Superconductivity and ferromagnetism in RbEu(Fe$_{\mathrm{1-x}}$Ni$_{\mathrm{x}})_{\mathrm{4}}$As$_{\mathrm{4}}$.

Hole-doped iron pnictides $A$EuFe$_{\mathrm{4}}$As$_{\mathrm{4}}$ ($A \quad =$ Rb, Cs) are ferromagnetic superconductors with bulk superconductivity at $T_{\mathrm{c}}\approx $ 35 K and Eu-spin ferromagnetism at $T_{\mathrm{m}}\approx $ 15 K. Here, we investigate the hole-compensation effect in RbEu(Fe$_{\mathrm{1-x}}$Ni$_{\mathrm{x}})_{\mathrm{4}}$As$_{\mathrm{4}}$ by electron doping through Ni substitution. We find that $T_{\mathrm{c}}$ decreases monotonically with increasing Ni concentration, and superconductivity vanishes at $x \approx $ 0.1 accompanying with the revival of spin-density wave. On the other hand, the Eu-spin ferromagnetism is very robust against the Ni doping, and to our surprise, $T_{\mathrm{m}}$ hardly changes. Consequently, a superconducting magnet, RbEu(Fe$_{\mathrm{0.925}}$Ni$_{\mathrm{0.075}})_{\mathrm{4}}$As$_{\mathrm{4}}$, in which $T_{\mathrm{c}}$ (\textasciitilde 5 K) is lower than $T_{\mathrm{m}}$, is obtained. The electronic phase diagram is concluded.