External-field control of Stoner ferromagnetism in TMDC monolayers.

Monolayer transition-metal dichalcogenides (TMDCs) realize a two-dimensional gapped Dirac fluid with strong spin-orbit coupling and spin-valley locking, providing a natural platform for examining exchange-driven magnetism under external control. Using the random-phase approximation with long-range Coulomb interactions, screening, and external electric and magnetic fields, we calculated the interacting ground-state energy. We extracted the spin polarization curve, the critical in-plane field for full spin alignment, and the nonlinear spin susceptibility. We find a predominantly continuous Stoner-type evolution over a broad density range. At the time, the first-order (Bloch-type) transition is pushed to extremely high carrier densities by the intrinsic gap and spin-valley coupling. As a result, magnetization grows smoothly with in-plane field, and the full-polarization field remains large across representative TMDCs. Both the critical field and nonlinear response are efficiently tunable by carrier density and a perpendicular electric field, which renormalizes the gap and spin splitting, thereby establishing TMDC monolayers as electrically controllable platforms for spin and valley functionalities.