Magnetic pigments for recording media

CrO₂ doped with 1--2 wt% Fe³⁺ on Cr⁴⁺ positions is one of the most important materials for magnetic recording in audio, data and video tapes. ⁵⁷Fe Mössbauer spectroscopy was applied to determine the level of iron doping in newly developed high coercivity CrO₂ particles. It was found that, compared with the conventional preparation process, the new particles contain increased amounts of Fe³⁺ ions in the CrO₂ crystal lattice giving rise to higher magnetocrystalline anisotropy. This is reflected in coercivities of up to over 900 Oe. All samples contain as a secondary iron-containing phase α-(Cr_1-xFe_x)₂O₃, the amount of which is higher in samples of the conventional preparation process than in those of the new BASF process. Details about the mechanism of the development of doped CrO₂ particles in both types of processes were obtained by means of Mössbauer, XRD and wet chemical investigations making possible an optimization of the new BASF process.