Nanocrystalline iron oxide and Ba ferrite particles in the superparamagnetism-ferromagnetism transition range with ferrofluid applications

Magnetic fluids based on Ba hexaferrite as well as iron oxide particles with enhanced anisotropy barriers show heating effects in ac magnetic fields which may be useful in technical processes as well as medical applications (magnetic hyperthermia). Such particles also allow the detection of biological binding reactions through an enhanced Neel relaxation time above the Brown relaxation. The loss processes and the relaxation times depend strongly on the mean particle size and the size distribution width. To influence and improve the mean size as well as the size distribution, new approaches to the preparation are promising, where nucleation and growth of the particles can be influenced independently or where further growth is possible on small given particles without further nucleation. We used a glass crystallization method for preparation of nanocrystalline Ba hexaferrite as well as magnetic iron oxide and a cyclic growth method based on 'conventional' precipitation for iron oxide preparation. Properties of the powders prepared, as well as water based ferrofluids, were analysed using x-ray diffraction, transmission electron microscopy and magnetic methods. Values of the specific loss power of the order of 100 W/g maghemite may be achieved with the option of further increase by improving the core size distribution.