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We exemplify some of the problems in designing novel functional materials with the substitution effects on colossal magnetoresistive (CMR) series CaCu3(Mn4-xFex)O12 (x = 0, 0.5, 1.0) and on the charge order phenomenon in half hole-doped manganites Bi0.25RE0.25Ca0.5MnO3 (RE = La, Nd, Ho, Er, Tm) and Bi0.5Ca0.5MnO3 (RE = La, Nd, Ho, Er, Tm) Bi0.5Ca0.5MnO3. All the CaCu3(Mn4-xFex)O12 samples are ferromagnetic and show a progressive decrease of Tc, saturation magnetization and Weiss temperatures, as the Fe contents increases, which suggests an increment of the antiferromagnetic interactions at the (Mn,Fe) octahedral sublattice and to a considerable canting of the Fe vs Mn spins. The magnetoresistance also decreases upon Fe-doping. In contrast, the Bi-RE samples remain semiconducting by entering antiferromagnetic state at low temperature and similarly to end members Bi0.5Sr0.5MnO3 and Bi0.5Ca0.5MnO3 are remarkable for their particularly stable charge order of the eg electrons. Although giving rise to important modifications in the resulting magnetic properties and leading at rather low Fe substitution level to full suppression of charge order in Bi0.5Ca0.5(Mn1-xFex)O3 the Fe substitution does not induce magnetoresistive effect.