Contrasting Ferromagnetism in FeS₂ Pyrites Induced by Cobalt Doping and by Electrostatic Gating

Pyrites are a family of transition metal disulfides that host a wide variety of electronic ground states. In particular, the ferromagnetic transition in Co-doped FeS₂ has been studied as a source of variable, experimentally accessible electron spin polarizations. Advances in ionic liquid gating have provided an alternative way to change the electron concentration via electrostatic gating instead of chemical doping, thus avoiding the introduction of impurity scattering centers. We use Density Functional Theory combined with a tight-binding model to compare electrostatic gating and chemical doping with cobalt. Using maximally localized Wannier functions to construct a tight-binding model, we calculate the magnetic susceptibility across the entire range of compounds Fe_1-xCo_xS₂. We find that electrostatic gating requires a higher electron concentration than the equivalent in Co-doping to induce ferromagnetism via a Stoner-like mechanism. We attribute this behavior to the formation of a narrow cobalt band near the bottom of the conduction band under doping. This band is not formed for electrostatic gating and wide, low DOS sulfur states are filled instead.