Interaction-driven domain-state transition in the meteorite cloudy zone: a hybrid micromagnetic approach to modelling remanence acquisition

The cloudy zone is a nanoscale intergrowth of tetrataenite islands in a paramagnetic antitaenite matrix, formed by spinodal decomposition and atomic ordering during slow cooling on a meteorite parent body. X-ray magnetic imaging has shown that the magnetic state of the cloudy zone can be used to detect the presence or absence of a magnetic field on the parent body. However, the mechanism of remanence acquisition and the quantitative relation between the signal measured by X-ray imaging and the intensity of the ancient field remains unknown.

Micromagnetic simulations demonstrate that the cloudy zone acquires remanence via a sequence of magnetic domain state transformations (vortex to two-domain to single-domain), driven by Fe-Ni ordering at 320 °C. The vortex to two-domain transition is a consequence of the large uniaxial magnetocrystalline anisotropy of tetrataenite. The two-domain to single-domain transition is a consequence of inter-island interactions: a positive feedback between domain wall displacement and increased interactions drives domain walls out of the islands.

Here we model the remanence acquisition process in ensembles of strongly interacting islands using a hybrid micromagnetic approach. First a parameterized model of the two-domain micromagnetic energy was developed and fitted to finite element micromagnetic simulations for isolated islands. This analytical model was then combined with an iterative solver for interacting particles within FORCulator. Close-packed ensembles of two-domain 100 nm spherical islands with varying island separation and external fields were investigated. The results show a linear relationship between ensemble magnetisation and external fields for particles separated by up to 200 nm, beyond which no signal is recorded. This suggests that interactions do not destroy the paleomagnetic signal - in fact, they are essential for propagating this signal across the cloudy zone. The magnitude of the remanence observed in the planetary field range is a good match to those observed experimentally using X-ray imaging. This model explains why the cloudy zone is viable candidate for paleomagnetic signal recovery and provides the first step towards a theoretical framework for obtaining reliable quantitative estimates of paleointensity from X-ray imaging data.