Oxidation of pseudo-single domain Fe3O4 particles and associated magnetic response examined by environmental TEM and off-axis electron holography

In order to interpret palaeomagnetic measurements reliably, the mechanisms that induce and alter chemical remanent magnetisation (CRM) in naturally occurring magnetic recorders must be fully understood. Current models of CRM processes only exist for the smallest uniformly-magnetised single domain (SD) grains. However, magnetic signals from rocks are often dominated by larger grains that contain non-uniform pseudo-SD (PSD) magnetisation states. Magnetite (Fe₃O₄) is the most magnetic naturally occurring mineral on Earth, carrying the dominant magnetic signature in rocks and providing a critical tool in paleomagnetism. The oxidation of Fe₃O₄ to other iron oxides, such as maghemite (γ-Fe₂O₃) and hematite (α-Fe₂O₃), is of particular interest as it influences the preservation of remanence of the Earth's magnetic field by Fe₃O₄. The complementary use of environmental transmission electron microscopy (ETEM) and off-axis electron holography techniques can be used to reveal local changes in magnetisation in minerals as they alter during in situ heating in a controlled oxidising atmosphere. Such experiments can provide direct information about the relationship between magnetic domain structure and chemical alteration features, phase boundaries and crystalline microstructure. In the present study, synthetic Fe₃O₄ particles with sizes in the PSD range (< 300 nm) were heated in situ in an ETEM under an oxygen atmosphere. Oxidation of the Fe₃O₄ particles was investigated using bright-field and dark-field imaging, electron diffraction and electron energy-loss spectroscopy. The associated alteration in CRM exhibited by individual Fe₃O₄ particles was investigated using off-axis electron holography, in the form of reconstructed magnetic induction maps.