Room-temperature ferromagnetism engineering in non-van der Waals Fe7S8 via molybdenum doping-induced sulfur vacancy control

Non-van der Waals magnetic materials with tunable room-temperature ferromagnetism (RTFM) are essential for next-generation spintronic devices. However, achieving quantitative control of defects in experiments remains challenging. Here, we demonstrate reversible RTFM modulation in wrinkled, nonstoichiometric Fe7S8 thin films through molybdenum (Mo) doping. Using polymer-assisted deposition, we synthesize dual-phase Fe7S8 with controlled Mo incorporation, which systematically reduces sulfur vacancy concentration and drives the composition toward stoichiometric Fe7S8. Remarkably, the saturation magnetization exhibits a non-monotonic trend, reaching a maximum of 1.06 emu/g for a nominal 15 at. % Mo-doped sample, attributed to enhanced Fe–S–Fe superexchange pathways due to vacancy passivation. Concurrently, Mo doping triggers the in situ formation of MoS2 nanosheets and MoS2@Fe7S8 heterostructures without secondary crystalline phases. Our results establish Mo doping as a dual-function strategy for both magnetic engineering and heterointerface creation in non-layered transition metal sulfides, opening avenues for defect-driven spin functionality beyond van der Waals magnets.