MAGNETIC-BEHAVIOR OF AGGREGATES OF SMALL GAMMA-FE2O3 PARTICLES

Aggregates of small (10 nm) gamma-Fe2O3 crystallites dispersed in Al2O3 Powder have been studied by magnetization measurements and Mossbauer spectroscopy. The magnetic behavior, ZFC magnetization, coercive field, remanent magnetization and magnetization in intermediate magnetic fields, has been shown to be sensitive to the packing fraction of the aggregates as the result of the dipolar interactions between the aggregates. Both the coercive field and the remanent magnetization decrease very quickly as the temperature increases owing to relaxation phenomena with relaxation time shorter than 1 min. On the other hand, Mossbauer spectra at room temperature does not show any quadrupolar doublet implying relaxation time longer than 10(-8) s. Such long relaxation times are interpreted as resulting from the relaxation of correlated volumes containing a large number of crystallites. Assuming that the coupling between the crystallites only results from dipolar interactions we show that the magnetic properties and the relaxation process are very sensitive to the size of the crystallites and to the shape of the aggregates.