gamma -Fe2O3 of improved quality from direct synthesis of acicular alpha -Fe2O3

Fe(OH)₃ is precipitated from an Fe(III)-containing solution leading to a gel with alpha -Fe₂O₃ nuclei. Both the size and the number of the nuclei are governed by the conditions during precipitation. The precipitate is heated under alkaline conditions after addition of organic complexing agent having a strong chelating capability for Fe(III). The complexing agents retard the growth of the alpha -Fe₂O₃ nuclei in the crystallographic a-directions, so that growth proceeds in the c-direction, leading to needles instead of platelets. It is impossible to obtained products which are thinner than the starting nuclei. A doping with tin was developed to reduce the size of the nuclei. This leads to thinner needles, which can be used in improved magnetic media with lower noise.