Micromagnetic modelling and single particle multipole expansions from Micromagnetic Tomography

Micromagnetic Tomography is a technique that combines X-ray micro tomography and scanning magnetometry data to obtain magnetic information of individual grains embedded in a sample. Recovering magnetic signals of individual grains in rock samples and synthetic samples provides a new pathway to study the rock-magnetic properties of remanent magnetizations that are crucial to paleomagnetic studies. This is possible by numerically inverting the surface magnetic signal for the magnetic potential of individual magnetic grains via their spherical harmonic expansion [1]. Resulting magnetic moment solutions are uniquely determined as dipole and higher order multipole moments, which has been proved in [2]. Furthermore, the higher order multipole signals in the magnetic particles are an indication that the grains carry complex magnetic orderings, such as multi-domain or vortex configurations [3]. In this work we show that the magnetic moment information can be used to constrain the internal magnetic configuration of individual grains using micromagnetic modelling. We first review the multipole expansion method used in Micromagnetic Tomography [3]. Further, we show three dimensional micromagnetic modelling results to predict the multipole signal of magnetic particles in different local energy minimum magnetization states. We show that for certain grains it is possible to uniquely infer the magnetic configuration from the inverted magnetic multipole moments. This result is crucial to discriminate single-domain particles from grains in more complex configurations. Our investigation proves the feasibility to select statistical ensembles of magnetic grains based on their magnetization states, which opens new possibilities to characterize stable paleomagnetic recorders in natural samples. 

[1] L. V. de Groot, K. Fabian, A. Béguin, M. E. Kosters, D. Cortés-Ortuño, R. R. Fu, C. M. L. Jansen, R. J. Harrison, T. van Leeuwen, A. Barnhoorn. Micromagnetic tomography for paleomagnetism and rock-magnetism. Journal of Geophysical Research: Solid Earth, 126:e2021JB022364, 2021.
[2] K. Fabian and L. V. de Groot. A uniqueness theorem for tomography-assisted potential-field inversion. Geophysical Journal International, 216(2):760–766, 2018.
[3] D. Cortés‐Ortuño, K. Fabian and L. V. De Groot. Single particle multipole expansions from Micromagnetic Tomography. Geochemistry, Geophysics, Geosystems, 22:e2021GC009663, 2021.