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Description
Maghemite (γ-Fe₂O₃) is a biocompatible ferrimagnetic iron oxide that crystallizes in an inverse spinel lattice and can be viewed as magnetite (Fe₃O₄) with a high concentration of iron vacancies. While the magnetic behaviour of maghemite and magnetite is similar, magnetite exhibits higher total magnetic moment. Iron oxide nanoparticles related to maghemite have numerous applications, including uses as contrast agents for magnetic resonance imaging, carriers for drug delivery or heat sources for local hyperthermia therapy as well as many others profiting purposes which arise from the combination of their magnetic properties, biocompatibility, biodegradability, low cytotoxicity and small size. Due to the structural similarities between magnetite and maghemite, several experimental techniques find it difficult to distinguish between them. For this reason, we employed the density functional theory (DFT) calculations to examine the local magnetic moments of individual atoms in two iron oxide nanoparticles. Our calculations for the nanoparticles were inspired by the bulk maghemite γ-Fe₂O₃, where tetrahedrally and octahedrally O-coordinated Fe sublattices have opposing orientations of local magnetic moments. Importantly, our results show that the nanoparticle surfaces create a far more complex magnetic states compared to the magnetic ordering found in bulk maghemite or magnetite. These complex magnetic states were defined by term "nested" ferrimagnetism, which is characterized by local magnetic moments of Fe atoms with mutually opposite orientations appearing even within each of the two (tetrahedral or octahedral) sublattices.
Financial support from the Czech Science Foundation, project "Properties of Nanopowders Prepared by a Pulsed Electron Beam Method in a Low-Pressure Gas" No. 21-31852J, is gratefully acknowledged.
Computational resources were provided by the e-INFRA CZ project (ID:90254), supported by the Ministry of Education, Youth and Sports of the Czech Republic. These resources were utilized through IT4Innovations National Supercomputing Center.
