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⊥ > 0.17, the shape factor), platy, and rhombohedral (both G⊥ < 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.