The hexagonal ferrite W=BaFe2+2Fe3+16O27 has a saturation magnetization about 10% higher than the widely used permanent-magnet material M=BaFe3+12O19 and the anisotropy field is about equal. This opens up the possibility of using W as a permanent-magnet material with a higher remanence Br and energy product BHm. This paper describes the sintering of W having a slight deviation from the stoichiometric composition and containing a trace of SiO2 at, e.g., 1220 °C in a reducing atmosphere to 92–95% of the x-ray density and containing well-oriented grains of 2–8 μm. Samples with coercive forces Hc=1.5–2.1, Br=4.4–4.7, and BHm=3.7–4.3 (B in kG, H in kOe) were obtained, which may be compared with Hc=2.1, Br=4.0, and BHm=3.4 found from our own measurements on high-quality commercial M. We found surprisingly that the pickup of a trace iron during grinding increases the sintering density considerably, and this is therefore essential for the preparation of useful material.
The hexagonal ferrite W=BaFe2+2Fe3+16O27, which is in equilibrium with oxygen under 0.015 atm at 1350°C, is oxidized in the range 250° to 1100°C in the same atmosphere to BaFe3+16+2x Fe22‐3x>xO27 with 2–3x<0.03. The metal‐deficient W structure is chemically unstable and decomposes after longer annealing at the same temperature. This difference in oxidation and decomposition rate is attributed to a difference in the diffusion rate of barium and iron ions.
It is experimentally demonstrated that two techniques allow the manufacture of a 100-µm-thick cover layer within the requirements of the digital video recording system, that uses a high numerical aperture of 0.85 at 650 nm wavelength. The first technique uses spin-coating of an ultraviolet curable resin and the second is the ultraviolet bonding of a 100-µm-thick
polycarbonate sheet. Both techniques guarantee thickness variations within ±3 µm, and display disc radial and tangential tilts
within sufficient margins.
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