Anisotropy-induced spin disorder in intergrown, ferrimagnetic Fe7S8 polytypes

Abstract

The monosulfides of the pyrrhotite omission series (Fe1−xS, 0<x≤0.125) are important remanence carriers for paleomagnetic reconstruction of the Earth's crust and extraterrestrial materials. The ferrimagnetic Fe7S8 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 Fe7S8 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.