Nanomagnetic intergrowths in Fe-Ni meteoritic metal: The potential for time-resolved records of planetesimal dynamo fields

作者：Bryson James F J^*; Church Nathan S; Kasama Takeshi; Harrison Richard J

来源：Earth and Planetary Science Letters, 2014, 388: 237-248.

DOI：10.1016/j.epsl.2013.12.004

• extraterrestrial magnetism
• tetrataenite
• cloudy zone
• high-resolution magnetic microscopy
• off-axis electron holography

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 (H-5) varies with the length scale of the matrix phase (L-m), with H-s %26gt; 1 T for L-m similar to 10 nm (approaching the intrinsic switching field for isolated single domain tetrataenite) and 0.2 %26lt;H-s %26lt; 0.6 T for L-m similar to 30 nm. The reduction in H-s with increasing L-c 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.

• 出版日期2014-2-15