Generating net moment inversions and demagnetization spectra at micro-meter resolution using the quantum diamond microscope (QDM)
Abstract
The development of the quantum diamond microscope (QDM) provides access to a set of novel paleo- and rock magnetic measurements. High spatial resolution paired with relatively high sensitivity makes QDM magnetic microscopy the ideal tool for characterizing moderately to strongly magnetized samples with heterogeneous magnetization properties. Such capabilities are especially relevant for meteorites and highly processed ancient Earth samples that contain multiple generation of ferromagnetic grains juxtaposed at <<1 mm scales. The ability to characterize the natural remanent magnetization (NRM) and/or demagnetization spectra of discrete grain populations can be critical for the recovery of reliable paleomagnetic information.
However, the small, 5-100 μm sample-to-sensor distance of the QDM, while providing high spatial resolution, also results in complex magnetic field maps with non-dipolar sources, from which accurate computation of the net magnetic moment cannot be done using existing techniques. Here we describe and validate two methods to analyze complex (non-dipolar) magnetic maps. Net moment inversion using a combination of upward continuation and dipolar model fitting. By studying synthetic maps with known net moments to quantify the errors introduced by sample non-dipolarity, we find that inversions based on least-squares dipole fits of upward continued data can recover the net moment of reasonably complex samples with <5% to 10% error, given the optimized amount of upward continuation. Comparison of experimental maps of identical samples obtained from the QDM and SQUID microscope demonstrates that this technique is capable of recovering net moment within the stated accuracies. Extracting coercivity or thermal unblocking spectra of map regions without explicit net moment inversion. For map sources that are too complex for the above treatment using upward continuation, we can quantify their demagnetization behavior by examining the degree of change in the map between demagnetization steps. In this protocol, we align maps from a demagnetization sequence of NRM or laboratory remanence, select a region of interest, and quantify deviations from initial map by pixel counting. As an example, we examined magnetic maps of alternating field (AF) demagnetization of large titanomagnetite grains in a martian nakhlite meteorite (MIL03346), allowing us to reconstruct the coercivity spectrum through the changes in field pattern between AF steps. Our results compare well with bulk sample determinations of median destructive field from AF demagnetization and coercivity from hysteresis loops.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMGP004..06V
- Keywords:
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- 1518 Magnetic fabrics and anisotropy;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1519 Magnetic mineralogy and petrology;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1540 Rock and mineral magnetism;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1594 Instruments and techniques;
- GEOMAGNETISM AND PALEOMAGNETISM