Field response of surface spins on Co-adsorbed γ-Fe2O3 (abstract)

Abstract

The Mössbauer spectrum of a small particle (250 Å diameter), partially reduced γ-Fe2O3 sample coated sequentially with 57Fe and Co was obtained in a 1.8-kOe field transverse to the γ-ray propagation direction at room temperature. The adsorbed 57Fe, less than a monolayer thick, provided half the Mössbauer signal. The data were fit with a distribution in hyperfine fields. The relative areas of the Mössbauer lines of each subspectrum are in the ratio 3:2p:1:1:2p:3, with p the common polarization; large p implies small HK.1 For several possibilities of B-site to A-site population ratios and variations in core isomer shifts, the resultant spectrum least-squares fit values of p were found to be about 1.4. Theoretical calculations1 of composite Mössbauer line intensities based on a model2 of a rigid random-spin shell and a core with its p determined by the anisotropy field obtained1 from magnetization vs applied field measurements yield a composite p≊1.2. Since this value is substantially smaller than experimentally observed, the outer shell spins are not rigidly pinned even in this weak applied field. A simple theoretical free-energy minimization calculation of a linear chain of ferrimagnetically coupled atoms in an external field applied along the hard axis shows that a high anisotropy ‘‘shell’’ will cause a parabolic dependence of sublattice spin angle vs position along the chain, with the maximum deviation from collinearity of a few degrees.<ks>