Spin orientation, structure, morphology, and magnetic properties of hematite nanoparticles

Monodisperse hematite (alpha-Fe2O3) nanoparticles were synthesized by forced hydrolysis of acidic Fe3+ solution. Rietveld analysis was applied to the X-ray powder diffraction data to refine the lattice constants and atomic positions. The lattice constants for a hexagonal unit cell were determined to be a similar to 0.50327 and c similar to 1.37521 nm. High resolution transmission electron microscopy was employed to study the morphology of the particles. Atomic scale micrographs and diffraction patterns from several zone axes were obtained. These reveal the high degree of crystallinity of the particles. A series of observations made on the particles by tilting them through a range of +/- 45 degrees revealed the particles to be micaceous with stacking of platelets with well defined crystallographic orientations. The Morin transition in these nanoparticles was found to occur at 210 K, which is lower temperature than 263K of bulk hematite. It was ascertained from the previous Mossbauer studies that the spin orientation for nano-sized hematite particle flips from 90 degrees to 28 degrees with respect to the c-axis of the hexagonal structure during the Morin transition, which is in contrast to that observed in bulk hematite where spin orientation flips from 90 degrees to 0 degrees. (c) 2015 AIP Publishing LLC.