Abstract
Nanoscale intergrowths unique to the cloudy zones (CZs) of meteoritic metal display novel magnetic behaviour with the potential to reveal new insight into the early development of magnetic fields on protoplanetary bodies. The nanomagnetic state of the CZ within the Tazewell IIICD iron meteorite has been imaged using off-axis electron holography. The CZ is revealed to be a natural nanocomposite of magnetically hard islands of tetrataenite (ordered FeNi) embedded in a magnetically soft matrix of ordered Fe3Ni. In the remanent state, each tetrataenite island acts as a uniaxial single domain particle with its [001] magnetic easy axis oriented along one of three 〈100〉 crystallographic directions of the parent taenite phase. Micromagnetic simulations demonstrate that switching occurs via the nucleation and propagation of domain walls through individual tetrataenite particles. The switching field (Hs) varies with the length scale of the matrix phase (Lm), with Hs > 1 T for Lm ∼10 nm (approaching the intrinsic switching field for isolated single domain tetrataenite) and 0.2<Hs<0.6 T for Lm ∼30 nm. The reduction in Hs with increasing Lc is caused by exchange coupling between the hard tetrataenite islands and the soft magnetic matrix, which lowers the critical field for domain wall nucleation, providing an explanation for previously observed coercivity variations throughout the CZ. Non-random distributions of the tetrataenite easy axes are observed locally throughout the CZ, suggesting a magnetic field could have been present during nanostructure formation. This observation demonstrates the potential for stable chemical transformation remanent magnetisation to be encoded by the nanostructure, with variations in the proportions of the six possible magnetisation states reflecting the intensity and relative direction of the magnetic fields present during cooling. According to recent cooling models, the cooling rate of meteoritic metal originating near the surface of differentiated planetesimals was such that the magnetic signal across the CZ could potentially record dynamo field intensity and direction variations over time (10–100 Ma), which would enable events such as magnetic reversals and the decay of an asteroid dynamo to be observed.
Highlights
An increasing body of paleomagnetic evidence suggests that, while molten, the cores of differentiated planetesimals generated magnetic dynamo fields with comparable strength to those of present-day Earth (Tarduno et al, 2012; Fu et al, 2012; Weiss et al, 2010)
The Tazewell IIICD iron meteorite was chosen for study based on previous electron microscopy studies (Goldstein et al, 2009; Reuter et al, 1987; Yang et al, 1997b) which demonstrate that the cloudy zones (CZs) had not been significantly affected by shock
Scanning transmission electron microscopy (STEM) images, energy dispersive spectroscopy (EDS) maps, diffraction patterns and electron holograms were captured on an FEI Titan 300 kV TEM, and bright field images were captured on an FEI Technai 300 kV TEM, both at the Centre for Electron Nanoscopy, Denmark Technical University, Copenhagen
Summary
An increasing body of paleomagnetic evidence suggests that, while molten, the cores of differentiated planetesimals generated magnetic dynamo fields with comparable strength to those of present-day Earth (Tarduno et al, 2012; Fu et al, 2012; Weiss et al, 2010). Magnetic properties could enable a time-resolved record of the dynamo field to be captured by meteoritic metal originating from near-surface regions of a differentiated asteroid. On cooling to ∼500 ◦C, large lamellae (∼50 μm wide in this study) of the magnetically soft Fe-rich body-centred cubic (bcc) phase kamacite nucleate and grow parallel to taenite {111} planes (Yang and Goldstein, 2005) (Fig. 1a). This process ejects Ni into the surrounding taenite, establishing concentration gradients that vary from 50% Ni at the kamacite/taenite interface to
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