Magnetic Hysteresis Properties of Magnetite: Trends With Particle Size and Shape.

Magnetic hysteresis measurements are routinely made in the Earth and planetary sciences to identify geologically meaningful magnetic recorders, and to study variations in present and past environments. Interpreting magnetic hysteresis data in terms of domain state and paleomagnetic stability are major motivations behind undertaking these measurements, but the interpretations remain fraught with challenges and ambiguities. To shed new light on these ambiguities, we have undertaken a systematic micromagnetic study to quantify the magnetic hysteresis behavior of room‐temperature magnetite as a function of particle size (45–195 nm; equivalent spherical volume diameter) and shape (oblate, prolate and equant); our models span uniformly magnetized single domain (SD) to non‐uniformly magnetized single vortex (SV) states. Within our models the reduced magnetization associated with SV particles marks a clear boundary between SD (≥0.5) and SV (<0.5) magnetite. We further identify particle sizes and shapes with unexpectedly low coercivity and coercivity of remanence. These low coercivity regions correspond to magnetite particles that typically have multiple possible magnetic domain state configurations, which have been previously linked to a zone of unstable magnetic recorders. Of all the hysteresis parameters investigated, transient hysteresis is most sensitive to particles that exhibit such domain state multiplicity. When experimental transient hysteresis is compared to paleointensity behavior, we show that increasing transience corresponds to more curved Arai plots and less accurate paleointensity results. We therefore strongly suggest that transient behavior should be more routinely measured during rock magnetic investigations. Plain Language Summary: Characterizing the magnetic properties and behavior of natural materials in the Earth and planetary sciences is key to identifying reliable magnetic recorders and variations in the environment. One standard method for achieving this is through room‐temperature measurements of magnetic hysteresis. However, the interpretation of magnetic hysteresis data remains one of the most challenging aspects of rock magnetism. To improve our understanding of magnetic hysteresis data, we have systematically investigated how the hysteresis properties of distributions of randomly oriented magnetite change as a function of particle size and shape and how this can help us quantify the contents of natural materials and identify rocks that may give unreliable magnetic signals. We model prolate, oblate and equant magnetite particles in the size range 45–195 nm. We show that magnetic hysteresis defines a clear boundary between simple uniform magnetic structures and more complex non‐uniform magnetic structures. We also identify the sizes and morphologies of magnetic particles that are likely to have unstable remanent magnetizations. These unstable particles are associated with distinctive hysteresis behavior, suggesting that hysteresis data can be used to identify rock samples dominated by such behavior. Key Points: Magnetic hysteresis is micromagnetically modeled for oblate, prolate and equant magnetite particles (45–195 nm)Hysteresis loop shape is highly dependent on particle size/shape due to complex switching behavior of single vortex statesTransient magnetic hysteresis is a powerful tool for identifying stable remanent magnetizations but is currently infrequently reported [ABSTRACT FROM AUTHOR]