SPIN CANTING, SURFACE MAGNETIZATION, AND FINITE-SIZE EFFECTS IN GAMMA-FE2O3 PARTICLES

Small gamma-Fe2O3 acicular particles (250-angstrom diam by 2000 angstrom) with and without surface coating of Fe-57 were examined by Mossbauer 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 gamma-Fe2O3 particles and, as a comparison, large gamma-Fe2O3 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 Mossbauer canting. Hyperfine-field tailing increases with temperature for the small particles, especially for the coated samples, indicating weaker surface exchange interactions.