What causes pyrrhotite's magnetic properties to change at low temperatures?

What is it about?

Pyrrhotite is an essential magnetic mineral in the Erath's crust and some meteorites. It is an iron sulfide (Fe7S8) that is magnetic because of missing Fe atoms (defects) in the crystal structure. At low temperatures pyrrhotite undergoes massive changes in its response to magnetic fields. Up to this day, we don't fully understand this effect, even though it was found in 1964. In this study, we are using several magnetic techniques to study this low-temperature transition (≈30 K). We show that while there is a considerable change in the directional dependence of the magnetic properties (see Volk et al. 2016), no apparent changes in the crystal structure can be found using small angle neutron diffraction. However, using magnetic hysteresis data, we can see that the phase transition temperature changes when a magnetic field is applied.

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Why is it important?

The phase transition (Besnus) in pyrrhotite is a well studied but less well-understood phenomenon. In the Earth and Planetary Sciences, the change in magnetic remanence at the phase transition is often used to detect pyrrhotite in rocks and meteorites. As such, the nature of the phase transition is crucial to understand. Furthermore, pyrrhotite can be seen as an analog to other crystal systems, such as Fe7Se8.

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