Electrical transport, magnetism, and magnetoresistance in ferromagnetic oxides with mixed exchange interactions: A study of the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">La</mml:mi></mml:mrow><mml:mrow><mml:mn>0.7</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Ca</mml:mi></mml:mrow><mml:mrow><mml:mn>0.3</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow…

In this paper we report the results of an extensive investigation of the La${}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{O}}_{3}$ system. Substitution of Mn by Co dilutes the double-exchange (DE) mechanism and changes the long range ferromagnetic order of ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ to a cluster glass-type ferromagnetic (FM) order similar to that observed in ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{CoO}}_{3}$. This happens even for the lowest Co substitution of $x=0.05$ and persists over the entire composition range studied (0.05$<~x<~$0.5). The Co substitution also destroys the metallic state and the resistivity increases by orders of magnitude even with a very small extent of Co substitution. The charge localization due to Co substitution is likely to have its origin in polaronic lattice distortion. The Co substitution also suppresses the colossal magnetoresistance (CMR) of the pure manganate $(x=0)$ over the entire temperature and composition range and it becomes very small for $x>~0.2$. We conclude that the DE interaction and the resulting metallic state is very ``fragile'' and hence even a small amount of Co substitution can destroy the FM order, the metallic state, and the CMR.