Micromagnetic Modeling of Thermoremanent Magnetization in Small Natural Pseudo-Single Domain Magnetite Particles

Earth and planetary rocks record magnetic variations and are crucial recorders to further our understanding of the formation and evolution of the Earth’s magnetic field. The acquisition and stability of remanent magnetization in rocks are controlled by the magnetization state of magnetic particles. Where the thermoremanent magnetization (TRM) of pseudo-single domain (PSD) magnetite particles probably dominates the natural remanence of many igneous rocks. These PSD particles are therefore important carriers of paleomagnetic information. Nevertheless, the extension of Néel's (1955) elegant analytical theory of single domain (SD) particles to larger PSD particles meets with substantial technical challenges. Understanding the unblocking of energy barriers between energetically favorable domain states is important when predicting the TRM behavior of natural particles. While categorizing different magnetization states is manageable for simple SD particles, it will become increasingly difficult with increasing particle size. Mapping the full energy landscape for PSD particles can be challenging and time-consuming. 

Here we present an automated process that can categorize all possible local energy minima (LEM) from micromagnetic modeling results. The automated process determines if magnetic states with approximately equal total energy are in the same reversible region. We incorporate calculations of energy barriers between LEMs for an assemblage of natural magnetite particles obtained by focused-ion-beam (FIB) nano-tomography. The technique was tested for a suite of PSD particles from single-vortex to multi-vortex states, where we systematically studied the energy landscape and relaxation time as a function of temperature. To map the energy landscape, LEM and energy barriers between all possible LEM were calculated for temperatures between room temperature and Curie temperature. Combining these results in a statistical model allows for predicting the TRM acquisition of individual particles and isotropic ensembles of equal particles. The results are discussed in terms of the TRM behavior of natural PSD magnetite, magnetic stability, and the implications for paleomagnetic research.