Suppression of Eu magnetic order in single crystals of the magnetically ordered superconductor RbEu_1-xCa_xFe₄As₄

The magnetically ordered, relatively low anisotropy superconductor RbEuFe₄As₄, with a high superconducting transition temperature T_c (~37 K) and a high magnetic ordering temperature T_m (~15 K), may host topological states and is an excellent platform for investigating interactions between magnetism and superconductivity. We show that light doping of Ca-for-Eu in single crystal material lowers T_m by ~1.5 K but does not affect T_c or the superconducting anisotropy, indicating limited coupling between magnetic order and superconductivity. However, there is a clear effect on the superconducting critical current measured via magnetic hysteresis loops in both pristine and doped material, suggesting novel pinning effects. Metamagnetic transitions are also observed in-field in the doped material.