Method of producing cobalt-modified magnetic particles

A method of producing cobalt-modified magnetic particles which comprises:

• (a) heating an aqueous suspension of amorphous ferric hydroxide at an elevated temperature from 100.degree. C. to 250.degree. C. at an alkaline pH in the presence of an effective amount of at least one growth regulating agent dissolved in the suspension, in the presence or in the absence of .alpha.-ferric oxide seed crystals of minor axes not more than 0.4 microns in average in amounts of 0.1-25 mole % in terms of Fe content in relation to the ferric hydroxide in the suspension, the growth regulating agent being selected from the group consisting of an organic phosphoric acid, a hydroxycarboxylic acid, a polybasic acid, an aminocarboxylic acid, an aminosulfonic acid, an aromatic sulfonic acid, a thiocarboxylic acid, a water soluble salt of any one of the acids, a water soluble ester of any one of the acids, a polyamine, a thioalcohol, a polyhydric alcohol and a .beta.-dicarbonyl compound, for a length of time sufficient to convert the amorphous ferric hydroxide into acicular .alpha.-ferric oxide,
• (b) converting the .alpha.-ferric oxide into .gamma.-ferric oxide by reduction thereof followed by oxidation, and
• (c) forming an aqueous suspension of the .gamma.-ferric oxide and ferrous hydroxide and cobaltous hydroxide in amounts of 0.5-50 mole %, respectively in relation to the Fe content of the .gamma.-ferric oxide in the suspension, said suspension having an alkaline pH and maintaining the suspension at a temperature from 30.degree. C. to 50.degree. C. for a time sufficient to form a cobalt modified .gamma.-ferric oxide;
• said cobalt modified .gamma.-ferric oxide when incorporated into a tape effecting a magnetic recording tape having an improved squareness ratio.