Magnetic properties of pure and Eu-doped hematite nanoparticles

Nanoparticles (NPs) of pure and Eu-doped hematite were prepared by a wet chemical technique; their structure, size and morphology were determined by XRD and transmission electron microscopy. The magnetic properties were measured in the 6–320 K temperature range by vibrating-sample magnetometry. Pure hematite NPs exhibit a pseudo-cubic shape with a size of about 74 nm; addition of trivalent Eu cations in different amounts (Eu/Fe atomic ratios 2.4 %) brings about a definite change in particle morphology with the development of rice-grain like NPs with aspect ratios of about 2.8. Trivalent Eu cations act as magnetic defects making the overall antiferromagnetic arrangement of hematite host less robust. A large defect magnetism arises below the Morin transition temperature. The resulting uncompensated moments on NPs (mostly arising from the NP cores) behave superparamagnetically before undergoing single-particle blocking at about 40 K. Instead, uncompensated moments in pure hematite NPs mostly occur at the NP surface and lead to standard defect ferromagnetism. Above the Morin temperature, the usual spin-canted ferromagnetism is observed in all samples, although it is somewhat inhibited by Eu addition.