Spin-Selective Charge Flow Drives Ultrafast Magnetic Transitions in Type-II Bipolar Magnetic Heterostructures

Ultrafast laser-driven changes in magnetic order provide a rich early-time platform for nonequilibrium physics. Established mechanisms and verified material candidates for ultrafast magnetic transitions in semiconducting magnets remain limited. Using density-functional theory (DFT) and real-time time-dependent DFT simulations, we demonstrate that spin-selective charge transfer across van der Waals layers, enabled by type-II bipolar band alignment, offers a robust way to control the magnetic order on the femtosecond timescale. In a representative CrSBr/CrSCl heterostructure, an antiferromagnetic-to-ferrimagnetic transition can be triggered within 40-56 fs under laser excitation above 4.2 eV, generating a net magnetic moment of ~1 µB per unit cell. The transition originates from a spin-flip charge transfer from spin-up valence states in CrSBr into spin-down conduction states in CrSCl. The resulting asymmetric demagnetization stabilizes residual spin polarization in CrSCl and induces a net magnetic moment. Additionally, electronic anisotropy facilitates an optically polarization-dependent charge redistribution, showing the main contribution from surface Cl atoms upon b-polarized excitation, while under a-polarized excitation the donation stems from interfacial Cr atoms. Our findings of spin-selective interlayer charge flow based on band alignment and symmetry breaking present a general mechanism for ultrafast optical control of magnetic order in low-dimensional materials.