Exfoliation of hematite: Morphological, structural and magnetic investigations

Abstract

Three natural specimens of hematite are characterized using X-ray diffraction, magnetometry, IR/Raman and Mössbauer spectroscopy, with focus on the Morin transition. All are exfoliated by grinding and prolonged sonication in dimethylformamide, followed by centrifuging to extract exfoliated debris from the supernatant. This material is characterized by electron microscopy and atomic force microscopy. There are three main Fe2O3 constituents: lamellae that are hundreds of nanometers in diameter and typically 1–10 nm thick, nanoparticles that are about 100 nm in size and ultra-fine nanoparticles smaller than 10 nm. Estimated yield of the lamellae is 10−3−10−4 by volume. All the iron in the debris is antiferromagnetically ordered at room temperature, including 30% that is superparamagnetic and contributes to a quadrupole doublet in the Mössbauer spectrum, but the characteristic weak net moment due to spin canting is absent. Unlike the normal hematite nanoparticles, there is no Morin transition and no net moment in the range of 100 to 300 K. The absence of a net moment is understandable because the lamellae cleave in planes that make small angles with the c-axis.