Micromagnetic Constraints on the Grain Size Dependence and Magnetic Stability of Sub‐Micron Monoclinic Pyrrhotite (Fe ₇ S ₈ )

Abstract We present the first micromagnetic simulations for sub‐micron monoclinic 4C pyrrhotite (), a common mineral in rocks and sediments and an important mineral in paleomagnetic studies. Previous experimental studies on the magnetic properties of pyrrhotite had limited control over granulometry and focused primarily on larger, micron‐scale grain sizes. We model particles here with a hexagonal prismatic habit and uniaxial and triaxial basal plane anisotropy in the 5 nm to range. Single domain (SD) structures remain the lowest energy state for particles up to ≈2 μm in size, although it is possible to nucleate multidomain (MD) states in particles as small as 100 nm. MD structures consist of domains aligned within the basal plane separated by Néel walls, often with internal Néel lines; no vortex states are observed. In hysteresis and first‐order reversal curve simulations, sub‐micron pyrrhotite particle magnetizations switch coherently, giving rise to uniaxial‐SD signatures within the basal (001) plane; triaxial switching is not observed because the field step used in our models is too large to visualize the expected signals. Estimated relaxation times predict that hexagonal monoclinic pyrrhotite prisms have a ≈15 nm superparamagnetic threshold size and are geologically stable at sizes above≈20 nm. We find generally good agreement between experimental data and numerical predictions that assume uniaxial basal plane anisotropy, although there is little grain‐size overlap between the two data types, and questions remain regarding the accuracy of experimentally observed material parameters for pyrrhotite.