Exchange constants in spinel ferrites

A three-sublattice analysis of M²⁺Fe₂³⁺O₄ (M=Li0.5⁺Fe0.5³⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺) spinel ferrites based on the molecular-field approximation has been used to obtain exchange constants from the MS vs T and χ^-1 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.