Abstract: A process for preparing acicular .alpha.-FeOOH by partially neutralizing and then oxidizing a ferrous salt solution in the presence of phosphoric acid or a salt thereof to form seed crystals of .alpha.-FeOOH and optionally further oxidizing said solution while neutralizing said solution with an alkali to grow said seed crystals, wherein pyrophosphoric acid or a salt thereof is used as said phosphoric acid and the seed crystals of .alpha.-FeOOH are formed at a temperature of 50.degree. to 100.degree. C. The .alpha.-FeOOH obtained according to the process of this invention is composed of fine particles and appreciably improved in acicularity and particle size distribution. Also, a magnetic recording medium made from the magnetic iron oxides derived therefrom in a usual way have a low noise characteristic and, are excellent in coercivity and other magnetic properties.

Abstract: A process is provided for coating the particle surfaces of a phosphor-containing magnetic iron oxide powder with a cobalt compound after subjecting said powder to an activation treatment with an aqueous alkaline and/or acidic medium. The cobalt-containing magnetic iron oxide powder obtained according to this process has excellent coercivity and thermal stability, and the magnetic tape produced by using said powder has not only high coercivity but also excellent squareness, orientability and remanent induction.

Abstract: A phosphorus-containing magnetic iron oxide powder is subjected to the activation with an aqueous alkaline medium under pressure and then coated with a cobalt compound or a cobalt compound and other metallic compound(s) to produce a cobalt-containing magnetic iron oxide powder. The magnetic iron oxide powder obtained according to this process finds its best application to the production of magnetic recording media.

Abstract: A process which comprises coating a cobalt compound or a combination of a cobalt compound and a ferrous compound on the surface of magnetic iron oxide particles and then heating the coated particles in the presence of water vapor. The cobalt containing ferromagnetic iron oxide produced by this process is excellent in coercivity and thermal stability. Further, magnetic tape produced from this iron oxide has a high coercivity and is excellent in squareness, orientability and remanent induction.