Size and shape effect on magnetic properties of α-Fe2O3 nanoparticles

The magnetic properties of alpha-Fe2O3 nanoparticles, prepared as powders by chemical route and as granular films by sputtering, have been investigated for different size (3 - 700 nm) and shape (rhombohedral acicular and spherical). The particle size and shape effect on the Morin transition (T-M = 263 K in the bulk system) has been investigated by analyzing the temperature dependence of the zero-held-cooled (ZFC) and held-cooled (FC) magnetization. For rhombohedral (30 - 350 nm) and spherical (10 - 50 nm) particles, the Morin temperature was found to decrease with decreasing particle size and increasing magnetic field. On the other hand, acicular particles (350 - 700 nm) do not show the Morin transition, unless annealed. In samples characterized by a large distribution of particle size, the ZFC and FC measurements show the superimposed effects of the superparamagnetic behaviour of very small particles and the Morin transition for the larger ones. Only the superparamagnetic component is observed in granular films consisting of very small spherical particles (3 nn), whose average blocking temperature is about 140 K.