Mixed Magnetocrystalline Anisotropy and the Magnetization of Monoclinic 4C Pyrrhotite

Ferrimagnetic, monoclinic 4C pyrrhotite (Fe$_7$S$_8$), as the iron deficient endmember of the pyrrhotite solid-solution series (Fe$_{1–x}$S, 0 < x ≤ 0.125), is the main sulfide mineral widely used in rock and paleomagnetic studies of Earth's crust and extraterrestrial materials. Its particular magnetocrystalline anisotropy (MCA), characterized by an easy plane and extreme hard c-axis, originates from ordered vacancies and Fe cations in octahedral sites of a hexagonal close packing of sulfur. Here, we combine a detailed crystallographic analysis together with magnetization and magnetic torque data of a natural 4C pyrrhotite to demonstrate the configurational effect of occupied and vacant Fe-sites on the MCA. The crystallographic analysis reveals two sub-systems with six-fold symmetry set off by 30° that mainly arise from the vacancy ordering. The Fe cations in the two sub-systems have the same spin-orbit coupling as inferred from the out-of-plane spin rotations, but they differ in their dipolar coupling. The interplay of these coupling mechanisms generates a mixed MCA with respect to the two crystallographic sub-systems. Such anisotropy has a critical effect on the magnetization properties and becomes evident by the occurrence of 12-fold magnetic torques. ... mehrThe intrinsic directional magnetization behavior and the characteristic low-temperature magnetic anomaly known as the Besnus transition of 4C pyrrhotite can be derived from the crystallographic structure on the atomic scale that governs the MCA. This finding underscores the relevance of vacancy order on the magnetization of 4C pyrrhotite and its fidelity as a key magnetic mineral in geoscience and cosmology.