Multi-scale three-dimensional characterization of iron particles in dusty olivine: Implications for paleomagnetism of chondritic meteorites

Abstract

Dusty olivine (olivine containing multiple sub-micrometer inclusions of metallic iron) in chondritic meteorites is considered an ideal carrier of paleomagnetic remanence, capable of maintaining a faithful record of pre-accretionary magnetization acquired during chondrule formation. Here we show how the magnetic architecture of a single dusty olivine grain from the Semarkona LL3.0 ordinary chondrite meteorite can be fully characterized in three dimensions, using a combination of focused ion beam nanotomography (FIB-nT), electron tomography, and finite-element micromagnetic modeling. We present a three-dimensional (3D) volume reconstruction of a dusty olivine grain, obtained by selective milling through a region of interest in a series of sequential 20 nm slices, which are then imaged using scanning electron microscopy. The data provide a quantitative description of the iron particle ensemble, including the distribution of particle sizes, shapes, interparticle spacings and orientations. Iron particles are predominantly oblate ellipsoids with average radii 242 ± 94 × 199 ± 80 × 123 ± 58 nm. Using analytical TEM we observe that the particles nucleate on sub-grain boundaries and are loosely arranged in a series of sheets parallel to (001) of the olivine host. This is in agreement with the orientation data collected using the FIB-nT and highlights how the underlying texture of the dusty olivine is crystallographically constrained by the olivine host. The shortest dimension of the particles is oriented normal to the sheets and their longest dimension is preferentially aligned within the sheets. Individual particle geometries are converted to a finite-element mesh and used to perform micromagnetic simulations. The majority of particles adopt a single vortex state, with "bulk" spins that rotate around a central vortex core. We observed no particles that are in a true single domain state. The results of the micromagnetic simulations challenge some preconceived ideas about the remanence-carrying properties of vortex states. There is often not a simple predictive relationship between the major, intermediate, and minor axes of the particles and the remanence vector imparted in different fields. Although the orientation of the vortex core is determined largely by the ellipsoidal geometry (i.e., parallel to the major axis for prolate ellipsoids and parallel to the minor axis for oblate ellipsoids), the core and remanence vectors can sometimes lie at very large (tens of degrees) angles to the principal axes. The subtle details of the morphology can control the overall remanence state, leading in some cases to a dominant contribution from the bulk spins to the net remanence, with profound implications for predicting the anisotropy of the sample. The particles have very high switching fields (several hundred millitesla), demonstrating their high stability and suitability for paleointensity studies.

Resolving the Origin of Pseudo‐Single Domain Magnetic Behavior

“…Vortex cores in these larger particles develop “winged” structures that protrude along directions in which domain walls nucleate (Figures c and d). Some multivortex scenarios simulated by Einsle et al () are similar to electron holograms in Figure and to the complex magnetization structures documented by Donnelly et al (). The presence of both multiple vortex cores and domain walls in larger particles suggests that the vortex to MD transition is gradual.…”

“…Thus, some aspects of the early domain imbalance moment explanation for PSD behavior (Dunlop, ; Shcherbakov, ; Stacey, ) could contribute to developing a more detailed understanding of the full range of magnetic behaviors in the vortex state. Overall, Einsle et al () concluded that particles in the single vortex state with high coercivity and large volume can have long‐term paleomagnetic stability and can be reliable paleomagnetic recorders.…”

“…Three‐dimensional micromagnetic model results reveal that as particle volume increases, vortex cores and domain walls coexist (e.g., Figures c and d) (Muxworthy, ). Einsle et al () found that in particles several hundred nanometers across, the magnetic structure is dominated by the single vortex state, which gives way to multiple vortex and domain wall structures in larger particles with ~1 μm long axes. Vortex cores in these larger particles develop “winged” structures that protrude along directions in which domain walls nucleate (Figures c and d).…”

“…When genuine MD behavior occurs, magnetizations will be dominated by domain wall pinning (Néel, ; Pike et al, ). The persistence of weak but stable remanences and the gradual decline in remanence with increasing particle size through the PSD size range is likely explained by the imbalanced nature of magnetic moments in complex magnetic structures within magnetic particles in the vortex state (Einsle et al, ). Available evidence suggests that these structures can record paleomagnetically meaningful information over long geological periods (Almeida et al, , ; Nagy et al, ).…”

“…The most important question for paleomagnetic analysis of materials in the magnetic vortex state concerns their magnetic recording capability. Single vortex state particles have been demonstrated to have high field and thermal stability close to the Curie temperature of magnetite (Almeida et al, , ; Einsle et al, ; Muxworthy et al, ; Nagy et al, ), which suggests that they are capable of recording paleomagnetic information over long geological periods. This confirms the paleomagnetic recording fidelity of PSD particles that has been postulated for over 55 years.…”