Low coercive iron oxide pigments and a process for their preparation

Cobalt coated, needle-shaped berthollide iron oxide pigments comprising:

• an iron oxide core having the composition (Fe.sub.x.sup.2+, Fe.sub.2.sup.3+) O.sub.3+x, wherein 0.3<x<0.75,
• a cobalt coating, the quantity of cobalt being from 0.1 to 1.5% by weight based on the weight of the core, and
• a berthollide Zn-containing compound coating composed of a berthollide compound having the composition (Fe.sub.x.sup.2+, Fe.sub.2.sup.3+) O.sub.3+x, wherein 0.3<x<0.75, and Zn, the berthollide Zn-containing compound coating being situated between the cobalt coating and the iron oxide core, the quantity of Zn in said coating being from 0.1 to 2% by weight based on the weight of the core,
• said cobalt coated, needle-shaped berthollide iron oxide pigments having characteristics that when the specific saturation magnetization is greater than 102mWb.m/kg and the coercive field strength is from 30 to 37 kA/m, the printing attenuation measured after incorporation of the pigments in an audio tape according to DIN IEC 94 Part 5 is greater than 50 dB, based on a magnetic layer thickness of 12 .mu.m and an ash content of 11 g/m.sup.2 ;
• and
• produced by the process comprising:
• dispersing the iron oxide core material in water to form a dispersion;
• adding an aqueous iron salt solution and an aqueous zinc salt solution to the dispersion;
• adding an alkaline solution to the dispersion to precipitate the iron and zinc salts, thereby forming the berthollide Zn-containing compound coating on the iron oxide core material;
• introducing air to the dispersion;
• and
• then coating the obtained material with a cobalt compound using a soluble cobalt salt in alkaline medium under a nitrogen atmosphere.