Anisotropy-induced spin disorder in intergrown, ferrimagnetic Fe$_7$ S$_8$ polytypes

The monosulfides of the pyrrhotite omission series (Fe$_{1−𝑥}$S, 0<𝑥≤0.125) are important remanence carriers for paleomagnetic reconstruction of the Earth's crust and extraterrestrial materials. The ferrimagnetic Fe$_7$S$_8$ polytypes are the endmembers, and their stacking modulations of full and vacant layers generate different magnetic anisotropy properties due to the cation-vacancy configurations. In this study, intergrown long-range ordered polytypes with four- and threefold modulation, i.e., 4C and 3C pyrrhotite, were prepared in a diffusion-driven process by quenching of a natural pyrrhotite crystal with randomized vacancies. In addition, a third constituent with coherence lengths of a few nanometers, denoted 3C*, was found that exhibits spin-glass behavior at about 10 K due to local magnetic anisotropies arising from vacancy-density variations. The concomitant occurrence of this nano-scale constituent with spin disorder and the long-range ordered polytypes indicate competitive diffusion-driven processes during Fe$_7$S$_8$ formation. Such information provides insight into the provenance and genesis of ferrimagnetic pyrrhotite in Earth and extraterrestrial systems and in a broader sense into vacancy-induced materials.