Aging phenomena in cobalt-modified iron oxides: influence of dilution and magnetic interactions

Aging of cobalt-modified iron oxides is shown to lead to changes of the coercivity of the particles. Microscopically, this is due to variations in the local structure of the cobalt ferrite surface layer under the influence of magnetic interactions with neighboring particles. By means of dilution with nonmagnetic material these interactions can be reduced and, consequently, in these diluted samples no aging is observed. Aging is dependent upon the topography of the surface layer. In contrast to the above-mentioned smooth particles, pigments with very rough surfaces do not show aging effects. This is interpreted as being due to very large stray fields in the rough surface layers which mask the additional effect of particle interactions