Spin canting in γ‐Fe2O3 particles (abstract)

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Abstract
It has been reported1 that the spin canting observed in γ‐Fe2O3 is confined to a thin surface layer. We have examined by Mössbauer spectroscopy acicular γ‐Fe2O3 particles, ∼250 Å in diameter, before and after coating them with 57Fe, which provided a surface layer 1.3 Å thick, if uniform. Measurements were made in applied fields Ha to 60 kOe collinear with the γ‐ray direction at temperatures T from 4.2 to 296 K. For the coated particles, 50% of the signal arose from the adsorbed 57Fe. For a given Ha and T, the relative heights of the Δm=0 transitions were the same for coated and untreated particles. This indicates that the spin canting, which is visible at 4.2 K even at 60 kOe, is a bulk phenomenon and is not confined to a thin surface layer. The spectra were fit with a distribution p(H) in hyperfine fields H. A tailing to low‐H values from the main (core) peaks was observed for both sets of particles, but was more extensive for the treated particles. Thus the tailing presumably is due to surface Fe with weaker exchange coupling. Fitting2 the magnetic moment vs Ha curves of the untreated sample, we obtained a paramagnetic high‐field susceptibility term which increased nearly linearly with decreasing T, as did the Mössbauer canting. Potential implications of these results for magnetization reversal in these particles will be discussed.

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