4$As$_4$ exhibits bulk superconductivity at $T_{\mathrm{sc}}=36.5$ K and ferromagnetic ordering in the Eu sublattice at $T_\mathrm{m}=15$ K. Here we present a hole-compensation study by introducing extra itinerant electrons via a Ni substitution in the ferromagnetic superconductor RbEuFe$_4$As$_4$ with $T_{\mathrm{sc}}>T_{\mathrm{m}}$. With the Ni doping, $T_{\mathrm{sc}}$ decreases rapidly, and the Eu-spin ferromagnetism and its $T_{\mathrm{m}}$ remain unchanged. Consequently, the system RbEu(Fe$_{1-x}$Ni$_x$)$_4$As$_4$ transforms into a superconducting ferromagnet with $T_{\mathrm{m}}>T_{\mathrm{sc}}$ for $0.07\leq x\leq0.08$. The occurrence of superconducting ferromagnets is attributed to the decoupling between Eu$^{2+}$ spins and superconducting Cooper pairs. The superconducting and magnetic phase diagram is established, which additionally includes a recovered yet suppressed spin-density-wave state.