Abstract
AbstractA wide variety of Earth and planetary materials are very good recorders of paleomagnetic information. However, most magnetic grains in these materials are not in the stable single domain grain size range but are larger and in nonuniform vortex magnetization states. We provide a detailed account of vortex phenomena in geologic materials by simulating first‐order reversal curves (FORCs) via finite‐element micromagnetic modeling of magnetite nanoparticles with realistic morphologies. The particles have been reconstructed from focused ion beam nanotomography of magnetite‐bearing obsidian and accommodate single and multiple vortex structures. Single vortex (SV) grains have fingerprints with contributions to both the transient and transient‐free zones of FORC diagrams. A fundamental feature of the SV fingerprint is a central ridge, representing a distribution of negative saturation vortex annihilation fields. SV irreversible events at multiple field values along different FORC branches determine the asymmetry in the upper and lower lobes of generic bulk FORC diagrams of natural materials with grains predominantly in the vortex state. Multivortex (MV) FORC signatures are modeled here for the first time. MV grains contribute mostly to the transient‐free zone of a FORC diagram, averaging out to create a broad central peak. The intensity of the central peak is higher than that of the lobes, implying that MV particles are more abundant than SV particles in geologic materials with vortex state fingerprints. The abundance of MV particles, as well as their single domain‐like properties point to MV grains being the main natural remanent magnetization carriers in geologic materials.
Highlights
Rocks can record information about the geomagnetic field intensity and direction, and preserve it over geologic timescales
We show that Single vortex (SV) and Multi vortex (MV) micromagnetic configurations control the geometry of first-order reversal curves (FORCs) signatures observed experimentally, and that they account for most of the features observed in samples with particles that span the entire vortex state grain size continuum
1) We have provided a detailed understanding of vortex-related phenomena in geologic materials by simulating FORCs using finite-element micromagnetic modeling of magnetite nanoparticles with realistic morphologies
Summary
Rocks can record information about the geomagnetic field intensity and direction, and preserve it over geologic timescales. Larger particles have non-uniform magnetization states, and have been traditionally classified as pseudo single domain (PSD), because of their transitional properties between SD and larger, multi domain (MD) grains. These intermediate-size grains have the capacity to acquire remanent magnetization efficiently, like SD particles, but have lower coercivities, akin to MD particles [Stacey, 1962, 1963]. For PSD magnetite, grain size ranges from around 100 nm to a few μm, depending on grain morphology These particles are not uniformly magnetized, but are not partitioned into magnetic domains either. Since vortex phenomena adequately explain the physics of the magnetization in these particles, Roberts et al [2017] have proposed replacing the term ’PSD state’, which is used purely functionally, with ’vortex state’
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