Origin of exotic ferromagnetic behavior in exfoliated layered transition metal dichalcogenides MoS2 and WS2

Bulk layered transition metal dichalcogenides (TMDs) show diamagnetic properties. When exfoliated, the materials' band gap increases and changes from an indirect band gap to a direct one. During the exfoliation, the TMDs may undergo a phase transition from 2H to 1T polymorph, which is likely electronically driven and accompanied by a metal-insulator transition. A significantly higher efficiency of the exfoliation was observed using sodium naphthalenide compared to butyllithium. Moreover we demonstrate that the exfoliation has a dramatic influence on the magnetic properties of two TMDs, MoS2 and WS2. These materials become partly ferromagnetic upon exfoliation, which is a highly unexpected behavior. Exotic ferromagnetism is generally observed on samples with a high degree of exfoliation, which indicates the association of this effect with defects formed on the edges of dichalcogenide sheets. Such an exotic ferromagnetic behavior, if properly understood and brought under material engineering control, shall open the door to new applications of these materials.