How the Magnetic Properties change when Pyrrhotite is cooled through its 30 K Phase Transition

What is it about?

The sulfide mineral pyrrhotite (Fe7S8) is magnetic and can be found in several classes of meteorites. At very low temperatures (30 K) the crystal goes through a phase transition, which is accompanied by stark changes of its magnetic properties. The magnetic information stored in all materials is sensitive to temperature changes. In paleomagnetism, high-temperature demagnetization is used to extract the magnetic field recorded by a rock to recover snapshots of the Earth's magnetic field. As we are trying to better understand the evolution of the magnetic fields in the early solar system, we use similar techniques on meteorites. One major difference between terrestrial and extraterrestrial rocks, apart from mineralogy and shock, are the temperatures the meteorite is exposed to. Space is cold and a meteorite can be cooled to temperatures lower than 130K. The questions we are answering in this article are: 1. Can we trust the magnetic signal that was recorded by pyrrhotite bearing meteorites, after they cooled in space and warmed back to room temperature after they landed on Earth? 2. What is the mechanism behind the phase transition?

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Why is it important?

As pyrrhotite is one of the major magnetic materials in several meteorite classes, especially the Martian meteorites, it can help us understand how the magnetic fields in the early solar system changed and how the dynamo in planets grow and die, as it was the case in Mars. However, in order to get answers to these questions, we have to be sure that the extracted information is accurate and can be trusted. Therefore, the low-temperature magnetic properties of pyrrhotite need to be well understood.

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