COERCIVITY ENHANCEMENT IN GAMMA-FE2O3 PARTICLES DISPERSED AT LOW-VOLUME FRACTION

Magnetic properties of well-defined gamma-Fe2O3 particles with differe nt morphology have been shown to be strongly dependent on the dispersi on method. The gamma-Fe2O3 particles have been prepared by reduction-o xidation of the corresponding alpha-Fe2O3 obtained by homogeneous hydr olysis in solution. Different degrees of dispersion of these particles in silica have been obtained by heterocoagulation (volume fraction si milar to 0.1) and by a sol-gel method (volume fraction similar to 0.00 1), and they have been compared with a sample consisting of well-compa cted particles. Hysteresis parameters show the influence of the partic le morphology and interactions on the magnetic behaviour of the sample s. gamma-Fe2O3 ellipsoidal particles dispersed in silica at 0.001 volu me fraction show a coercivity enhancement of 50%, while for spherical particles the coercivity decreases. When alpha-Fe2O3 particles are dis persed in silica before their transformation to gamma-Fe2O3 at similar volume fraction, a contrary effect was observed. In this case, spheri cal particles show an increase in coercivity of 150% while the coerciv ity for ellipsoidal particles decreases. The coercivity increase, in t he first case, is in accordance with an inter-particle interaction red uction, whereas, in the last case, the enhancement of the coercivity i s attributed to anisotropy contributions due to stress, presumably at the particle-matrix interface. (C) 1998 Elsevier Science B.V. All righ ts reserved.