Perpendicular magnetic alumite discs were obtained using an anodic oxidation method.The additional new pore widening process was effective to control the coercive force. The electro-deposited fine iron needles were single crystals and the growth direction was [110]. The recording characteristics of 3.5 and 5.25 inch rigid discs were evaluated. A recording density D50of 50 KBPI was attained for the disc with a media thickness 1.5 µm. The environmental test under an atmosphere of of 40 °C 90 % R. H. over 1000 hr affirmed the stability of the alumite disc with 250A SiO2overcoat.
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Tsuya et al. (1986) studied this question.