Field response of surface spins on Co-adsorbed γ-Fe2O3

Abstract

The Mössbauer effect has been employed to determine spin orientations at the surface and in the core of magnetic recording particles in applied fields. The samples consisted of a partially reduced γ-${\mathrm{Fe}}_2$ ${\mathrm{O}}_3$ (A), A coated with ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ (B), B coated with Co (C), and a sample (D) obtained by coating A with natural Fe followed by Co. About one-half of the Mössbauer signal of samples B and C thus came from atoms on the surfaces of the particles. Samples C and D showed the strong enhancement of coercivity typical for Co-coated surfaces. In 60 kOe longitudinal applied field at 4.2 K, sample C showed much less canting than expected from the previously hypothesized random spin surface. First-quadrant moment versus field data obtained at 296 K were analyzed within the Stoner-Wohlfarth model to obtain anisotropy field distributions. Samples C and D showed nearly identical distribution parameters. The Mössbauer spectra of sample D and the surface-enriched sample C exhibited essentially the same canting in a 1.8-kOe transverse applied field at 296 K. The similarity of low-field anisotropy parameters and Mössbauer canting angles is again inconsistent with the random-spin surface-shell model. Using the experimental data, simple free-energy minimization calculations were done for a linear chain of ferrimagnetically coupled atoms in external fields applied along the hard axis. These show that a high-anisotropy Co shell will cause a parabolic dependence of sublattice spin angle versus position along the chain, with the maximum deviation from collinearity a few degrees, as observed. Coercivities calculated from the anisotropy field distributions assuming coherent rotation are only about 10% larger than experimental values. The Co coating reduces the hyperfine-field tailing found on the particulate surface. We report a Mössbauer spectrum characteristic of slightly reduced γ-${\mathrm{Fe}}_2$ ${\mathrm{O}}_3$.