The Besnus transition in 4C pyrrhotite revisited

Ferrimagnetic, monoclinic 4C pyrrhotite (Fe7S8) is the only iron sulphide with high relevance for palaeomagnetism and rock magnetism that can be identified in rock materials by its characteristic low-temperature anomaly. Despite its relevance in natural magnetism and the many magnetic studies over the last decades, the physics and the crystallography behind this anomaly, also denoted Besnus transition, is a matter of debate. In this study, we analyse the static and dynamic magnetization associated with the Besnus transition in conjunction with low-temperature structural data of 4C pyrrhotite reported in the literature. The correlation between the Fe–Fe bonds causing spin-orbit coupling and the dynamic magnetic properties show that the magnetic characteristics of the Besnus transition stem from the interaction of two magnetocrystalline anisotropy systems triggered by thermally induced structural changes on an atomic level in monoclinic 4C pyrrhotite. This refutes the widespread view that the Besnus transition is caused by a crystallographic change from monoclinic to triclinic.