Micromagnetic Modes of Anisotropy of Magnetic Susceptibility in Natural Magnetite Particles

Abstract

Abstract Anisotropy of magnetic susceptibility (AMS) is commonly used to assess sedimentation, deformation, tectonics, rock fabric, and texture. Using focused‐ion beam nanotomography, we develop a micromagnetic method to investigate the AMS of individual magnetite inclusions in silicates across the transition between single‐domain (SD) to multidomain behavior. We calculate individual AMS tensors by modeling the magnetization response of a particle to weak applied fields in three orthogonal directions. The main AMS mode of elongated SD particles is not a homogeneous magnetization rotation, but focused alignment of spins at their edges and tips. In single‐vortex particles, vortex displacement is the dominant AMS mode, which focuses the largest magnetization changes in a planar region containing the vortex core, and perpendicular to the direction of vortex motion. In multi‐vortex structures a combined motion of all vortex centers can lead to high degrees of anisotropy when some motion patterns are energetically much easier to achieve than others. Plain Language Summary Many rocks have physical properties that vary according to the direction in which these are measured. Such rocks are called anisotropic. A fast and simple measurement which can uncover such anisotropy is the magnetization response to applied fields in different directions. This anisotropy of magnetic susceptibility (AMS), or AMS, is used in numerous applications to detect hidden information about geological processes. However, the interpretation of AMS data requires additional knowledge about the small magnetic mineral grains in the rock, because very small grains behave exactly opposite (inverse) to very large grains (normal). Here, we use hundreds of slices of electron microscopy images to reconstruct the exact shape of hundreds of such tiny magnetite grains. Then we run computer models to predict how these grains respond to applied magnetic fields. These data reveal how magnetic grains react to magnetic fields, and help us to understand better why small and large grains behave differently. Now we have a much better insight into the physical processes that are measured during AMS determinations made in laboratories all over the world. Key Points Micromagnetics and nanotomography elucidate the physical anisotropy of magnetic susceptibility (AMS) modes of natural magnetite particles The data explain the transition from inverse to normal AMS fabric with increasing grain size A systematic characterization of AMS modes in single‐domain and pseudo‐single domain magnetite particles is established