Coexistence of superconductivity and magnetism*

RbEuFe₄As₄ and CsEuFe₄As₄ are members of the recently discovered 1144 superconducting family, they exhibit the unusual combination of superconductivity (T_c ~ 35 K) and ferromagnetism (T_m ~ 15 K). We performed a series of measurements under compression including x-ray diffraction, magnetic susceptibility, and electrical resistivity. Compression monotonically suppresses T_c and enhances T_m. Near 7 GPa, T_c onset and T_m become comparable, at higher compression evidence of bulk superconductivity gradually disappears. Room-temperature x-ray diffraction measurements provide evidence of a transition from tetragonal to a half-collapse-tetragonal structure near 10 GPa for RbEuFe₄As₄ and near 12 GPa for CsEuFe₄A₄. The cross-over from ferromagnetic superconductor (T_c > T_m), to superconducting ferromagnet (T_m > T_c) occurs before the structural transition, suggesting that RbEuFe₄As₄ and CsEuFe₄As₄ are an ideal system for studying the interplay between superconductivity and magnetism.