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

Abstract:

During formation, magnetic minerals record the direction and intensity of the Earth’s magnetic field. Paleomagnetists use this information to investigate, for example, past tectonic plate motion and geodynamo evolution. However, subsequent to formation the constituent magnetic minerals are commonly exposed to a range of weathering conditions and environments. One of the most common weathering processes is maghemization, which is the oxidation of magnetite (Fe₃O₄) at ambient temperatures, i.e., the slow oxidation of Fe₃O₄ to maghemite (γ-Fe₂O₃), and is known to alter the original remanent magnetization.

Of the constituent magnetic minerals, particles in the single domain (SD) grain size range (< 100 nm) are regarded as ideal paleomagnetic recorders because of their strong remanence and high magnetic stability, with potential relaxation times greater than that of the age of the Universe. However, magnetic signals from rocks are often dominated by small grains with non-uniform magnetization that exhibit magnetic recording fidelities similar to those of SD grains (termed pseudo-SD (PSD)).

In this context, the effect of maghemization on the magnetic properties of Fe₃O₄ grains in the PSD size range is investigated as a function of annealing temperature. X-ray diffraction and transmission electron microscopy confirms the precursor grains as Fe₃O₄ ranging from ~ 150 nm 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 ºC to investigate their oxidation to γ-Fe₂O₃. High-angle annular dark field imaging and localized electron energy-loss spectroscopy reveals slightly oxidized Fe₃O₄ grains, heated to 140 ºC, 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 ºC, but subsequently decreases after heating above 200 ºC; this magnetic behavior is attributed to the growth of a γ-Fe₂O₃ shell with magnetic properties distinct from the Fe₃O₄ core.