The Potential of the Quantum Diamond Microscope and Micromagnetic Tomography for Paleomagnetic Measurements

The Quantum Diamond Microscope holds the potential to revolutionize our field of research. It enables measurements on micrometer scale, going beyond the measurements on bulk rocks that we as a field of research have been using so far. An inherent limitation of QDM observations, however, is that it measures magnetic flux in one direction, usually perpendicular to the surface of a sample. A QDM image is therefore a set of one-dimensional measurements, characterizing the magnetic flux in a plane very close to a polished surface of a sample. Following the potential theory of Kellogg, this geometry precludes making reliable three-dimensional interpretations of these magnetic measurements unless additional information is added.

The required additional constraints can be in the form of imposed boundary conditions, or additional data. If the magnetic sources are assumed to be well separated and of dipolar nature, it is possible to determine the magnetic moment and location of the magnetic sources. This works well in samples with a very low concentration of very small magnetic particles that behave single-domain-like, such as speleothems. In the vast majority of geological materials, however, the sources are not sufficiently well isolated, and complex magnetic anomalies as measured from especially the larger particles are expressions of multidomain states in the grains. Such samples require additional data to reliably determine the magnetic moments of the magnetic grains in them.

Micromagnetic Tomography combines QDM data with information on the location of the magnetic sources in the sample material that is obtained by NanoCT scans. NanoCT measurements can identify iron-oxide bearing grains by their attenuation contrast that is much higher than the other minerals present in geological materials. Currently it is possible to routinely acquire NanoCT scans with a resolution down to 350 nm, enabling the identification of potentially magnetic grains well into the range of vortex domain states.

In this contribution we will present the potential and pitfalls of the interpretation of QDM data, the current state-of-the-art of Micromagnetic Tomography, and their practical applications for paleomagnetic studies on various types of geological samples.