Effects of thermal treatments on structural and magnetic properties of acicular alpha-Fe2O3 nanoparticles

The effect of transformations induced by annealing at different temperatures (450 < T-a < 1000 degrees C) on the structure and magnetic properties of acicular alpha-Fe2O3 particles (major axis: 330 +/- 50 nm; minor axis: 70 +/- 10 nm) have been investigated by X-ray diffraction, TEM, SEM and magnetization measurements. The particles were prepared by hydrolysis and polimerization in an aqueous solution of FeCl3. In the as-prepared sample, particles are constituted by small units (3-5 nm) separated by a less dense interlayer, while keeping the same crystallographic orientation. With increasing T-a, the sample microstructure evolves towards a collapse of the small units and then to a coalescence of particles. Coherently, the spin reorietation Morin temperature T-M increases with T-a, the highest value (T-M = 259 K) being comparable to that of the bulk sample.