  The effect of electron doping by trivalent charge state rare-earth ion (RE =
La, Ce, Pr, and Nd) substitutions on the superconductivity in BaFe2As2 was
examined using epitaxial films. Each of the RE substitutions suppressed the
resistivity anomaly associated with the magnetic/structural phase transitions,
leading to the resistivity drops and superconductivity transitions. Bulk
superconductivity was observed at the maximum onset critical temperature
(Tconset) of 22.4 K for La-doping and 13.4 K for Ce-doping, while only broad
resistivity drops were observed at 6.2 K for Pr-doping and 5.8 K for Nd-doping
but neither zero resistivity nor distinct Meissner effect were observed at
least down to 2 K. The decrease in Tconset with increasing the number of RE 4f
electrons cannot be explained in terms of the crystalline qualities or
crystallographic structure parameters of the BaFe2As2 films. It was clarified,
based on resistivity-temperature analyses, that magnetic scattering became
increasingly significant in the above order of the RE dopants. The negative
magnetoresistance was enhanced by the Ce- and Pr-doping, implying that the
decrease in Tc originates from magnetic pair breaking by interaction of the
localized 4f orbitals in the RE dopants with the itinerant Fe 3d orbitals.
