Modification of the Upper Critical Field of RbEuFe₄As₄ due to Eu-Magnetism

ReEuFe₄As₄ is a Fe-base superconductor with a high transition temperature of T_c~37 K that enters into a magnetic state at 15 K in which Eu-4f moments are helically ordered. In this state, in-plane ferromagnetically ordered Eu-planes undergo 90 deg rotation between layers resulting in a commensurate helix with a period of 4 c-axis lattice constant. This structure is easily polarized in fields of the order of several kOe resulting in a field-induced ferromagnet with a moment of 7 µ_B/(unit cell) coexisting with superconductivity. Nevertheless, most measurements to date have yielded no or only very weak coupling between superconductivity and magnetism. Using specific heat measurements in fields up to 14 T, we show that the H_c2-line exhibits clear downwards curvature in a temperature range close to T_c where Ginzburg-Landau theory applies, amounting to a shift by δT_c ~ 0.4 K in 14 T ||c. This effect can be captured quantitatively in a model in which the exchange fields from the field-induced polarization of the Eu-moments renormalize T_c(H).