Vacancy ordering schemes govern the antiferromagnetism in a modulated pyrrhotite structure

Superstructures with modulated vacancy order are a characteristic trait of the pyrrhotite solid-solution series (Fe1–xS, 0 < x ≤ 0.125), and they strongly affect the magnetocrystalline anisotropy that causes Fe-spin rotation. The effect of a stochastic vacancy order and the resulting structural heterogeneity at the atomic level on the magnetization is not resolved yet in detail. Here, we demonstrate that a thermally tailored antiferromagnetic pyrrhotite made up of fivefold and sixfold stacking modulations with five different Fe-vacancy ordering variants forms two distinct vacancy ordering schemes that govern the spin rotation. A field-dependent thermomagnetic anomaly between 220 and 170 K and a secondary hysteresis loop in the magnetization curve in a field of around 3 Tesla provide compelling evidence for two different Fe-spin rotation patterns. The simultaneous occurrence of the magnetic anomaly and two enthalpy changes link the magnetocrystalline anisotropy to structural distortions of Fe–Fe bonds in the vacancy-ordering schemes, thereby explaining the presence of the two spin rotation patterns. This finding highlights magnetic effects induced by vacancy order and underscores the relevance of its architecture in a key sulfide material extensively used in geoscience and cosmology and with potential for technical applications.