Magnetic Domain State and Anisotropy in Monoclinic 4C Pyrrhotite (Fe ₇ S ₈ ) From First‐Order Reversal Curve Diagrams

Abstract First‐order reversal curve (FORC) diagrams are an important tool for assessing the domain state and anisotropy type of magnetic particles, as well as for detecting magnetostatic interactions among particles, in paleoclimatic, geomagnetic, tectonic, and planetary studies. We present here FORC diagrams for diverse natural monoclinic 4C pyrrhotite (Fe 7 S 8 ) samples, including sieved particle size fractions. Experimental FORC distributions for natural pyrrhotite have a dominant asymmetric “kidney shaped” feature for fine particles in the stable single domain (SD) size range. As particles coarsen to the multi‐domain state, FORC distributions diverge vertically at low coercivities. Vertical spreading in FORC distributions is also an intrinsic response of magnetic particle systems with multiaxial anisotropy even for magnetostatically noninteracting SD particles. The observed kidney shaped FORC distributions are attributed to twin domain and magnetic moment pinning along easy directions that follow the hexagonal parent structure to produce an apparent triaxial magnetocrystalline anisotropy signal. Similar FORC distributions have been reported for pyrrhotite, hematite, and smythite, which all have basal plane triaxial anisotropy. Central ridge‐type signatures, which are typical of noninteracting uniaxial SD particle assemblages, have been reported for some pyrrhotite samples, but are not documented here. Our results explain FORC features for pyrrhotite and provide a catalog for comparison with other studies.