NOVEL EQUIAXED MAGNETITE MEDIA WITH HIGH COERCIVITY AND THERMAL-STABILITY

Equiaxed magnetite media with coercivity of up to 1560 Oe, saturation and residual magnetizations of up to 83 and 50 emu/g, respectively, and squareness ratio of up to 0.61 have been developed by the modification of Mn and Co, and by proper annealings. They were prepared by coprecipitation of Fe, Mn, and Co ions from an aqueous solution, by suitable alkali such as triethylamine, to adjust the pH, followed by calcination, reduction, oxidation, step-annealings, and final reduction. Oxidation leads to a defect spinel ferrite structure, whose coercivity can be greatly enhanced by a step-annealing treatment, and high coercivity can be retained after the final reduction to magnetite, whose coercivity can not be raised at all by the step-annealing. These media have a temperature coefficient of coercivity as low as 0.20-0.33%/-degrees-C, hence are potential candidates for high density recording.