ANALYSIS OF THE OXIDATION PROCESS AND MECHANICAL EVOLUTION IN NANOSIZED COPPER SPINEL FERRITES - ROLE OF STRESSES ON THE COERCIVITY

Isothermal oxidations of the iron(II) and copper(I) cations in nanosiz ed copper spinel ferrites which were oxidized in cation-deficient spin els have been studied by thermogravimetric analysis. The low temperatu re kinetics of oxidation of each oxidizable cation has been explained on the basis of a diffusion-induced stress effect, these stresses bein g generated in the ferrite particles by the chemical gradient induced during the oxidation process where the cations at the surface are more oxidized than those in the bulk. A reaction mechanism coupling the ch emical and the mechanical effects has been proposed. When the Cu-ferri tes contain some Co-cations, it has also been revealed that the stress es have a significant influence on the enhancement of the coercivity. (C) 1997 Elsevier Science S.A.