Interplay Among Stoichiometry, Phase Behavior, and Magnetism in the Altermagnet Candidate Troilite FeS

Troilite FeS has drawn recent attention as a potential collinear antiferromagnet exhibiting altermagnetic characteristics, including a spontaneous anomalous Hall effect (Takagi et al., 2024). Due to its high Néel temperature (590 K), this system is of particular interest. Complexities with FeS abound, however, including proximity to ordered-vacancy pyrrhotite FeS_1+x phases with uncompensated magnetic order, a ~410 K phase transition between high- and low-symmetry phases, and extreme sensitivity to stoichiometry. Here, we systematically investigate the bulk synthesis of FeS with tightly controlled S/Fe stoichiometry. We find that fine tuning the S:Fe loading during synthesis enables room-temperature stabilization of both the high- and low-symmetry phases, with high phase purity. Supplementing room-temperature powder X-ray diffraction and energy-dispersive X-ray spectroscopy characterization, we report on temperature-dependent X-ray diffraction and magnetometry measurements, exploring the full relationship between stoichiometry, phase behavior, and the various important magnetic transitions in FeS. This lays a vital foundation for further studies of altermagnetic behavior in this compound.