Magnetic powders for use in the manufacture of magnetic recording media are produced by a borohydride process in which aqueous solutions of ferrous sulfate and sodium borohydride are mixed in a magnetic field. The coercive force of the magnetic powder produced as a result of this method is controlled...http://www.google.fr/patents/US4394160?utm_source=gb-gplus-shareBrevet US4394160 - Making magnetic powders
