Spin canting, surface magnetization, and finite-size effects in γ-Fe2O3particles

Small γ-${\mathrm{Fe}}_2$ ${\mathrm{O}}_3$ acicular particles (250-Å diam by 2000 Å) with and without surface coating of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ were examined by Mössbauer spectroscopy in large (to 60 kOe) longitudinal applied fields. As has been previously seen, the Fe moments are not aligned in large fields. However, unlike previous results, the canting is found to be a finite-size effect rather than a surface property. Magnetization versus field curves for the uncoated small γ-${\mathrm{Fe}}_2$ ${\mathrm{O}}_3$ particles and, as a comparison, large γ-${\mathrm{Fe}}_2$ ${\mathrm{O}}_3$ particles were analyzed with log-normal anisotropy field distributions within the Stoner-Wohlfarth model. The small particles show a large paramagnetic susceptibility that increases with decreasing temperature. Using a Langevin-function analysis, one finds that the susceptibility is not due to superparamagnetism, and the susceptibility correlates well with the Mössbauer canting. Hyperfine-field tailing increases with temperature for the small particles, especially for the coated samples, indicating weaker surface exchange interactions.