Journal of Magnetism and Magnetic Materials 310 (2007) 107–115 Reduction behavior of barium hexaferrite into metallic iron nanocrystallites M. Bahgat à , M. Radwan, M.M. Hessien Central Metallurgical Research and Development Institute (CMRDI), P.O. Box 87 Helwan, Cairo 11421, Egypt Received 25 May 2006; received in revised form 16 July 2006 Available online 30 August 2006 Abstract Barium hexaferrite (BaFe 12 O 19 ) powder was prepared through the ceramic route by calcination of a stoichiometric mixture of barium carbonate and iron oxide at 1200 1C for 2 h. Compacts of synthesized BaFe 12 O 19 powder were isothermally reduced in hydrogen atmosphere at 600–1000 1C. Based on thermogravimetric analysis, the reduction behavior of BaFe 12 O 19 and reaction kinetics and mechanism were studied. The initial ferrite powder and the various reduction products were characterized by X-ray diffraction analysis,a scanning electron microscope and a vibrating sample magnetometer to reveal the effect of hydrogen reduction on composition, microstructure and magnetic properties. The reduction rate was found to be controlled by interfacial chemical reaction mechanism. Metallic iron was formed in nanosized regimes while grain growth and coalescence were observed at higher reduction temperatures. Magnetic measurements show diluted magnetic properties for the formed metallic iron, whereas coercivity values were in the range 14.1–55 Oe and saturation magnetization varied from 24.79 to 36.15 emu/g. These values have been ascribed to the formation and growth of nanosized metallic iron particles as the reduction treatment proceeded. r 2006 Elsevier B.V. All rights reserved. Keywords: Barium hexaferrite; Reduction; Nanocrystallite; Metallic iron; Diluted magnetic 1. Introduction The synthesis of ultrafine magnetic powders for applica- tions such as magnetic recording and magnetic fluids has received wide attention in recent years. Metallic iron particles have been used as magnetic media to prepare metal tapes due to their high magnetic moment and coercivity. Many investigators [1–3] studied the prepara- tion of metallic ultrafine particles and its magnetic properties based on various techniques like laser-induced pyrolysis, mechanochemical processing, etc. Carpenter [4] studied the preparation of stable magnetic nanoparticles composite based on iron. It was clearly found that these nanocomposite materials offer enhanced magnetic properties over other composites based on the presence of iron oxide. Uniform and stable a-Fe nanopar- ticles of around 40 nm in width and axial ratios from five to seven have been obtained from hematite (a-Fe 2 O 3 ) without any additive by heating under a hydrogen atmosphere [5]. There is a minimum temperature (400 1C) and a minimum hydrogen flow (20 l/h) to reduce the hematite particles to metal in a reasonable time (4 h), preserving the size and the shape of particles. The main change in the material is related to the crystallite size of metal particles, which increases as reduction proceeds. The magnetic properties of nanoparticles compressed into pellets and dispersed in the composites were studied by Baker et al. [6] and Kuhn et al. [7]. The particles were observed to exhibit increased coercivity and exchange bias. The exchange bias was observed to increase with oxide shell thickness. The magnetism in nanoparticle composites was studied as a function of nanoparticle loading. It was observed that when particles were dispersed into the nanocomposite, coercivity was increased, suggesting a heightened aniso- tropy barrier. Similarly, the magnetic relaxation results indicate that the composites exhibit significantly reduced relaxations through the entire temperature range, as ARTICLE IN PRESS www.elsevier.com/locate/jmmm 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.07.032 à Corresponding author. Tel.: +2 02 5513854; fax: +2 02 5010639. E-mail address: m_bahgat70@yahoo.com (M. Bahgat).