Co-coated acicular magnetite particles and process for producing the same

Co-coated acicular magnetite particles each comprising:

• an acicular magnetite particle as the core particle;
• a lower coating layer 4 to 50 .ANG. thick of Co-containing spinel ferrite, which is formed on the surface of said acicular magnetite particle;
• and
• an upper coating layer 2 to 20 .ANG. thick of Co- and Mg-containing spinel ferrite, which is formed on said lower coating layer,
• said particles having an aspect ratio (major axial diameter/minor axial diameter) of not less than 4, a coercive force of 580 to 900 Oe, a saturation magnetization of 77 to 90 emu/g, a reduction ratio of the Fe.sup.2+ content after said Co-coated acicular magnetite particles were permitted to stand at a temperature of 60.degree. C. and a humidity of 90% for 14 days of not more than 51%, and a change of the coercive force after said Co-coated acicular magnetite particles were permitted to stand at a temperature of 60.degree. C. and a humidity of 90% for 14 days of -10 to +5%,
• the Fe.sup.2+ content of the Co-coated acicular magnetite particles being 10 to 24 wt %, the cobalt content thereof being 0.1 to 10 wt % (calculated as Co), and the magnesium content thereof being 0.2 to 3.5 wt % (calculated as Mg),
• said particles obtained by a process comprising the steps of:
• (1) heating and stirring a suspension obtained by adding an aqueous alkali hydroxide solution and an aqueous solution of a cobalt salt to an aqueous dispersion of acicular magnetite particles;
• (2) adding an aqueous solution of a magnetism salt to said suspension in the process of the cobalt coating reaction;
• and
• (3) continuing to heat and stir the resulting mixture completing said cobalt coating reaction, thereby forming an upper coating layer composed of Co- and Mg-containing ferrite having a spinel structure on the surface of the lower coating layer composed of Co-containing ferrite having a spinel structure, which is formed on the surface of each acicular magnetite particle.

Co-coated acicular magnetite particles each comprising:

• an acicular magnetite particle as the core particle;
• a lower coating layer 4 to 50 .ANG. thick of Co-containing spinel ferrite, which is formed on the surface of said acicular magnetite particle;
• and
• an upper coating layer 2 to 20 .ANG. thick of Co- and Mg-containing spinel ferrite, which is formed on said lower coating layer, and
• said particles having an aspect ratio (major axial diameter/minor axial diameter) of not less than 4, a coercive force of 580 to 900 Oe, a saturation magnetization of 77 to 90 emu/g, a reduction ratio of the Fe.sup.2+ content after said Co-coated acicular magnetite particles were permitted to stand at a temperature of 60.degree. C. and a humidity of 90% for 14 days of not more than 51%, and a change of the coercive force after said Co-coated acicular magnetite particles were permitted to stand at a temperature of 60.degree. C. and a humidity of 90% for 14 days of -10 to +5%,
• the Fe.sup.2+ content of the Co-coated acicular magnetite particles being 10 to 24 wt %, the cobalt content thereof being 0.1 to 10 wt % (calculated as Co), and the magnesium content thereof being 0.2 to 3.5 wt % (calculated as Mg),
• said particles obtained by a process comprising the steps of:
• (1) heating and stirring a suspension obtained by adding an aqueous alkali hydroxide solution, an aqueous solution of a cobalt salt and an aqueous solution of a ferrous salt to an aqueous dispersion of acicular magnetite particles;
• (2) adding an aqueous solution of a magnesium salt to said suspension in the process of the cobalt coating reaction;
• and
• (3) continuing to heat and stir the resulting mixture so to complete said cobalt coating reaction, thereby forming an upper coating layer composed of Co- and Mg-containing ferrite having a spinel structure on the surface of the lower coating layer composed of Co-containing ferrite having a spinel structure, which is formed on the surface of each acicular magnetite particle.