Aging phenomena of cobalt-modified iron oxides in the absence of Fe 2+ ions

Cobalt-modified iron oxide particles can change their coercivity as a function of time. The greatest effect is found for aging treatments in air, resulting in 3-5-kA/m lower coercivities for loose powders. The opposite behavior is found for tapes and pressed powder samples; here coercivity increases with time by approximately the same amount. The effect is independent of the presence of Fe²⁺ ions in the sample; thus oxidation of ferrous ion can be excluded as the cause. The phenomena are interpreted in terms of local structural changes within the cobalt-ferrite surface layer that are induced by the presence of oxygen