Control moiré magnetic moment interactions by light polarizations

The ability of controlling many-body interactions between carriers is essential to explore correlated phenomena and access complex electronic phase diagram. Transition metal dichalcogenide (TMD) moiré superlattice emerges to be a powerful platform for studying correlated physics due to its highly tunable many-body Hamiltonian. Recently, a non-equilibrium, light induced ferromagnetic states was observed in WSe₂/WS₂ heterobilayers, pointing to dynamic optical control of electron correlations. Here we report light polarization controlled magnetic interactions between localized moments in WSe₂/WS₂ moiré superlattices. Using a continuous-wave pump-probe reflective magnetic circular dichroism (RMCD) measurements, we show that ferromagnetic states can be induced by linearly polarized pump, i.e., no optical orientation of carrier spins are needed. However, under circularly polarized pump, we find that the RMCD response is drastically enhanced at low excitation regime, and near completely suppressed when the photo-excited exciton-spin density is comparable to moiré cell density. Our results reveal a collaboration-competition relation between excitons mediated spin-spin interaction and valley-pseudo magnetic field effect in the formation of ferromagnetic states.