A halide free route to the manufacture of microstructurally improved M ferrite (BaFe12O19 and SrFe12O19) fibres

In hexagonal ferrite precursor fibres halide was found to be retained up to high temperatures, delaying the formation of the BaM (BaFe12O19) and SrM (SrFe12O19) phases, and resulting in a product with a grain size of 0.5–1 μm. This paper reports a halide free route for these fibres, using a nitrate stabilised sol precursor, from which the M phases formed at lower temperatures with smaller grain sizes. This sol was characterised and compared to the halide based sol, the evolution of the crystalline phases was studied and the hysteresis loops of the M ferrite fibres assessed by VSM. It was shown that retained halide inhibited the formation of the ferrite phase, as both SrM and BaM formed at a temperature 200 °C lower than in the halide containing precursor fibres. The grain sizes were accordingly lower, the SrM grains being below 100 nm, although the crystallite sizes were only slightly smaller than those reported for the halide containing fibres fired to 1000 °C, and the magnetic properties were also similar. The BaM fibres had Ms=58.4 kA m2 g−1 and Hc=401 kA m−1, while the SrM fibres had Ms=65.0 kA m2 g−1 and Hc=440 kA m−1, comparable to small grained polycrystalline powdered samples. The M ferrite phase was formed directly from haematite, with no BaFe2O4 observed as an intermediate, and the pure ferrites were made from stoichiometric precursors.