PREPARATION AND PROPERTIES OF PARTICLES FOR MAGNETIC RECORDING

Abstract

Selected contributions in the fields of iron oxide, metal and CrO2 magnetic pigments are presented. Several wet chemical synthesis of undoped and cobalt bulk doped isometric Fe3O4-particles from Fe(OH)2 and iron(III) compounds are compared with regard to the influence of composition and geometrical dimensions of the ferric coreactant on the particle size and the magnetic properties of the product. Oxidation of cobalt bulk doped magnetite particles leads to pigments with multiaxial magnetic behaviour. The structure of passivation layers on metal pigments is analyzed by using different techniques. Thicknesses of 4 - 5 nm are measured 0.5 to 1 nm of which are due to surface coatings with phosphate compounds which are used to prevent sintering of the particles. Several reduction methods for the preparation of elongated metal pigment are compared. Details of the reaction mechanism of the hydrothermal synthesis of needle shaped CrO2 in the presence of antimony are discussed. CrO2 pigments qualitatively show the same behaviour with regard to the dependence of coercivity on particle size as described earlier for other materials. Maximum coercivity of 60 kA/m for Sb-Fe-doped CrO2 is obtained with crystallite sizes (220) of 23 nm. Chemically stabilized CrO2 pigments are protected by layers of 0.7 to 1.3 nm thickness only. For use in magnetic recording media both, metal pigment and Cr2O2 can thus be described by a core-shell-structure.