There are various known processes for increasing the coercive field strength of magnetic iron oxides.
According to U.S. Pat. No. 4,297,395, pigments showing increased coercive field strength are obtained from .alpha.-FeOOH precursor compounds which have been doped with cobalt. A coercive field strength of around 620 Oe is obtained with 1.0% of cobalt for an FeO-content of 20%. However, these pigments have the disadvantages of low copying attenuation and inadequate magnetic stability.
Another method of obtaining high coercive field strengths is described in U.S. Pat. No. 4,122,216. In this process, a layer of cobalt ferrite is crystallized epitaxially onto a core of magnetic iron oxide in a strongly alkaline medium. Compared with the cobalt doped pigments, these pigments show considerably better copying attenuation values and greater magnetic stability. One disadvantage of these pigments however, lies in the fact that large quantities of the very expensive element, cobalt, are required for obtaining the desired coercive field strength.
GB-PS No. 2.060.592 describes a process in which the magnetic iron oxide is dispersed in water in a mildly alkaline medium. Cobalt is then deposited onto its surface. The cobalt-coated iron oxides thus obtained have a higher coercive field strength than products which are dispersed in water having a mildly acidic pH-value (pH 4.2). However, the increase obtainable in coercive field strength and the possible saving of cobalt are minimal.
Accordingly, the object of the present invention is to provide magnetic iron oxide pigments having increased coercive field strength which are not attended by any of the above-mentioned disadvantages for equivalent cobalt contents.