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

Trevor P Almeida,Wyn Williams,Adrian R Muxworthy,Cathrine Frandsen,Rafal E Dunin‐Borkowski,Christian Damsgaard,Takeshi Kasama,Timothy J Pennycook

doi:10.1002/2015gc005858

Abstract

AbstractThe 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.

Highlights

During formation, magnetic minerals record the direction and intensity of the Earth’s magnetic field
This study has investigated the maghemization of Fe3O4 grains in the PSD size range and the effect of this oxidation process on their magnetic properties
Complementary methods of X-ray diffractometry (XRD) and Mossbauer spectroscopy followed this phase transformation as a function of temperature and showed the onset of oxidation was most pronounced at 1608C, with a reaction temperature of 2208C producing pure c-Fe2O3

Summary

Introduction

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. 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 (titano)magnetite at ambient temperatures, i.e., the slow oxidation of (titano)magnetite to (titano)maghemite [van Velzen and Zijderveld, 1995]. Of the constituent magnetic minerals, particles in the single domain (SD) grain size range (

Results

Discussion

Conclusion