Toward a model for Co-surface-treated Fe-oxides

A series of Co-surface-treated particles was prepared from an acicular γ-Fe₂O₃ containing 15.5% Fe²⁺ . The samples contained 4, 8 and 16% Co of total cation content. For all treated samples, H_c at 300 K increased by 250% over that of the precursor. Magnetic and structural measurements were made to determine the mechanism for this H_c increase. Surface treatment produced an epitaxial layer with spinel structure. It was determined that the epitaxial layer consisted of an initial monolayer or less of magnetically ordered Co²⁺ ions covered by the spinel Co₃O₄ which orders antiferromagnetically at 40 K. This structure suggests that the magnetic behavior could arise from surface anisotropy, the neutralization of easy reversal sites, or the reduction of spin canting at the particles´ surfaces