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Abstract The minerals carrying the magnetic remanence in geological samples are commonly a solid solution series of iron‐titanium spinels known as titanomagnetites. Despite the range of possible compositions within this series, micromagnetic studies that characterize the magnetic domain structures present in these minerals have typically focused on magnetite. No studies systematically comparing the domain‐states present in titanomagnetites have been undertaken since the discovery of the single vortex (SV) structure and the advent of modern micromagnetism. The magnetic properties of the titanomagnetite series are known to vary strongly with composition, which may influence the domain states present in these minerals, and therefore the magnetic stability of the samples bearing them. We present results from micromagnetic simulations of titanomagnetite ellipsoids of varying shape and composition to find the size ranges of the single domain (SD) and SV structures. These size ranges overlap, allowing for regions where the SD and SV structures are both available. These regions are of interest as they may lead to magnetic instability and “partial thermal remanent magnetization (pTRM) tails” in paleointensity experiments. We find that although this SD + SV zone occupies a narrow range of sizes for equidimensional magnetite, it is widest for intermediate (TM30‐40) titanomagnetite compositions, and increases for both oblate and prolate particles, with some compositions and sizes having an SD + SV zone up to 100s of nm wide. Our results help to explain the prevalence of pTRM tail‐like behavior in paleointensity experiments. They also highlight regions of particles with unusual domain states to target for further investigation into the definitive mechanism behind paleointensity failure. 

Abstract Micromagnetic modeling allows the systematic study of the effects of particle size and shape on the first‐order reversal curve (FORC) magnetic hysteresis response for magnetite particles in the single‐domain (SD) and pseudo‐single domain (PSD) particle size range. The interpretation of FORCs, though widely used, has been highly subjective. Here, we use micromagnetics to model randomly oriented distributions of particles to allow more physically meaningful interpretations. We show that one commonly found type of PSD particle—namely the single vortex (SV) particle—has far more complex signals than SD particles, with multiple peaks and troughs in the FORC distribution, where the peaks have higher switching fields for larger SV particles. Particles in the SD to SV transition zone have the lowest switching fields. Symmetrical and prolate particles display similar behavior, with distinctive peaks forming near the vertical axis of the FORC diagram. In contrast, highly oblate particles produce “butterfly” structures, suggesting that these are potentially diagnostic of particle morphology. We also consider FORC diagrams for distributions of particle sizes and shapes and produce an online application that users can use to build their own FORC distributions. There is good agreement between the model predictions for distributions of particle sizes and shapes, and the published experimental literature. 

Abstract The ability of rocks to hold a reliable record of the ancient geomagnetic field depends on the structure and stability of magnetic domain‐states contained within constituent particles. In paleomagnetic studies, the Day plot is an easily constructed graph of magnetic hysteresis parameters that is frequently used to estimate the likely magnetic recording stability of samples. Often samples plot in the region of the Day plot attributed to so‐called pseudo‐single‐domain particles with little understanding of the implications for domain‐states or recording fidelity. Here we use micromagnetic models to explore the hysteresis parameters of magnetite particles with idealized prolate and oblate truncated‐octahedral geometries containing single domain (SD), single‐vortex and occasionally multi‐vortex states. We show that these domain states exhibit a well‐defined trend in the Day plot that extends from the SD region well into the multi‐domain region, all of which are likely to be stable remanence carriers. We suggest that although the interpretation of the Day plot and its variants might be subject to ambiguities, if the magnetic mineralogy is known, it can still provide some useful insights about paleomagnetic specimens' dominant domain state, average particle sizes and, consequently, their paleomagnetic stability.