Coercivity enhancement in γ-Fe2O3 particles dispersed at low-volume fraction

Magnetic properties of well-defined gamma-Fe2O3 particles with different morphology have been shown to be strongly dependent on the dispersion method. The gamma-Fe2O3 particles have been prepared by reduction-oxidation of the corresponding alpha-Fe2O3 obtained by homogeneous hydrolysis in solution. Different degrees of dispersion of these particles in silica have been obtained by heterocoagulation (volume fraction similar to 0.1) and by a sol-gel method (volume fraction similar to 0.001), and they have been compared with a sample consisting of well-compacted particles. Hysteresis parameters show the influence of the particle morphology and interactions on the magnetic behaviour of the samples. gamma-Fe2O3 ellipsoidal particles dispersed in silica at 0.001 volume fraction show a coercivity enhancement of 50%, while for spherical particles the coercivity decreases. When alpha-Fe2O3 particles are dispersed in silica before their transformation to gamma-Fe2O3 at similar volume fraction, a contrary effect was observed. In this case, spherical particles show an increase in coercivity of 150% while the coercivity for ellipsoidal particles decreases. The coercivity increase, in the first case, is in accordance with an inter-particle interaction reduction, whereas, in the last case, the enhancement of the coercivity is attributed to anisotropy contributions due to stress, presumably at the particle-matrix interface. (C) 1998 Elsevier Science B.V. All rights reserved.