CORRELATION BETWEEN OXIDATION-STATES OF TRANSITION-METAL IONS AND VARIATION OF THE COERCIVITY IN MIXED-VALENCE DEFECT SPINEL FERRITES

Due to the very high dispersion of mixed-valence spinel ferrites prepa red by 'soft chemistry' it becomes possible to oxidize in the spinel l attice, not only ferrous ions but also different transition metal ions (Cr3+, Mn2+, Mn3+, Mo3+, Mo4+, V2+, V3+, Cu+), by 'soft' oxidation be tween 150 and 500 degrees C, under formation of non-stoichiometric spi nels cation vacancies. The oxidation state with the coordination and t he oxidation temperature of the cations have been determined in copper -manganese ferrites by derivative thermogravimetry (DTG), based on the specific solid-state reactivity of cations in the redox reaction. For Go-modified non-stoichiometric spinels, which present interesting mag neto-optical properties due to their exceptional high coercivity (1000 -3500 Oe) and whose cation distribution has been determined by the met hod described above, close correlations have been found between the ox idation state of the spinel and the variation of the coercivity.