Towards nanometre-scale imaging of paleomagnetic recorders

Quantum Diamond Microscopy (QDM) has opened new avenues for palaeomagnetism by enabling magnetic imaging at micrometre-scale spatial resolution, bridging the gap between bulk rock measurements and grain-scale magnetic observations. Micromagnetic Tomography (MMT) experiments integrate QDM data with micro- or nano-CT–derived grain geometries to determine magnetic moments of individual grains within a sample. Adding spatial information to the inversion problem makes it possible to calculate magnetic moments of individual iron-oxide grains even in samples with high grain concentrations, complex domain states, and overlapping magnetic signals. This enables the magnetic contribution of individual particles to be quantified, compared through consecutive (de)magnetization steps, and tested for stability. MMT therefore provides direct experimental access to grain-scale magnetic moments and goes beyond what bulk or surface-integrated measurements can reveal.

The goal of MMT has always been to isolate and select contributions of only the most reliable recorders in a rock sample. Nevertheless, MMT faces a fundamental challenge in spatial scales: wide-field QDM imaging achieves spatial resolutions of ~1 µm, while the practical resolution of micro- and nano-CT similarly limits the detection of magnetic particles to sizes of ~1 µm and larger. Magnetite and titanomagnetite grains in this size range are typically characterized by multidomain behaviour and are therefore often magnetically unstable, limiting their usefulness as reliable paleomagnetic recorders. As a result, current grain-scale approaches predominantly probe particles that are least suitable for preserving stable remanent magnetisations.

Accessing the information stored in smaller, submicron, vortex-state grains that are reliable recorders of the Earth’s magnetic field requires moving beyond wide-field QDM imaging. Improvements in spatial resolution of wide-field QDMs are fundamentally restricted by the optical diffraction limit, motivating a transition to Quantum Scanning Microscopy (QSM). In QSM, a single nitrogen-vacancy centre functions as an atomic-scale magnetometer, enabling nanometre-scale spatial resolutions that are ideal for magnetic imaging of vortex-state grains.

Here we present the first results of QSM stray-field imaging applied to a volcanic rock sample, in combination with slice-and-view FIB-SEM analysis of the same sample to characterise the particles’ geometries. These measurements demonstrate the feasibility of detecting magnetic signals at length scales inaccessible to wide-field QDM and current MMT techniques, while highlighting both the opportunities and technical challenges associated with pushing paleomagnetic observations into the nanoscale. Together, these developments provide a path forward towards resolving the magnetic behaviour of the particles that are most relevant for reliable paleomagnetic recording in rock samples.