The Besnus Transition in Single‐Domain 4C Pyrrhotite

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. Key Points A combined crystallographic and magnetic approach is presented to identify and characterize single domain 4C pyrrhotite The multi‐ to single‐domain threshold in 4C pyrrhotite is attributed to the coherent scattering domains in a range between 220 and 30 nm The dynamic magnetic response of the Besnus transition is a diagnostic tool to identify single‐domain 4C pyrrhotite in geological systems