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Recent micromagnetic simulations have found that particles in the transitional zone between the single domain (SD) and single vortex (SV) zone are prone to thermal and external magnetic field instabilities that could adversely affect the accuracy of interpretations of paleomagnetic recordings. In this study, we attempt to evaluate the internal magnetization characteristics of these magnetically unstable (MU) particles and the influence on paleomagnetic observations by simulating the magnetic behavior of 68–104 nm truncated octahedral magnetite particles using the MERRILL modeling software. We found that: (a) The size region of the "MU zone" for grains of truncated octahedron shape is different from cubic octahedrons and spheres, indicating that the zone may be controlled by the geometry and shape of particles; (b) The MU zone has a range of 79–97 nm region, which is dominated by a hard‐axis aligned single vortex (HSV); and (c) MU particles are unstable as a function of temperature and have low coercive fields. Finally, the numerical fitting of hysteresis parameters for experimental data suggests that the influence of such MU particles in samples should not be ignored, especially for samples with fine‐grained magnetic minerals as the primary magnetic recording carriers. This research has extended our understanding of the behavior of the "MU zone" and its significance on paleomagnetic records. Plain Language Summary: Magnetic minerals in geological samples can record geomagnetic information over billions of years, which makes it possible to study the evolution of the Earth. However, this ability of magnetic minerals is greatly affected by particle size and the range we call these "magnetically unstable (MU) zone." Recently, micromagnetic simulations found that the particles in the "MU zone" can be easily affected by slight thermal disturbances, which means a detailed investigation of the "MU zone" is very important to improve the accuracy of geological information interpretations. This study presents a detailed evolution of the MU zone with grain size and temperature, and will deepen our understanding of the magnetic characteristics of the MU zone and its significance on paleomagnetic records. Key Points: The magnetically unstable (MU) zone for equant truncated octahedra is ∼20 nm wide and is geometry and shape dependentThe unstable grains are characterized by low coercivities owing to their multiple possible states with small intervening energy barriersThe domain states of MU grains vary with temperature, and their influence on geological samples should not be ignored [ABSTRACT FROM AUTHOR]

Isothermal remanent magnetization (IRM) curves are used widely in rock magnetic and environmental magnetic studies to characterize magnetic properties of natural samples. Previous theoretical and experimental investigations indicate that magnetostatic interactions can strongly affect the IRM behavior. In order to quantitatively investigate this effect, we modeled IRM curves and hysteresis properties for magnetite assemblages with variable morphologies (grain size, elongation, and shape) using the finite‐element micromagnetic modeling code MERRILL. We also conducted an analytical study of IRM behavior for noninteracting single domain grains utilizing the Stoner–Wohlfarth model. It is found that multiple Gaussian components do not necessarily reflect multiple magnetic mineral populations. For example, magnetostatic interactions can produce coercivity distributions that resemble results of some samples containing a detrital and extracellular (D + EX) and a biogenic soft or hard (BS/BH) coercivity components. Simulated IRM curves for different morphologies of interacting flattened grains are similar, indicating that the effect of interactions can overshadow the effect of grain morphologies. Moreover, our micromagnetic and analytical calculations consistently indicate that the IRM curves for noninteracting fine‐grained assemblages are mostly determined by grain shape (elongated or flattened) and elongation distributions of the magnetic mineral assemblages. Our simulations present possible outcome of IRM curves for magnetic mineral assemblages with controlled grain size, shape and microstructures, which provides important theoretical constraints on characterizing complex magnetic mineral components in natural samples through IRM curves and hysteresis. Plain Language Summary: Tiny magnetic mineral grains, often in the nanometer and micrometer size, in geological samples retain critical information about past environmental changes and geomagnetic field variations. Knowing the properties of magnetic minerals is crucial to decode magnetic mineral records. Decomposing isothermal remanent magnetization (IRM) curve into multiple magnetic mineral parts is a powerful technique, but is often complicated by the common presence of magnetic interactions where magnetic mineral grains are clumped to each other. This process is similar to attracting and repelling of magnets, but occurs at very small, microscopic scale. Here we have applied computer‐modeling to calculate how neighboring magnetic grains affect the IRM curves. Our simulation results provide new physical constraints on analyzing IRM data, which is important to make robust environmental and geophysical interpretations. Key Points: Micromagnetic calculations elucidate potential origin of multiple Gaussian remanent coercivity componentsThe elongation distribution and grain morphologies strongly affect the shape of IRM curves for noninteracting fine‐grained particlesMagnetostatic interactions can overshadow the effect of grain morphologies and can also cause multiple Gaussian coercivity components [ABSTRACT FROM AUTHOR]