We investigate the anisotropic superconducting and magnetic properties of single-crystal ${\mathrm{RbEuFe}}_4{\mathrm{As}}_4$ using magnetotransport and magnetization measurements. We determine a magnetic ordering temperature of the Eu moments of $T_m=15\mathrm{K}$ and a superconducting transition temperature of $T_c=36.8\mathrm{K}$. The superconducting phase diagram is characterized by high upper critical field slopes of $-70$ and $-42$ kG/K for in-plane and out-of-plane fields, respectively, and a surprisingly low superconducting anisotropy of $\mathrm{\Gamma }=1.7$. Ginzburg-Landau parameters of ${\kappa }_c\sim 67$ and ${\kappa }_{ab}\sim 108$ indicate extreme type-II behavior. These superconducting properties are in line with those commonly seen in optimally doped Fe-based superconductors. In contrast, Eu magnetism is quasi-two dimensional (2D), as evidenced by highly anisotropic in-plane and out-of-plane exchange constants of 0.6 K and $<0.04$ K. A consequence of the quasi-2D nature of the Eu magnetism are strong magnetic fluctuation effects, a large suppression of the magnetic ordering temperature as compared to the Curie-Weiss temperature, and a kinklike anomaly in the specific heat devoid of any singularity. Magnetization curves reveal a clear magnetic easy-plane anisotropy with in-plane and out-of-plane saturation fields of 2 and 4 kG.

• Received 11 May 2018

DOI:https://doi.org/10.1103/PhysRevB.98.104503