A three-sublattice analysis of M +Fe2'+ 04 (M = Lio, +Feo, +, Mn +, Fe +, Co +, Ni +, Cu +) spinel ferrites based on the molecular-field approximation has been used to obtain exchange constants from the Mz vs T and g ' vs T experimental data. The magnitude and sign of the exchange constants have been examined using Anderson's theory of superexchange. The magnitudes of the transfer integrals for different exchange routes have been generally found to be in agreement with the chemical theory of covalency.

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The recent attempt to account for the existence of several magnetic phases in insulating perovskite manganites in terms of the interplay between the orbital and spin orders produced by the localized eg and t2g electrons is extended to the metallic state which is obtained on doping. It is shown that the activation energy for electrical conductivity obtained on the basis of the correlated polaron transport depends on the product of the spin and bond order parameters which changes rapidly near the magnetic transition temperature and accounts for the existence of colossal magnetoresistance in this system.

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