Influence of magnetic ordering on the vortex matter in RbEuFe₄As₄ single crystals

Eu-containing Fe-based superconductors are ideal systems to study the interplay of superconductivity and magnetic order. The superconductor RbEuFe₄As₄ has T_c~37K and undergoes a magnetic ordering of the Eu at T_m~15K. We studied vortex pinning and dynamics in RbEuFe₄As₄ single crystals as a function of temperature (T), magnetic field and orientation (Θ) by angular dependent magnetization. The critical current density (J_c) of the pristine crystals is low, indicative of small disorder, and weakly anisotropic. Introduction of aligned columnar defects (CDs) by heavy ion irradiation increases J_c, but produces almost no changes in T_c and T_m. In the crystals with CDs, J_c(Θ) exhibits a lockin phase, the fingerprint of correlated pinning. Both pristine and irradiated crystals show clear evidence of the influence of the magnetic order on the vortex matter. In pristine crystals, the rate of decrease in J_c with T slows down in the proximity of T_m. Irradiation enhances this effect, with J_c(T) becoming nonmonotonic and developing a local maximum near T_m. In both cases, flux creep rates exhibit local minima around the magnetic transition, indicating an increase in the vortex pinning energy.