Molecular-orbital solution of magnetic exchange in spinel ferrites

A one-electron molecular-orbital model of magnetic exchange was applied to the spinel ferrite family A[B2]O4. Hybridized metal-oxygen eigenfunctions were used to compute estimates of the transfer integrals between orbital states of ions at sites i and j from which the molecular-field coefficients (Nij) are derived. A set of coefficients was first determined from magnetization data for the standard Mn ferrite cation distribution Fe0.23+Mn0.82+[Mn0.22+Fe1.83+]O4, and then used for extrapolation to the fully inverted Fe3+[Mn2+Fe3+]O4. It is concluded that the large content of Mn2+ in the A sublattice combined with its smaller valence charge in the standard cation distribution is responsible for its low Curie temperature (575K). The model was then used to examine other inverted spinel ferrites, including Fe3+[Fe2+Fe3+]O4 (magnetite), where ferromagnetic exchange between t2g states influences the NBB coefficient. Computations reveal that the coefficients vary according to theory within the spinel family and that exchange among the 3d orbitals of divalent cations is significantly weaker than that of trivalent cations because the smaller ionization potentials and ionic lattice energies lead to smaller transfer integrals. The calculated results are consistent with traditional reasoning based on thermomagnetism properties.