STRUCTURAL AND MAGNETIC-PROPERTIES OF ALPHA-FE2O3 NANOPARTICLES

The magnetic properties of alpha-Fe2O3 nanoparticles of different shap es (spherical, rhombohedral and acicular), prepared as powders by a ch emical route, have been investigated. The particle size effect on the Morin transition (T-M = 263 K in the bulk system) have been studied by analyzing the temperature dependence of the zero-field-cooled (ZFC) a nd field-cooled (FC) magnetization, For spherical (average diameter be tween 10 and 50 nm) and rhombohedral (edge between 30 and 350 nm) part icles, the Morin temperature was found to decrease with decreasing par ticle size and increasing magnetic field. On the other hand, acicular particles (major axis between 300 and 700 nm, minor axis between 70 an d 100 nm) do not show the Morin transition, unless annealed, (C) 1998 John Wiley & Sons, Ltd.