Ferromagnetism and Colossal Magnetoresistance from the Coexistence of Comparable Charge and Spin Density Orders

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.