Monoclinic pyrrhotite (Fe7S8) owes its ferrimagnetism to an ordered array of Fe vacancies. Its agnetic properties change markedly around 30K, in what is known as the Besnus transition. Plausible explanations for the Besnus transition are either due to changes in crystalline anisotropy from a transformation in crystal symmetry or from the establishment of a two-phase system with magnetic interaction between the two phases. To help resolve this discrepancy, we measured hysteresis loops every 5° and backfield curves every 10° in the basal plane of an oriented single crystal of monoclinic pyrrhotite at 300 K and every 2K from 50K through the Besnus transition until 20K. Between 50 and 30K, hysteresis loops possess double inflections between crystallographic a-axes and only a single inflection parallel to the a-axes. Magnetization energy calculations and relative differences of the loops show a sixfold symmetry in this temperature range. We propose that the inflections stem from magnetic axis switching, which is both field and temperature dependent, in a manner somewhat analogous to an isotropic point where magnetocrystalline constants change their sign. The Besnus transition is best characterized by changes in magnetic remanence and coercivity over a 6° temperature span(28-34K)with a maximum rate of change at 30 K. A surprising yet puzzling finding is that the coercivity ratio becomes less than unity below the transition when fourfold symmetry arises. Because the changes in magnetic parameters are linked to the crystal structure, we conclude the Besnus transition owes its origin to a distortion of the crystallographic axes below 30 K rather than an apparition of a two-phase system.An isothermal magnetization of natural pyrrhotite cycled from room temperature to successively lower temperatures through the Besnus transition decreases 2-4 times less than equivalent grain sizes of magnetite, with less than a 10 per cent loss in remanence between 300 and 150 K in pseudo-single-domain (PSD) pyrrhotite.As PSD monoclinic pyrrhotite carries the magnetic remanence in some meteorites, it is likely that low-temperature cycling in space then warming to ambient conditions at the Earth's surface will have only a minor influence on palaeointensity values derived from those meteorites.
Bibliographical noteFunding Information:
This work was supported by Deutsche Forschungsgemeinschaft project WA3402/1-1 and GI712/6-1 under the auspices of SPP1488, Planetary Magnetism. We thank Rupert Hochleitner of the Munich Mineralogical State Collection (Mineralogische Staatssammlung M?nchen) and Jean Pohl for providing the samples. Helpful reviews by Ann Hirt and Myriam Kars improved this paper. Editorial handling by Eduard Petrovsky is appreciated. Parts of thisworkwere carried out in theCharacterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program. We thank Nicholas Seaton for his help in obtaining EBSD and EDS data. Part of this work was performed as a Visiting Fellow at the Institute for Rock Magnetism (IRM) at the University of Minnesota. The IRM is a US National multiuser facility supported through the Instrumentation and Facilities program of the National Science Foundation, Earth Sciences Division, and by funding from the University of Minnesota.
- Magnetic and electrical properties
- Magnetic mineralogy and petrology
- Phase transitions
- Rock and mineral magnetism