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 $T_{\mathrm{s}\mathrm{c}}=36.5$ 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 $T_{\mathrm{s}\mathrm{c}}>T_{\mathrm{m}}$. With the Ni doping, $T_{\mathrm{s}\mathrm{c}}$ 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 $T_{\mathrm{m}}>T_{\mathrm{s}\mathrm{c}}$ for $0.07\le x\le 0.08$. 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.