  We report a complete multicomponent mean-field-theory for the coexistence and
competition of charge ordering (CO), antiferromagnetic (AFM) and ferromagnetic
(FM) spin ordering in the presence of a uniform magnetic field. Doping the AFM
or CO state always generates a ferromagnetic component. Itinerant FM, AFM and
CO, necessarily coexist and compete in a particle-hole asymmetric system.
Melting of large AFM-CO orders by small magnetic fields and the related
phenomenon of Colossal Magnetoresistance (CMR) may arise whenever the CO and
AFM order parameters have similar magnitude and momentum structure. Hole doping
favors FM metallic states and CMR while electron doping favors AFM-CO states in
agreement with the phase diagram of perovskite manganites.
