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

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 $0.07leq xleq0.08$. 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.