Superconductivity and magnetism in RbEu(Fe_1-x Co _x )₄As₄

We studied the cobalt-doping effect on superconductivity and magnetism in a hole self-doped RbEuFe₄As₄ magnetic superconductor which shows superconductivity at [Formula: see text] 36.5 K and Eu²⁺ -spin ordering at [Formula: see text] 15 K. The Co solubility limit in RbEu(Fe_1-x Co _x )₄As₄ achieves x  =  0.21 for the solid-state reaction at 880 °C. With increasing x, [Formula: see text] decreases gradually, and superconductivity eventually disappears at [Formula: see text]. A spin-density-wave transition at [Formula: see text] 35-40 K is recovered for [Formula: see text], which can be understood in terms of the hole-depletion and the disorder effects. On the other hand, [Formula: see text] remains unchanged despite the Co doping and, consequently, an intriguing superconducting ferromagnet without Meissner state is realized in the range of 0.125 [Formula: see text] 0.155. Our results indicate that the Eu²⁺ spins essentially decouple with superconductivity over a wide doping range, making the coexistence of superconductivity and ferromagnetism possible in the 1144-type system.