Magnetic properties of mixed cobalt-zinc ferrite nanoparticles

Solid solutions of nonstoichiometric mixed cobalt-zinc ferrite nanoparticles Co_0.73yZn_{0.73(1 - y)}Fe_2.18□_0.09O₄, were prepared in order to study their magnetic properties as a function of cobalt content y. The saturation magnetization changes with increasing y due to the various occupancies of cations in tetrahedral and octahedral sites and/or to an increase in the disorder of the ferrimagnetic structure. The cobalt content and particle size effects on the coercive field were studied. The increase in the ratio between remanent and saturation magnetizations indicates that, with increasing y, a transition takes place from uniaxial to cubic anisotropy. Field cooled (FC) curves drastically change with the cobalt content. At zero cobalt, this curve is characterized by a well-defined peak that disappears progressively with increasing y. This is attributed to the particles themselves and neither to their interactions nor to experimental procedures. The fit of the zero FC (ZFC) curve indicates an unrealistically high simulated saturation magnetization, consistent with a drastic field-dependent behavior of the ZFC-FC curves.