Superconductivity and Ferromagnetism in AEuFe4As4 (A $=$ Rb and Cs).

Superconductivity (SC) and ferromagnetism (FM) are mutually antagonistic collective phenomena in solids, and therefore, bulk SC rarely coexists with FM in a single material. Here we report evidence of coexistence of bulk SC and full FM in 1144-type iron pnictides~AEuFe4As4 (A$=$Rb and Cs), variants of EuFe2As2 in which~every alternate Eu layer is replaced by non-magnetic Rb/Cs layer. Both materials show bulk SC at 35-36 K and Eu-spin ferromagnetic ordering at ca. 15 K. The robustness of SC and FM in AEuFe4As4 suggests that the expected mutual suppression between SC and FM can be minimized via a certain mechanism, which may shed light on the mechanism of iron-based SC.