Theory of surface anisotropy and coercivity in ferrimagnetic γ-Fe₂O₃

We propose a quantum mechanism for surface magnetic anisotropy in a special class of spinel ferrite. One-ion anisotropy arises from the orbitally degenerate state of a 3dε{lunate}-electron bound to an Fe3+ core on a spinerl B-site near a (110)-facet. Dependence of electron energy on the orientation of the B-site trigonal symmetry axis with respect to the facet plane leads to a maximum in surface anisotropy versus surface charge. This explains published experimental dependences of the coercive field on the amount of polymeric (NaPO3)n·Na2O deposited onto chemically reduced acicular particles of γ-Fe2O3. The experimental magnitudes of coercivity change are understood if individual polyphosphate molecules oxidize such anisotropic subsurface sites with considerable quamtum efficiency. © 1992.