  Strong light-matter interactions within nanoscale structures offer the
possibility of optically controlling material properties. Motivated by the
recent discovery of intrinsic long-range magnetic order in two-dimensional
materials, which allows for the creation of novel magnetic devices of
unprecedented small size, we predict that light can couple with magnetism and
efficiently tune magnetic orders of monolayer ruthenium trichloride (RuCl3).
First-principles calculations show that both free carriers and optically
excited electron-hole pairs can switch monolayer RuCl3 from the proximate
spin-liquid phase to a stable ferromagnetic phase. Specifically, a moderate
electron-hole pair density (on the order of 10^13 cm-2) can significantly
stabilize the ferromagnetic phase by 10 meV/f.u. in comparison to the zigzag
phase, so that the predicted ferromagnetism can be driven by optical pumping
experiments. Analysis shows that this magnetic phase transition is driven by a
combined effect of doping-induced lattice strain and itinerant ferromagnetism.
According to the Ising-model calculation, we find that the Curie temperature of
the ferromagnetic phase can be increased significantly by raising carrier or
electron-hole pair density. This enhanced opto-magnetic effect opens new
opportunities to manipulate two-dimensional magnetism through non-contact,
optical approaches.
