Effect of maghemization on the magnetic properties of nonstoichiometric pseudo-single-domain magnetite particles

The effect of maghemization on the magnetic properties of magnetite (Fe₃O₄) grains in the pseudo-single-domain (PSD) size range is investigated as a function of annealing temperature. X-ray diffraction and transmission electron microscopy confirm the precursor grains as Fe₃O₄ ranging from 150 to 250 nm in diameter, whilst Mössbauer spectrometry suggests the grains are initially near-stoichiometric. The Fe₃O₄ grains are heated to increasing reaction temperatures of 120-220°C to investigate their oxidation to maghemite (γ-Fe₂O₃). High-angle annular dark field imaging and localized electron-energy loss spectroscopy reveal slightly oxidized Fe₃O₄ grains, heated to 140°C, exhibit higher oxygen content at the surface. Off-axis electron holography allows for construction of magnetic induction maps of individual Fe₃O₄ and γ-Fe₂O₃ grains, revealing their PSD (vortex) nature, which is supported by magnetic hysteresis measurements, including first-order reversal curve analysis. The coercivity of the grains is shown to increase with reaction temperature up to 180°C, but subsequently decreases after heating above 200°C; this magnetic behavior is attributed to the growth of a γ-Fe₂O₃ shell with magnetic properties distinct from the Fe₃O₄ core. It is suggested there is exchange coupling between these separate components that results in a vortex state with reduced vorticity. Once fully oxidized to γ-Fe₂O₃, the domain states revert back to vortices with slightly reduced coercivity. It is argued that due to a core/shell coupling mechanism during maghemization, the directional magnetic information will still be correct; however, the intensity information will not be retained. Key Points: Maghemization of Fe₃O₄ grains is confirmed through XRD, Mossbauer, and EELS Maghemization of Fe₃O₄ grains occurs through formation of a core-shell structure Magnetic behavior is considered due to core-shell exchange coupling