for x ≤ 0.04 in pulsed fields up to 65 T*

The superconducting phase diagram of single-crystal RbEu(Fe_1-xNi_x)₄As₄ for x = 0.02, 0.03, 0.04 has been measured in pulsed magnetic fields up to 65 T using a proximity diode oscillator technique. Doping lowers T_c and slightly lowers the superconducting anisotropy, but does not affect the Eu magnetic order. Upon doping, the curvature of phase boundaries for both H || ab and H || c remains remarkably consistent, indicating that the suppression of T_c is doping driven, not disorder driven. As in the parent compound, Pauli limiting dominates H_c2^ab, whereas orbital limiting dominates H_c2^c. The resultant high Maki parameter α for H|| ab is consistent with a possible FFLO state at low temperatures, but none is seen in the accessible field range.