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.
Rbumb.-Dans les phtnomenes de relaxation et de rtsonance dans les ferrites et grenats polycristallins, les contributions qui dtpendent de la dimension proviennent d'effets dus aux courants pelliculaires, aux inhomogenkites d'aimantation et aux defauts de surface ; au contraire Ies contributions indtpendantes de la dimension sont likes A I'anisotropie et A la porositt. Les contri
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