A STUDY ON THE ANISOTROPY OF COBALT-SURFACE-MODIFIED γ-Fe2O3 PARTICLES

The coercivity of cobalt-surface-modified γ-Fe2O3 particles is 8000 to 32000 A/m greater than that of the original γ-Fe2O3. The origins of uniaxial anisotropy and the mechanisms of the increase of coercivity of Co-γ-Fe2O3 and CoFe-γ-Fe2O3 particles are studied in this paper. The directional order (preferred orientation) of the cobalt ions appears on the surface of γ-Fe2O3, raises the surface uniaxial anisotropy and thus increases the coercivity of Co-γ-Fe2O3. The squareness ratios of various samples of CoFe-γ-Fe2O3 (Co/γ-Fe2O3 = 2 -16wt%) are obtaihed to be about 0.5. The coercivity of CoFe-γ-Fe2O3 is increased by the uniaxial anisotropy and multiaxial magnetocrystalline anisotropy strengthened after CoFe-coating. The origins of uniaxial anisotropy strengthened are suggested. The first is that directional order (preferred orientation) of cobalt ions on the surface of γ-Fe2O3 results in surface uniaxial anisotropy. The second is that the uniaxial anisotropy results from preferred alignment of preferred axis of cobalt ferrite microcrystal in surface layer of γ-Fe2O3.