  Copper and Nickel impurities have been doped into the iron pnictide
superconductor Ba0.6K0.4Fe2As2. Resistivity measurements reveal that Cu and Ni
impurities suppress superconducting transition temperature T_c with rates of
\Delta T_c/Cu-1% = -3.5 K and \Delta T_c/Ni-1% = -2.9 K respectively.
Temperature dependence of Hall coefficient R_H of these two series of samples
show that both Cu-doping and Ni-doping can introduce electrons into
Ba0.6K0.4Fe2As2. With more doping, the sign of R_H gradually changes from
positive to negative, while the changing rate of Cu-doped samples is much
faster than that of Ni-doped ones. Combining with the results of
first-principles calculations published previously and the non-monotonic
evolution of the Hall coefficient in the low temperature region, we argue that
when more Cu impurities were introduced into Ba0.6K0.4Fe2As2, the removal of
Fermi spectral weight in the hole-like Fermi surfaces is much stronger than
that in the electron-like Fermi surfaces, which is equivalent to significant
electron doping effect. DC magnetization and the lattice constants analysis
reveal that static magnetic moments and notable lattice compression have been
formed in Cu-doped samples. It seems that the superconductivity can be
suppressed by the impurities disregard whether they are magnetic or nonmagnetic
in nature. This gives strong support to a pairing gap with a sign reversal,
like S^\pm. However, the relatively slow suppression rates of T_c show the
robustness of superconductivity of Ba0.6K0.4Fe2As2 against impurities, implying
that multi-pairing channels may exist in the system.
