Crystallisation of BaFe12O19 hexagonal ferrite with an aid of B2O3 and the effects on microstructure and magnetic properties useful for permanent magnets and magnetic recording devices

doi:10.1016/0304-8853(89)90073-5

Journal of Magnetism and Magnetic Materials

Volume 82, Issue 1, November 1989, Pages 129-150

https://doi.org/10.1016/0304-8853(89)90073-5 Get rights and content

Abstract

Polycrystalline Ba-ferrite (BaFe₁₂O₁₉) using B₂O₃ (up to 1 mol) as a sintering aid has been prepared by the usual solid state reaction of BaCO₃ and Fe₂O₃. The B₂O₃, which exists in molten form during the reaction (operated at 1000–1400°C), dissolves the reactants in intimate contact. The dissolved solid and molten B₂O₃ both diffuse into the system to facilitate the reaction, BaCO₃+6Fe₂O₃ ⇋ BaFe₁₂O₁₉+CO₂, of ferrite formation. The reaction is very sensitive to B₂O₃ addition and temperature (T_s) employed for the sintering. The distortion of Ba-ferrite lattice, that usually appeared due to sintering at higher temperature, T_s⩾1300°C, is considerably reduced by the use of B₂O₃(x) ⩽0.3 mol addition. The distortion however appears for the intermediate B₂O₃ content of 0.3 mol ⩽x<0.7 mol. A model for the reaction based on the experimental results of X-ray diffractometry and microstructure of the samples is proposed. It accounts successfully for the crystallisation of magnetic particles and variation in the magnetic anisotropies. The effect of B₂O₃ additions to Ba-ferrite is reflected by the increase of magnetisation M_s (by ≈13%) and the decrease of coercivity H_c (to as small as 5 Oe), subjected to the sintering especially at higher T_s. The materials obtained with M_s as large as 79 emu/gandH_c=4−3 kOe (at 25°C) are suitable for the use as permanent magnets, whereas those comprising small H_c of ≈1 kOe can be useful for magnetic recording applications. The increase of M_s is explained by invoking the fact that B₂O₃(B³⁺) substitutes on Fe³⁺ (tetrahedral) sites according to BaFe_12−yB_yO₁₉, with y up to 0.4 (equivalent to x ≈ 0.2 mol). The substitution (B³⁺→Fe³⁺) causes distortion in the tetrahedral sites within the crystal unit cell as evidenced by the EPR spectroscopy. We also developed an empirical relation describing the EPR linewidth (ΔH) in Fe³⁺ ferrimagnetic resonance at g ≈ 2.5 and the various magnetic anisotropy factors. This enables us to analyse for the anisotropy factors by using the observed values for ΔH, H_c and M_s.

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In the late 1980s [12–14], our group has used crystallization of vitreous glasses such as BaOFe2O3B2O3 that yields ferrites and borates of small fibrils. In another experiment, using B2O3 as an additive, which promotes solid state reaction between BaO and Fe2O3, a selective phase of barium ferrite BaFe12O19 was made of tailored shapes [15]. Some studies had been carried by authors’ on the iron borate glasses with tailored magnetic as well as dielectric properties [16,17].
A well-known canted antiferromagnetic Fe₃BO₆ with functionalized properties is an important material useful for light energy carrier, electrodes, gas sensors, and biological probes. In this investigation, a facile synthesis is explored in order to obtain Fe₃BO₆ of small crystallites in a specific shape of nanoplates by self-combustion method. To access (i) whether Fe₃BO₆ is formed at the as-prepared stage and (ii) how it stands stable with a residual carbon surface layer, thermal gravimetric (TG) analysis has been carried out by heating 10–20 mg powder (as-prepared) at a typical 10 K/min heating rate over 300–1100 K under air or argon atmosphere. The electronic absorption, infrared (IR) and Raman spectra studied for the Fe₃BO₆ sample in this investigation elucidate how the density of states of the phonons and valence electrons confine in small crystallites. IR and Raman bands in the oxygen polygons also confer the results of forming Fe₃BO₆ with a bonded surface carbon layer. A stable bonded surface layer supports thermal stability of small crystallites and it affects other useful functional properties.
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A nanoscale iron borate is rarely prepared except Fe3BO5 made as nanorods using a hydrothermal method [16], or a solution phase reaction [9], and Fe3BO6 spherical nanoparticles made from a rheological phase reaction [7]. Quite good numbers of studies are carried in authors' laboratory on the iron borate glasses [17–21], in which small fibrils precipitate as ferrites and/or borates on thermal annealing a glass specimen at moderate temperature (above the glass-transition temperature Tg) in air. The crystallites so obtained in general embed in a residual vitreous phase of a different electronic structure and refractive index.
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Crystallisation of BaFe12O19 hexagonal ferrite with an aid of B2O3 and the effects on microstructure and magnetic properties useful for permanent magnets and magnetic recording devices

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Crystallisation of BaFe₁₂O₁₉ hexagonal ferrite with an aid of B₂O₃ and the effects on microstructure and magnetic properties useful for permanent magnets and magnetic recording devices