The Besnus Transition in Single‐Domain 4C Pyrrhotite

Abstract

The discontinuous change in magnetic properties at a temperature of about 32 K, denoted as the Besnus transition, is a diagnostic feature for detecting 4C pyrrhotite in geological systems. The transition is grain size dependent, and it has been assumed that the single‐domain (SD) to multidomain (MD) threshold is in the micron size range. Here we present a combined crystallographic and magnetic study of a pure phase, SD 4C pyrrhotite that was produced by ball milling of a MD precursor. The mechanical treatment generated varying grain sizes of less than 20 μm, which are solely in a SD magnetic state. It also decreases the coherent scattering domains of the grains from 216 nm in the MD sample to 30 nm, which is taken to set limits for the MD/SD threshold in 4C pyrrhotite grains. The alternating current susceptibility response associated with the Besnus transition is diagnostic for distinguishing SD and MD 4C pyrrhotite. The unambiguous identification of SD 4C pyrrhotite provides a better insight into its occurrence in geological systems.

Polycrystalline texture causes magnetic instability in greigite

“…3b, inset). Both curves exhibit a concave shape with a maximum at T ≈ 160 K. Such behaviour is known for pyrrhotite (Fe 7 S 8 ) and was explained by the interplay between the Zeeman energy (E z ) and the magnetocrystalline anisotropy energy 35 . For the greigite flakes the departure from the Bloch law, however, can be attributed to the absence of saturation and to the finite size of the crystallites and their interfaces, which affect the local chemical potential 36,37 .…”

“…After the cycling, the gain in magnetization at 300 K (ΔM 300K ) is 12.9%. In general, SD particles exhibit similar cooling and warming curves whereas MD particles show demagnetization during the cycling due to domain wall dynamics 35,38 . Amplitudedependent ac susceptibility up to μ 0 H ac ≤ 1.5 mT at 300 K provides no evidence of domain-wall motions in our polycrystalline greigite flakes, i.e., the domain walls associated with the interfaces are relatively stiff.…”

Microbially Induced Anaerobic Oxidation of Magnetite to Maghemite in a Hydrocarbon‐Contaminated Aquifer

“…Iron reduction is known to sometimes produce secondary iron sulfides (pyrrhotite, greigite, and pyrite), depending on available electron donors and the prevailing conditions (Hansel et al., 2003). The monoclinic 4C pyrrhotite polytype possesses a magnetic transition temperature of ∼32 K (Koulialias et al., 2019; Volk et al., 2018) which was absent in low‐temperature magnetometry measurements. Further, pseudohexagonal pyrrhotite displays the “lambda transition” at temperatures between 200 and 250°C which was absent in high‐temperature susceptibility measurements on core samples, suggesting no evidence of magnetic iron sulfides (Figure S13 in Supporting Information ).…”

Magnetic structure and exchange interactions in pyrrhotite end member minerals: hexagonal FeS and monoclinic Fe7S8

“…More recent works have dealt mostly with the low temperature magnetic transition around 30 K also known as the Besnus transition, which is considered to allow the easy detection of the mineral in natural samples [15][16][17][18][19][20][21]. William Herbert et al [22] conducted diffusion measurements in Fe 1−x S as a function of temperature in the range 170 • C-400 • C. The measured activation energy for diffusion in the paramagnetic and structurally disordered pyrrhotite was 0.83 eV, which was found substantially lower compared to the activation energy of 1.18-1.30 eV in the fully magnetic ordered state.…”

“…Furthermore, Fe 1−x S nano-wires and nano-disks that display a particular type of AFM to FIM transition upon heating have been investigated and proposed for technological purposes such as phase-change magnetic memory devices [23][24][25]. Overall, pyrrhotites have attracted a considerable interest in fundamental magnetism over the past decades as a result of the diverse magnetic properties displayed through the iron deficiency coupled with many existing superstructures [20].…”