The magnetism of micro-sized hematite explained

Several sets of micro-sized synthetic hematite samples were systematically studied by infrared absorption spectroscopy, X-ray diffraction, thermogravimetric and low-temperature magnetic analysis. These hematite samples could be divided into three distinctive morphologies of pseudocubic (G(perpendicular to) > 0.17, the shape factor), platy, and rhombohedral (both G(perpendicular to) <0.14). Pseudocubic hematite contains more vacancies than other morphologies due to the presence of OH in the crystal. Accordingly, the pseudocubic hematite exhibits a wider Morin transition and higher magnetic coercivity. Both the low temperature changes in coercivity and first-order reversal curve indicate that the pseudocubic hematite is dominated by a combination of magnetocrystalline and magnetoelastic anisotropies. In contrast, because of the low vacancy content, hematite with non-pseudocubic morphologies is governed primarily by the magnetocrystalline anisotropy. Such a simple morphology dependence successfully links microstructure to the bulk magnetic properties of micron-sized hematite and hence improve our understanding of the magnetism of hematite and its possible application in elucidating the origin of hematite in natural environments. (C) 2010 Elsevier B.V. All rights reserved.