The oxidation at low temperature of
fine-grained ferrites containing oxidizable cations such as
Cu+, Fe2+, Mn2+, Mo3+, V3+, has
led to new mixed-valent cation deficient ferrites
with a large number of vacancies. It has
been demonstrated by DTG analysis that each
cation oxidizes at a specific temperature
in close relation with the cation-oxygen
distance. The kinetics oxidation of each oxidizable
cation has been explained on the basis
of a diffusion-induced stress effect
when these stresses are generated by the chemical gradient
induced during the oxidation process.
When fine-grained ferrites contain some
Co-cations, it has been revealed that the stresses have a
significant influence on the enhancement of the coercivity.