Crystallisation of BaFe 12O 19 hexagonal ferrite with an aid of B 2O 3 and the effects on microstructure and magnetic properties useful for permanent magnets and magnetic recording devices

Abstract

Polycrystalline Ba-ferrite (BaFe 12O 19) using B 2O 3 (up to 1 mol) as a sintering aid has been prepared by the usual solid state reaction of BaCO 3 and Fe 2O 3. The B 2O 3, which exists in molten form during the reaction (operated at 1000–1400°C), dissolves the reactants in intimate contact. The dissolved solid and molten B 2O 3 both diffuse into the system to facilitate the reaction, BaCO 3+6 Fe 2 O 3 ⇋ BaFe 12 O 19+ CO 2, of ferrite formation. The reaction is very sensitive to B 2O 3 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 B 2 O 3( x) ⩽0.3 mol addition. The distortion however appears for the intermediate B 2O 3 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 B 2O 3 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/ g and H 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 B 2O 3(B 3+) substitutes on Fe 3+ (tetrahedral) sites according to BaFe 12−yB yO 19, with y up to 0.4 (equivalent to x ≈ 0.2 mol). The substitution (B 3+→Fe 3+) 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 (Δ H) in Fe 3+ 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 Δ H, H c and M s.