Preparation and characterisation of cobalt and cobalt-zinc ferrites for magnetorheological materials

The aim of this study was to study the structure, morphology, magnetic, and magnetorheological properties of cobalt and cobalt-zinc ferrite powders to be used as a functional component of magnetorheological fluids. Ferrites of cobalt CoFe2O4 and cobalt-zinc Co0.65Zn0.35Fe2O4 were obtained by combined hydrolysis of inorganic metal salt in aqueous solutions followed by thermal treatment of the precipitates. The ferrites were studied by means of X-ray phase analysis, scanning electron microscopy, IR spectroscopy, and magnetometry. The synthesised ferrites are polydispersed powders with the size of primary particles of 300–400 nm and the size of the coherent scattering regions of 22–33 nm. They demonstrate a high shear stress in magnetorheological suspensions, which is 2.5 times higher than that of the nanosized particles. High-temperature annealing results in a significant increase in the specific magnetisation of the powders, as well as the shear stress in suspensions prepared on their basis. Doping cobalt ferrite with zinc leads to an increase in the specific magnetisation and rheological characteristics. The studied materials have a high shear stress in suspensions (~ 2.5 kPa at 650 mT) and can be used as functional fillers for magnetorheological materials.