  Versatile features of impurity doping effects on perovskite manganites,
$R_{0.6}$Sr$_{0.4}$MnO$_{3}$, have been investigated with varying the doing
species as well as the $R$-dependent one-electron bandwidth. In
ferromagnetic-metallic manganites ($R$=La, Nd, and Sm), a few percent of Fe
substitution dramatically decreases the ferromagnetic transition temperature,
leading to a spin glass insulating state with short-range charge-orbital
correlation. For each $R$ species, the phase diagram as a function of Fe
concentration is closely similar to that for $R_{0.6}$Sr$_{0.4}$MnO$_{3}$
obtained by decreasing the ionic radius of $R$ site, indicating that Fe doping
in the phase-competing region weakens the ferromagnetic double-exchange
interaction, relatively to the charge-orbital ordering instability. We have
also found a contrastive impact of Cr (or Ru) doping on a spin-glass insulating
manganite ($R$=Gd). There, the impurity-induced ferromagnetic magnetization is
observed at low temperatures as a consequence of the collapse of the inherent
short-range charge-orbital ordering, while Fe doping plays only a minor role.
The observed opposite nature of impurity doping may be attributed to the
difference in magnitude of the antiferromagnetic interaction between the doped
ions.
