Exfoliation of hematite: Morphological, structural and magnetic investigations

Three natural specimens of hematite are characterized using X-ray diffraction, magnetometry, IR/Raman and Mossbauer 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 Mossbauer 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.

Automated summary

Three natural hematite specimens were characterized with a focus on the Morin transition, revealing the presence of lamellae, nanoparticles, and ultra-fine nanoparticles. The material exhibited antiferromagnetic order at room temperature, with some superparamagnetic iron contributing to a quadrupole doublet in the Mossbauer spectrum. However, the characteristic weak net moment due to spin canting was absent.