Abstract
Meteorites contain a record of their thermal and magnetic history, written in the intergrowths of iron-rich and nickel-rich phases that formed during slow cooling. Of intense interest from a magnetic perspective is the "cloudy zone," a nanoscale intergrowth containing tetrataenite-a naturally occurring hard ferromagnetic mineral that has potential applications as a sustainable alternative to rare-earth permanent magnets. Here we use a combination of high-resolution electron diffraction, electron tomography, atom probe tomography (APT), and micromagnetic simulations to reveal the 3D architecture of the cloudy zone with subnanometer spatial resolution and model the mechanism of remanence acquisition during slow cooling on the meteorite parent body. Isolated islands of tetrataenite are embedded in a matrix of an ordered superstructure. The islands are arranged in clusters of three crystallographic variants, which control how magnetic information is encoded into the nanostructure. The cloudy zone acquires paleomagnetic remanence via a sequence of magnetic domain state transformations (vortex to two domain to single domain), driven by Fe-Ni ordering at 320 °C. Rather than remanence being recorded at different times at different positions throughout the cloudy zone, each subregion of the cloudy zone records a coherent snapshot of the magnetic field that was present at 320 °C. Only the coarse and intermediate regions of the cloudy zone are found to be suitable for paleomagnetic applications. The fine regions, on the other hand, have properties similar to those of rare-earth permanent magnets, providing potential routes to synthetic tetrataenite-based magnetic materials.
Highlights
Meteorites contain a record of their thermal and magnetic history, written in the intergrowths of iron-rich and nickel-rich phases that formed during slow cooling
If the cooling rate is too fast, insufficient time is available for the Fe–Ni ordering to take place, and the soft ferromagnetic phase taenite is retained that is unsuitable for meteorite | paleomagnetism | cloudy zone | tomography |
The size of the tetrataenite islands decreases systematically from 150 nm adjacent to the rim to less than 10 nm at a distance of several microns away from the rim. This change from “coarse” to “fine” island sizes is caused by a decrease in the local Ni concentration with increasing distance from the rim, which in turn lowers the temperature at which spinodal decomposition initiates during cooling
Summary
Meteorites contain a record of their thermal and magnetic history, written in the intergrowths of iron-rich and nickel-rich phases that formed during slow cooling. Despite notable successes of this new “nanopaleomagnetic” approach [8,9,10,11], precisely how paleomagnetic information is recorded by the cloudy zone remains unknown, and further questions about the timing of primary remanence acquisition during cooling, and the susceptibility of the cloudy zone to acquire secondary remanence postcooling, remain open Providing answers to these questions is an essential step in the quest to develop a quantitative theory linking the magnetic state of the cloudy zone to the intensity of the parent-body magnetic field. Small island sizes promote uniformly magnetized (i.e., single-domain) states, while the L10 ordered tetragonal symmetry of tetrataenite generates high magnetic coercivity (up to 2 T) These two properties combine to make the cloudy zone a potent carrier of paleomagnetic information in meteorites as well as a potential sustainable replacement for rare-earth permanent magnet materials [3, 4]. We discover the mechanism by which paleomagnetic information becomes encoded into the cloudy zone and, inspired by our findings, point toward potential pathways to optimize synthetic analogues of the cloudy zone for industrial applications
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