Maghemite

Maghemite is a ferrimagnetic iron(III) oxide mineral with a defect-spinel structure. Its cation vacancies distinguish it from the closely related spinel mineral magnetite.

Iron(III) oxide: Iron(III) oxide is a class of compounds containing iron in the +3 oxidation state and oxygen. Maghemite is a specific iron(III) oxide with a spinel-related crystal structure.
Magnetic sublattice: A magnetic sublattice is a set of crystallographic sites whose magnetic moments share an ordering direction. Iron moments on distinct maghemite sites oppose one another without fully cancelling.
Superparamagnetism: Superparamagnetism is the rapid thermal fluctuation of a nanoparticle’s magnetic moment, yielding no persistent remanence over measurement times. Small maghemite particles can enter this regime, aiding their dispersion and magnetic response.
Lodestone: Lodestone is naturally magnetized magnetite capable of attracting iron. Its natural magnetism is often mistaken for maghemite’s, though lodestone is chiefly magnetite.
Spinel structure: The spinel structure is a crystal arrangement with cations occupying tetrahedral and octahedral sites in a close-packed anion lattice. Maghemite retains this framework while leaving some cation sites vacant.
Superexchange: Superexchange is an indirect magnetic interaction transmitted between ions through a nonmagnetic bridging ion. Iron–oxygen–iron interactions help align maghemite’s opposing magnetic sublattices.
Magnetic resonance imaging: Magnetic resonance imaging is a medical imaging technique that uses nuclear magnetic resonance signals in a strong magnetic field. Maghemite nanoparticles can act as contrast agents by changing nearby proton relaxation.
Maghemite nanoparticles: Maghemite nanoparticles are nanoscale particles of γ-Fe₂O₃ with size-dependent magnetic and surface properties. Nanoscale specimens are the form most often engineered for biomedical and environmental uses.
Cation vacancy: A cation vacancy is an unoccupied site where a positively charged ion would normally sit in a crystal. Vacancies balance maghemite’s charge after iron is oxidized to iron(III).
Verwey transition: The Verwey transition is a low-temperature structural and electronic transition characteristic of magnetite. Maghemite lacks magnetite’s mixed-valence transition because its iron is ferric.

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