  We investigate the effect of Ni doping on the Fe-site in single crystals of
the magnetic superconductor RbEuFe$_4$As$_4$ for doping concentrations of up to
4%. A clear suppression in the superconducting transition temperature is
observed in specific heat, resistivity and magnetization measurements. Upon
Ni-doping, the resistivity curves shift up in a parallel fashion indicating a
strong increase of the residual resistivity due to scattering by charged dopand
atoms while the shape of the curve and thus the electronic structure appears
largely unchanged. The observed step $\Delta C/T_c$ at the superconducting
transition decreases strongly for increasing Ni doping in agreement with
expectations based on a model of multi-band superconductivity and strong
inter-band pairing. The upper critical field slopes are reduced upon Ni doping
for in- as well as out-of-plane fields leading to a small reduction in the
superconducting anisotropy. The specific heat measurements of the magnetic
transition reveal the same BKT behavior close to the transition temperature
$T_m$ for all doping levels. The transition temperature is essentially
unchanged upon doping. The in to out-of-plane anisotropy of Eu-magnetism
observed at small magnetic fields is unaltered as compared to the undoped
compound. All of these observations indicate a decoupling of the Eu magnetism
from superconductivity and essentially no influence of Ni doping on the Eu
magnetism in this compound.
