= 0 and 0.05) superconductor to non-magnetic disorder induced by 2.5 MeV electron irradiation*

Temperature dependent electrical resistivity and London penetration depth were measured in single crystals of CaK(Fe_1-xNi_x)₄As₄ (x = 0 and 0.05, as determined by wavelength dispersive spectroscopy) before and after 2.5 MeV electron irradiation of different doses. We observed increase of residual resistivity and suppression of the superconducting transition T_c, while the low temperature behavior of the penetration depth remained exponential, consistent with a robust s_± pairing. Overall, the x=0 compound is quite similar to a slightly overdoped BaK_1-yK_yFe₂As₂ with y ≈ 0.5. The Ni-doped composition, x=0.05, can be mapped onto the underdoped BaK_1-yK_yFe₂As₂ with y ≈ 0.2 by using its T_c value and shows similar increase of the superconducting gap anisotropy, confirming that Ni doping moves the composition toward the electron doped side.