Transition warming and cooling remanences in pyrrhotite and hematite

Abstract

Magnetic minerals acquire a remanent magnetization when cooled or warmed in an applied field H through a phase transition. This paper reports the first observations of transition cooling and warming remanent magnetizations (TrCRM and TrWRM) in pyrrhotite (Fe7S8) and hematite (αFe2O3), with phase transitions at 32 K (Besnus transition) and 250 K (Morin transition). TrWRM was produced by warming in a 2 mT field from 10 to 300 K and TrCRM by cooling in 2 mT from 300 to 10 K. In both experiments, magnetization M was measured at 5 K intervals. The largest changes in M occurred over a 20 K interval flanking the Besnus or Morin transition but substantial changes also occurred away from any transition, presumably due to continuous changes in anisotropy. The triclinic low-temperature phase of pyrrhotite has high remanence efficiency compared to the room-temperature monoclinic phase. When H was zeroed at 300 K, only 35-40% of M survived as TrWRM, but in TrCRM experiments when H was zeroed at 10 K, practically 100% of M was preserved. After zero-field cooling to 10 K, 50-60% of TrWRM was retained but <15% of TrCRM survived zero-field warming to 300 K. In hematite, both TrWRM of the room-temperature spin-canted phase and TrCRM of the low-temperature defect phase exist, as shown by net remanence production following complete warming-cooling or cooling-warming cycles between 10 and 300 K. Details of remanence acquisition/demagnetization in crossing the Morin transition could not be resolved, however. Symmetries were observed for both minerals. Single-domain size pyrrhotite crystals had mirror-image field-offand field-on warming curves below 32 K, during TrWRM production/TrCRM loss at the 32 K transition, and up to ≈150 K. Submicron single-domain hematite had almost complete symmetry between field-offand field-on curves during cooling as well as warming, both below and above the Morin transition but not in the transition region itself. This is the first time that mirror-image symmetries, observed previously for pseudo-single-domain and multidomain magnetite, have been documented for mineral grains in a single-domain state.