Hard magnetic barium ferrite-iron composite materials
Artur Witkowski, Waldemar Kaszuwara, Marcin Leonowicz Politechnika Warszawska, Wydział Inżynierii Materiałowej, ul. Wołoska 141, 02-507 Warszawa
Quarterly No. 2, 2005 pages 85-90
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abstract Permanent magnet nanocomposites are the group of hard magnetic materials, which exhibit enhanced remanence due to both the nanoscale grain structure and presence of soft magnetic phase. The phenomenon of enhanced remanence was comprehensively described for Nd-Fe-B and Sm-Fe-N alloys containing Fe precipitates. Different opinions exists in terms of possibilities to obtain enhanced remanence in ferrite permanent magnets. It was proved that about 5% of Fe fine powder, added to the barium ferrite, leads to improvement of the remanence. But this is rather a result of the existence of inclusions having high saturation magnetisation than the magnetic exchange interactions between hard and soft magnetic phases. Because the basic condition of for the remanence enhancement is nanoscale grains structure, the effect of mechanical milling time of barium ferrite and Fe powder mixture on grain size and magnetic properties was investigated. The dependence between milling time, powder particle size and magnetic properties was studied (Figs 1-4). Grain size of the Fe crystallites, after milling for 192 h, amounds to about 20 nm. However, long time mechanical milling leads to partial amorphousation of the barium ferrite phase. Powders milled for 192 h beside the barium ferrite contained Fe and Fe2O3 phases (Figs 5, 6). This resultes in decrease of the of magnetic properties (Figs 3, 4). Annealing of the milled powders at a temperature 750oC/1 h can not remit the magnetic properties because of formation of the mixture of BaFe2O4 and Fe3O4 (Fig. 9). Concluding, it was found that the mechanical milling allows to obtain nano grain Fe, however, changes of the phase constitution leads to decrease of the magnetic properties of the composites. Key words: barium ferrite, hard magnetic nanocomposite, mechanical alloying