Vortex pinning and magnetic phase diagram of EuRbFe4As4 iron-based superconductor

We performed systematic AC susceptibility and magnetic moment measurements to investigate the vortex dynamics and pinning in $EuRbFe_4As_4$?--> single crystals as a function of temperature, frequency, and DC magnetic field. The vortex solid-liquid line was determined, and it can be fitted well with $H(T_p) = H(0)(1-t_p)^\beta$?--> using β = 1.74 ± 0.1-1.91 ± 0.1, where T_p is the peak position on an imaginary part of the AC susceptibility plot ( $\chi''$?--> ), $t_p = T_p/T_c$?--> for H $\parallel$?--> ab. It indicates a rather high pinning strength of the vortex system. The activation energy U₀ was determined from thermally activated flux creep theory and reached 6700 ± 1100 K at low fields, suggesting strong vortex pinning. A field dependence of U₀(H $\parallel$?--> ab) ∼ H^a with a = 0.47(4) suggests thermally activated plastic pinning or caused by planar defects. Magnetic moment measurements also confirmed strong pinning in the $EuRbFe_4As_4$?--> superconductor, and the superconducting response gives the main contribution to the M(H) hysteresis. Additionally, we show long-range ordering of the Eu^{2 +} sublattice according to the AC susceptibility measurement data.