On the Magnetism Behind the Besnus Transition in Monoclinic Pyrrhotite

Abstract

In monoclinic 4C pyrrhotite (ideal formula Fe7S8), ordered vacancy distribution forms a superstructure with strong ferrimagnetism that makes this mineral a major remanent magnetization carrier in the Earth's crust. The pronounced decrease in isothermal remanence magnetization at about 32 K, known as Besnus transition, is a characteristic trait that marks the low-temperature transition in 4C pyrrhotite, and it is used as a key to identify this mineral phase in rock samples. Here we take a nearly pure single pyrrhotite crystal (Fe6.97S8) from the Swiss Alps to study its Besnus transition in a broad mineral-magnetic approach that combines detailed structural analysis with static and dynamic magnetization experiments. All the magnetic properties inferred from the experimental data are discussed in the context of a recent model that explains interacting anisotropy fields in the 4C pyrrhotite caused by the vacancy arrangement as the origin of the Besnus transition.