Surface modification of gamma-Fe2O3 particles with cobalt containing spinel

The surface of acicular gamma-Fe2O3 particles was modified using a Co2+ containing aqueous suspension. Since octahedrally coordinated Co2+ ion in the spinel lattice produces strong magnetocrystalline anisotropy, the coercivity of the particles was enhanced. It was found from the XPS analysis that, in the case of the particles pre-modified with some Al3+ oxide or oxyhydroxide, a large amount of Co2+ ions did not enter the spinel phase but remained in the hydroxide phase. As the temperature of the modification reaction was raised, the transformation of Co2+ into the spinel lattice increased, resulting in the enhancement of coercivity.