Abstract
Pyrrhotite, Fe7S8, is a commonly occurring carrier of magnetic remanence and has a low temperature transition, the Besnus transition, involving a change in spin state. Variations of the thermodynamic, magnetic and elastic properties through this transition at ∼33 K in a natural sample of 4C pyrrhotite have been tested against a group theoretical model for coupling between order parameters relating to Fe/vacancy ordering (irrep U1(1/2,0,1/4)) and magnetic ordering (irreps m and m). Magnetoelastic coupling is weak but the pre-existing microstructure of ferroelastic and magnetic domains, that develop as a consequence of Fe/vacancy and ferrimagnetic ordering during slow cooling in nature (P63/mmc → C2′/c′), causes subtle changes in the low temperature transition (C2′/c′ → P). The Besnus transition involves a rotation of magnetic moments out of the a–c plane of the monoclinic structure, but it appears that the transition temperature might vary locally according to whether it is taking place within the pre-existing domain walls or in the domains that they separate. Evidence of metamagnetic transitions suggests that the magnetic field–temperature phase diagram will display some interesting diversity. Low temperature magnetic transitions in minerals of importance to the palaeomagnetism community have been used to identify the presence of magnetite and haematite in rocks and the Besnus transition is diagnostic of the existence of pyrrhotite, Fe7S8.
Highlights
Minerals which hold a remanent magnetic signal for tens to hundreds of millions of years provide a material record against which models of the evolution of the Earth’s magnetic eld and past tectonic plate motions have been tested
We present new heat capacity, magnetic, elastic and anelastic data collected as a function of temperature and magnetic eld to show that the Besnus transition in a natural pyrrhotite crystal with composition Fe7S8 appears to occur in stages which depend on the orientation of the eld with respect to different crystallographic twins arising from the hexagonal → 4C monoclinic transition
Magnetism and elasticity data through the temperature interval ∼30–35 K presented here for a natural sample of pyrrhotite with composition close to Fe7S8 are consistent with a discrete phase transition
Summary
Minerals which hold a remanent magnetic signal for tens to hundreds of millions of years provide a material record against which models of the evolution of the Earth’s magnetic eld and past tectonic plate motions have been tested. The primary objective of the present study was to investigate magnetoelastic and order parameter coupling associated with the low temperature transition at ∼35 K in a natural sample of pyrrhotite with composition Fe7S8. This transition has drawn comparisons with the Morin transition in hematite and the Verwey transition in magnetite [17,18,19,20,21], with a suggestion of in uence due to the Jahn–Teller effect [22]. Magnetoelastic and order parameter coupling behaviour associated with the hexagonal (P63/mmc)–monoclinic (C2′/c′) transition in the same sample is described in a separate paper (Haines et al 2020a [43])
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