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

Abstract

The effect of maghemization on the magnetic properties of magnetite (Fe 3 O 4 ) 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 Fe 3 O 4 ranging from $150 to $250 nm in diameter, whilst M€ ossbauer spectrometry suggests the grains are initially near-stoichiometric. The Fe 3 O 4 grains are heated to increasing reaction temperatures of 120-2208C to investigate their oxidation to maghemite (c-Fe 2 O 3 ). High-angle annular dark field imaging and localized electron-energy loss spectroscopy reveal slightly oxidized Fe 3 O 4 grains, heated to 1408C, exhibit higher oxygen content at the surface. Off-axis electron holography allows for construction of magnetic induction maps of individual Fe 3 O 4 and cFe 2 O 3 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 1808C, but subsequently decreases after heating above 2008C; this magnetic behavior is attributed to the growth of a c-Fe 2 O 3 shell with magnetic properties distinct from the Fe 3 O 4 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 c-Fe 2 O 3 , 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.

Models of Maghematization: Observational Evidence in Support of a Magnetic Unstable Zone

“…(2014) and Almeida et al. (2015). Note that the interaction effects have been considered both in the weighted multilayer and 2‐layer models.…”

“…In particular, the M rs /M s as a function of oxidation parameter is almost indistinguishable from the experiment results. Another group of systematic experimental data is from Almeida et al (2015), with grains size ranging from ∼150-250 nm. The variation of hysteresis parameters versus oxidation is not as evident as those for finer grains , resembling the modeling results for larger grains with an ESV state (Figure 3).…”

“…The variation of hysteresis parameters versus oxidation is not as evident as those for finer grains , resembling the modeling results for larger grains with an ESV state (Figure 3). and experiments results from and Almeida et al (2015). Note that the interaction effects have been considered both in the weighted multilayer and 2-layer models.…”

“…In equidimensional magnetite this transition zone is predicted to be between ∼84 nm and 100 nm (Nagy et al, 2017), and is close to the size region of the elongated grains modeled here (equivalent to a spherical volume diameter of 99 nm-149 nm). The rapid variation in B c and M rs /M s reflects proximity to the unstable zone and as the effective composition of the grain changes during oxidation and Almeida et al (2015) attributed the long rise and dramatic drop in hysteresis parameters with oxidation to the existence and disappearance of coupling effects between the magnetite core and maghemite shell. In this study, the strong interface anisotropy of the numerical model has been minimized by an approximately continuous oxidation transition zone, yet the predicted trend in hysteresis parameters remains unchanged.…”

Multiphase magnetic systems: Measurement and simulation

“…31 The SD critical grain size of maghemite is ~60 nm and it is ~50-84 nm for magnetite, 32 although these values can depend on particle shape. 33 Almeida et al 34 have observed almost identical FORC behavior for magnetite and maghemite with similar particle size. Therefore, their similar behaviors makes it difficult to exclusively assign the low-H C component to one of these phases.…”