Assemblies of cigar-shaped ferrite nanocrystals: orientation of the easy magnetization axes

Mesoscopic structures made of cigar-shaped maghemite (γ-Fe2O3) nanocrystals differing by their sizes are described. The structures were prepared by slow evaporation of a dilute suspension of nanocrystals, to which could be applied a magnetic field parallel to the substrate. If a magnetic field was applied, the nanocrystals rotated their long axis along the magnetic field direction to form ribbons whereas without a field the nanocrystals remained deposited on the substrate with a random orientation. The aligned nanocrystals is responsible of the anisotropy of the ribbons as evidence by the hysteresis loops. Moreover, a high demagnetizing field is observed when the magnetization measurements are made with an applied field normal to the ribbons.