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-(Cr1-xFex)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.